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Road safety effects of roundabouts: A meta-analysis
Abstract
This paper presents a meta-analysis of the road safety effects of converting junctions to roundabouts. 44 studies containing a total of 154 estimates of effect were included. Based on a meta-regression analysis, converting junctions to roundabouts is associated with a reduction of fatal accidents of about 65% and a reduction of injury accidents of about 40%. The mean effect on property-damage-only accidents is ambiguous. Summary estimates of effect are robust for fatal and injury accidents, but vary depending on the model of meta-analysis and the treatment of outlying data points for property-damage-only accidents. A trim-and-fill analysis suggests a weak tendency for publication bias, with modest influence on summary estimates of effect. It is concluded that roundabouts are very effective in reducing traffic fatalities.
... Modern roundabouts require traffic from all sides to yield circulating traffic, with sufficient horizontal curvature to slow entry and circulation speeds, and they are widely touted as an effective safety intervention to replace traditional intersections to lower accident rates and severities. This is in part due to their reduction in the number and types of conflicts between vehicles and/or pedestrians moving in different directions through the intersection [1,2]. In the meantime, forecasting roundabout traffic accidents and identifying factors determining the severity of injuries to road users is an essential component of accident prevention decision-making and avoiding casualties or damage. ...
... In order to fit a Multivariable Grey Model to, and to predict the value of the time series variables of X (0) and X (1) , we can also introduce the following two vectors of continuous time functions that correspond to the model fitted values with regards to X (0) and X (1) , respectively. ...
... In order to fit a Multivariable Grey Model to, and to predict the value of the time series variables of X (0) and X (1) , we can also introduce the following two vectors of continuous time functions that correspond to the model fitted values with regards to X (0) and X (1) , respectively. ...
We propose a multivariate Grey-Markov model to quantify traffic accident risk from different causality factors in roundabouts that is uniquely suited for the scarce and stochastic traffic crash data from roundabouts. A data sample of traffic crashes occurring in roundabouts in the U.S. State of Michigan from 2016 to 2021 was collected to investigate the capabilities of this modeling methodology. The multivariate grey model (MGM(1,4)) was constructed using grey relational analysis to determine the best dimensions for model optimization. Then, the Markov chain is introduced to address the unfitness of stochastic, fluctuating data in the MGM(1,4) model. Finally, our proposed hybrid MGM(1,4)-Markov model is compared with other models and validated. This study highlights the superior predictive performance of our MGM(1,4)-Markov model in fore-casting roundabout traffic accidents under data-limited conditions, achieving a 3.02% accuracy rate, in contrast to the traditional GM(1,1) model at 8.30% and the MGM(1,4) model at 4.47%. Moreover, incorporating human, vehicle, and environmental risk factors into a multivariate crash system yields more accurate predictions than merely aggregating crash counts.
... Roundabouts are of particular interest because they are considered safer due to their ability to slow through traffic and reduce the number of conflict points. In fact, several studies indicate that roundabouts, both conventional and unconventional, are effective in reducing collisions resulting in injuries or fatalities [1][2][3][4]. The widespread use of roundabouts is also justified by their positive perception by road users [5,6]. ...
... Although accidents at roundabouts are generally less severe, Daniels et al. [7] shows a tendency for more severe accidents to occur at night at roundabouts. Several studies have provided strong evidence that drivers' ability to avoid collisions is impaired in low light conditions [2,[8][9][10]. In addition, nighttime crashes have been shown to decrease after the installation of overhead lighting [2] and during longer days, such as the summer months. ...
... Several studies have provided strong evidence that drivers' ability to avoid collisions is impaired in low light conditions [2,[8][9][10]. In addition, nighttime crashes have been shown to decrease after the installation of overhead lighting [2] and during longer days, such as the summer months. Several researchers [11][12][13][14] developed an ordered probit model considering a variety of independent variables and concluded that darkness increases injury severity. ...
Roundabouts have become a common type of intersection design in many countries. The area of the center island can be used to install a photovoltaic system to power local loads such as lighting systems. The objective of this study is to evaluate the degree of self-sufficiency that a roundabout can achieve depending on the availability of the area for the installation of a photovoltaic system and the energy demand for lighting. The methodology is divided into five steps aimed at calculating the parameters required to characterize the roundabouts from the point of view of the electricity that can be generated by the photovoltaic systems installed, and then to evaluate the energy consumption required to operate the different system solutions for lighting. The mini roundabouts are not considered as a location for the photovoltaic system; in fact, the minimum diameter must be between 29 and 34 m. Considering the available irradiance in Italy, systems with monocrystalline silicon modules are sufficient to ensure energy self-sufficiency at diameters of 24 m or more. Systems with polycrystalline silicon modules are suitable to ensure energy self-sufficiency at diameters of 25/26 m or more. Photovoltaic (PV) technology continues to make progress in increasing efficiency, such as bifacial PV modules. This means that even smaller roundabouts could be eligible for a PV system sized to meet local electricity needs.
... In structural improvement, we looked at interventions that brought about the change in road network, signage etc. In this category one review assessed the safety effects of street lights [47], another one looked at the effects and efficiency of digital countdown timers [50], and the remaining two reviews focused on the effectiveness of converting intersections into roundabouts [48,49]. Of the four reviews, only one review by Fu et al. [50] included primary studies from high income, uppermiddle and lower-middle countries, while all the other reviews only included primary studies from high income countries. ...
... Of the four reviews, only one review by Fu et al. [50] included primary studies from high income, uppermiddle and lower-middle countries, while all the other reviews only included primary studies from high income countries. A lot of overlap existed between two studies conducted by Elvik [48,49] as all the studies used in the systematic review by [48] were also included in the 2017 review. Table 5 below gives systematic reviews which were considered in this category. ...
... Other reviews in this category included Elvik's 2003 and 2017 [48,49] studies in which the effects of converting intersections to roundabouts in order to improve on road safety was examined. Elvik found that roundabouts were associated with a reduction of 30 to 50% in the number of injury accidents, and fatal accidents were reduced by 50 to 70% [48]. ...
Background
Road traffic crashes (RTCs) are among the eight-leading causes of death globally. Strategies and policies have been put in place by many countries to reduce RTCs and to prevent RTCs and related injuries/deaths.
Methods
In this review, we searched the following databases Ovid Medline, Embase, Cochrane Database of Systematic Reviews, Epistemonikos, Web of Science, and LILACS for reviews matching our inclusion criteria between periods January 1950 and March 2020. We did not apply language or publication restrictions in the searches. We, however, excluded reviews that focused primarily on injury prevention and reviews that looked at crashes not involving a motor vehicle.
Results
We identified 35 systematic reviews matching our inclusion criteria and most of the reviews (33/35) included studies strictly from high-income countries. Most reviews were published before 2015, with only 5 published between 2015 and 2020. Methodological quality varied between reviews. Most reviews focused on enforcement intervention. There was strong evidence that random breath testing, selective breath testing, and sobriety checkpoints were effective in reducing alcohol-related crashes and associated fatal and nonfatal injuries. Other reviews found that sobriety checkpoints reduced the number of crashes by 17% [CI: (− 20, − 14)]. Road safety campaigns were found to reduce the numbers of RTCs by 9% [CI: (− 11, − 8%)]. Mass media campaigns indicated some median decrease in crashes across all studies and all levels of crash severity was 10% (IQR: 6 to 14%). Converting intersections to roundabouts was associated with a reduction of 30 to 50% in the number of RTCs resulting in injury and property damage. Electronic stability control measure was found to reduce single-vehicle crashes by − 49% [95% CI: (− 55, − 42%)]. No evidence was found to indicate that post-license driver education is effective in preventing road traffic injuries or crashes.
Conclusion
There were many systematic reviews of varying quality available which included studies that were conducted in high-income settings. The overview has found that behavioural based interventions are very effective in reducing RTCs.
... List of Studies Included in the Meta-Analysis-Based Research Conducted by Elvik (2003) [63]. More recently, a meta-analysis study investigated the road safety effects of converting junctions into roundabouts [65]. The objective of this study was to update the meta-analysis study conducted in the 2000s [63], as the data used in this older study was not only outdated, but it also focused on non-US studies. ...
... These studies were conducted between 1975 and 2014. Based on a total of 154 estimates of effect, this more recent meta-analysis research [65] showed that converting junctions into roundabouts was associated with a reduction in fatal accidents of about 65% and a reduction in injury accidents of about 40%. Based on these results, this comprehensive research concluded that roundabouts are very effective in reducing traffic fatalities. ...
... List of Studies Included in the Meta-Analysis-Based Research Conducted by Elvik (2017)[65]. ...
Several research studies conducted in North America and Europe have consistently shown that converting signalized intersections into roundabouts leads to safety benefits. These studies have led North American and European transportation agencies to convert hundreds of signalized intersections into roundabouts over the last few decades. Meanwhile, the Emirate of Abu Dhabi, the largest emirate in the United Arab Emirates (UAE), has replaced many of its long-standing roundabouts with signalized intersections (i.e., controlled by traffic lights) despite the fact that no UAE-based scientific study comparing the safety performance of these two intersection types can be found in the literature. Hence, the objectives of this paper are to (i) identify factors that significantly contribute to fatal intersection-related crashes and (ii) assess the in-service safety performance of signalized intersections and roundabouts. It is anticipated that the findings from this research will provide road designers and decision-makers with much needed scientific guidance on which factors contribute to fatal intersection crashes as well as on whether converting existing roundabouts into signalized intersections improves public safety or not in Abu Dhabi. The findings from this research should also benefit neighboring Gulf Cooperation Council (GCC) countries considering their similarities in road design, traffic characteristics, and driving culture.
... The United States and a number of European countries have converted hundreds of signalized intersections into roundabouts in the recent past. Research based on 44 intersection safety studies carried out between 1975 and 2014 in different European countries, the United States, and Australia, concluded: "It is therefore clear beyond reasonable doubt that roundabouts improve road safety and are particularly effective in reducing fatal accidents" (Elvik, 2017). ...
... However, a different phenomenon has happened in the United Arab Emirates (UAE). That is, the UAE has opted to replace many of its long-standing roundabouts with signalized intersections in the past few years (Rousseau, 2018;Wam, 2019), though a significant amount of research has shown that this move may pose a mobility safety threat to the motoring public in the UAE ( Elvik, 2017). Meanwhile, there has been no local research aimed at comparing the in-service safety performance of these two intersection types in order to assess whether the UAE's continued push for signalized intersections is supported. ...
Intersection crashes have accounted for a large portion of all fatal road crashes worldwide. Much research has been devoted to comparing safety levels among different intersection types, such as signalized intersections and roundabouts. While previous research efforts have been insightful, none have focused on crashes occurring in the United Arab Emirates (UAE). This is worth noting because while several states in the United States (US), as well as countries in Europe, have replaced many of their signalized intersections with roundabouts as an attempt to curb severe injuries and improve safety, the UAE appears to be following the opposite trend: replacing many of its long-standing roundabouts with signalized intersections. The objective of this research is to compare the in-service safety performance of signalized intersections and roundabouts in the Emirate of Abu Dhabi (AD), the largest Emirate in the UAE. The study used 6 years’ worth of road crash data. Crash severity levels between the two intersection types were compared, while controlling for factors such as posted speed limit, number of vehicles involved in the crash, and vehicle class. After controlling for previously described factors, the in-service safety performances of signalized intersections and roundabouts in the Emirate of AD are comparable. The authors discuss how future research could focus not only on identifying the factors that might have potentially contributed to the present study’s findings that differ from studies conducted in the past, but also on conducting before-after studies as data becomes available.
... Leave-one-out analysis will be used, which removes one study and recalculates the mean effect estimate and corresponding statistics for the purposes of sensitivity analysis. Specifically, the mean effect estimate is calculated g times with one effect estimate removed, or g-1 effect estimates (Elvik, 2017;e.g. Greenhouse and Iyengar, 2009). ...
... There also appeared to be some level of heterogeneity present in all funnel plots which would visually appear as studies outside of the 95% confidence interval or funnel (Elvik, 2017;. The results of the heterogeneity test, as well as the I-squared statistic as shown in Table 2 also suggest that the levels of heterogeneity are statistically significant and substantial. ...
A meta-analysis was performed to review the potential effects of reversible lanes on traffic safety. A systematic review resulted in ten U.S studies, with 155 estimates of safety effects, that were selected for inclusion in the analysis. These studies employed either a simple before-after comparison or a cross-sectional comparison methodology. Study results were extracted by transforming the collision rates and frequencies of these studies into a log-odds ratio.
The results of the meta-analysis suggest that the odds of a collision occurring are 30.9% higher for roads with reversible lanes when compared to roads without the treatment. The odds of a property-damage-only collision are predicted to be 16.6% higher, and injury collisions are predicted to be 34.9% higher with reversible lanes compared to no-treatment. The odds of a collision occurring during the peak period are expected to be 46.2% higher, while the odds of an off-peak period collision occurring are expected to be 12.8% higher. These results were statistically significant, with the exception of the off-peak period collision result.
A meta-regression was performed, which regresses variables related to study type, collision type and operational characteristics of the study locations on the extracted log odds ratio. Peak period operations were positively and significantly correlated with an increase in crashes. Presence of left-turn restrictions and/or delineator/barrier were negatively correlated with crashes, whereas the presence of dynamic traffic control and static traffic control were positively correlated with crashes. Finally, cross sectional studies tend to find greater effects than before-after studies.
... Intersections with roundabouts in all speed environments had the lowest KSI metrics of all intersection types. This is unsurprising, given that roundabouts may be used to replace other intersection types to improve safety (Devlin et al., 2011;Elvik, 2017) including to pedestrians (Retting et al., 2003) by reducing vehicle speed, as a method of traffic calming (Bunn et al., 2008). A recent meta-analyses showed that converting other intersection types to roundabouts is associated with a reduction of approximately 65% in fatal crashes and 40% in injury crashes (Elvik, 2017), concluding that roundabouts are very effective in reducing fatal crashes. ...
... This is unsurprising, given that roundabouts may be used to replace other intersection types to improve safety (Devlin et al., 2011;Elvik, 2017) including to pedestrians (Retting et al., 2003) by reducing vehicle speed, as a method of traffic calming (Bunn et al., 2008). A recent meta-analyses showed that converting other intersection types to roundabouts is associated with a reduction of approximately 65% in fatal crashes and 40% in injury crashes (Elvik, 2017), concluding that roundabouts are very effective in reducing fatal crashes. ...
As convergence points for road users approaching from multiple directions, intersections have more opportunities for conflicts, thus higher crash risk than other parts of the road network. Given the limited resources available for road safety, it is important to identify high risk intersections so that they can be prioritised for infrastructure improvement. This study used a three-stage approach to identify intersections in Perth, Western Australia: using Road Trauma Risk Analysis, then Comparative Safety Performance Analysis and finally ranking the intersections by the KSI (Killed and Serious Injury) metric. These methodologies were developed by Main Roads Western Australia. Crash data from 2011 to 2015 were used in the analyses. The results identify the top high risk intersections for each intersection type (by speed environment and control type). Recommendations are made for extensions to this process to improve identification of high risk intersections, and the use of a taxonomy to identify candidate treatments.
... Over time, the number of vehicles on the road increases, leading to severe traffic glitches, including environmental damage, traffic congestion, and road accidents [1], [2]. The studies [3], [4] provide statistics for road accidents, injuries, and deaths per year in different countries worldwide. According to a WHO report, 1.35 million people lose their lives yearly due to road accidents [5]. ...
In recent years researchers have shown significant interest in vehicular networks to augment road safety by providing real-time messaging services among vehicles. This work aims to provide a detailed analysis of emergency message dissemination techniques for the Internet of Vehicles (IoV). We explored position-based data dissemination techniques for emergency message dissemination, which is considered the best routing method because it does not rely on predestination entries of the route. Position-based schemes encounter some challenges, such as delay and accurate positioning. Existing survey papers of IoV focused on architecture, technologies, and layers. However, this article examines a brief comparison of subtypes of position-based emergency message routing, beacon-oriented and beacon-less techniques. In the end, we presented the basic challenges of emergency message dissemination; moreover, future directions are highlighted to promote the development of new protocols for emergency message dissemination to enhance the efficiency of IoV in a better way.
... Roundabouts are well-known for their ability to increase traffic safety at a particular kind of intersection. The results from a meta-analysis [1] (see also the older [2]) indicate that the conversion to a roundabout decreases the number of fatalities by 65%, and the number of crashes with injuries by 40% (see also [3]). The effect on property damage, however, was unclear. ...
Roundabouts are well-known for their ability to improve upon traffic safety, especially for motorized traffic. An in-depth analysis on this topic is known from previous work. It was found that different types of roundabouts have different levels of safety. The work at hand is a replication study for a previous study in this regard. It uses a mix of traditional and a Machine Learning (ML)-based approach, expands on the previous results and replicates some of the previous findings. This was possible especially by using a factor of 10 more roundabouts in the analysis, with considerably less manual intervention. Furthermore, this study could also draw some additional conclusions regarding the safety of bicyclists, which were not included in the original study. Finally, by using cross-validation techniques, a kind of minimal model could be established that needs fewer factors and achieves better prediction quality than straightforward glm models.
... The number of vehicles using these public roads is slowly increasing through the years [5] and as a side effect the amount of traffic incidents is also increasing. Safety measures such as traffic calming infrastructure, roundabouts or speed bumps [7,8,11] on public roads are often introduced to cope with this issue and have a significant impact on the number of road fatalities [6]. Unfortunately, although these imposed measures might be highly beneficial for regular road usage, it is usually the opposite when they are used for road cycling races. ...
Road cycling is a cycling discipline in which riders ride on public roads. Traffic calming measures are made to make public roads safer for everyday usage for all its users. However, these measures are not always yielding a safer cycling racecourse. In this paper we present a methodology that inspects the safety of roads tailored to road bicycle racing. The automated approach uses computer vision and geospatial analysis to give an indicative racecourse safety score based on collected, calculated and processed multimodal data. The current version of our workflow uses OpenStreetMap (OSM), turn detection and stage type / bunch sprint classification for the geospatial analysis and uses road segmentation and an extensible object detector that is currently trained to detect road cracks and imperfections for visual analysis. These features are used to create a mechanism that penalizes dangerous elements on the route based on the remaining distance and the generated penalties with its relative importance factors. This results in a comprehensive safety score along with a detailed breakdown of the most concerning passages on the course which can be used by race organizers and officials to help them in the iterative process to create an engaging, yet safe course for the riders.
... Overall, Carmel's roundabouts are associated with reductions in injury crashes of 47%, which signifies a safety improvement. The results are consistent with the findings of past research in the United States and internationally that roundabouts are successful in reducing injury crashes; for instance, a meta-analysis of 44 studies estimated an average reduction of 35-45% in injury crashes associated with converting traditional intersections to roundabouts (Elvik, 2017). ...
Introduction:
Roundabouts are a proven safety countermeasure for intersection safety. This study examined the safety effects of roundabout conversions in Carmel, Indiana, also known as the "Roundabout City." Doing so is of particular interest because Carmel has a high density of roundabouts and its drivers understand their effectiveness and are familiar with navigating them. This study also adds to the current state of knowledge about innovative double-teardrop roundabouts (i.e., linked roundabouts with teardrop-shaped central islands).
Method:
Negative binomial models accounting for correlation within site pairs were applied to evaluate the safety effects of converting conventional intersections to roundabouts on total crashes, injury crashes, and property-damage-only (PDO) crashes between study sites and control sites for different roundabout types (single-lane, multi-lane, and double-teardrop). We compared crash data from a 2-year period before the installation of the roundabouts with the 2-year period after the conversions.
Results:
Injury crashes were 47% lower than what would have been expected without the roundabout conversions. The effects were strongest at the double-teardrop roundabouts, where injury crashes were significantly reduced by 84% and total crashes by 63%. Single-lane roundabouts experienced significant decreases of 51% in total crashes and 50% in PDO crashes (and a nonsignificant decrease of 50% in injury crashes). Multi-lane roundabouts were associated with increases in total and PDO crashes but a 15% decrease in injury crashes, though all were nonsignificant.
Conclusions:
Overall, the City of Carmel's roundabout program is associated with reductions in injury crashes, which indicates improvements to safety. Single-lane and double-teardrop roundabouts are associated with improvements in the occurrence and severity of crashes.
Practical applications:
Double-teardrop roundabouts should be considered for installation at interchange terminals to improve highway safety.
... In the BR dataset, "Road Type" is These variables are potentially relevant to describe iterations between traffic and pedestrians, since they detail whether the road is close to roundabouts, presents signalized and non-signalized intersections, and other types of junctions. For the purpose of the current analysis, such variables allow a better understanding of the accident and its environment, and eventually provide authorities with relevant information for developing strategies to mitigate the occurrence and effects of accidents (Elvik, 2017). Another missing information in the BR datasets is road speed limit. ...
This paper proposes a novel framework to identify the most informative variables for clustering bus priority corridors according to their similarities regarding system and operational aspects. Although each bus corridor has its peculiarities, understanding the similarities (e.g., system, physical and operational aspects) between corridors of
different regions of the world can help researchers and transit specialists to draw up strategies tailored to improving the traffic in the cities. For that matter, we integrate a novel metric for measuring clustering quality to the omit-one-variable-out-at-a-time selection procedure. The proposed method relies on three steps: (i) collect and preprocess data describing bus corridors; (ii) define the number of clusters to be generated based on a hierarchical approach; and (iii) iteratively group bus corridors, and eliminate less relevant clustering variables. When applied to a dataset comprised of 296 bus priority corridors from 45 countries and described by 44 variables, the proposed framework retained only four variables (i.e., brand and/or logo, station spacing, enhanced stations, and operating speed) responsible for the best stratification of corridors. Four clusters were formed and qualitatively assessed regarding their similarities in terms of system, physical and operational aspects. Corridors were grouped
into basic corridors (cluster 1), improved corridors (cluster 2), Bus Rapid Transit (BRT) and Bus with High Level of Service (BHLS) systems (cluster 3), and express, limited-stop services (cluster 4).
Keywords: Bus priority corridors · Variable selection · Clustering analysis · BRT · BHLS.
... The mean cause on property-damage-only accidents is uncertain (Elvik, 2017). The traffic safety audit inspects an active or planned road in conditions of which a cluster of independent experienced experts gave a ending about a potential accident risk in turn to prevent accident-prone situations (Huvarinen, Svatkova, Oleshchenko, & Pushchina, 2017). ...
Indian roads have a sizeable amount of accidents happening in them. The transportation problems faced by various nations across the world have increased manifold, necessitating search for methods or alternatives that ensure efficient, safe, feasible and faster means of transport. Government of India is taking many efforts to minimize the road accidents. Amongst all traffic accident, road traffic accident bears largest number of human life and tends to be most serious problem world over. Road accidents are largely predictable and preventable, provided rational analysis and counter measures. The number of accidents in India is increasing rapidly every year due to addition of more number of vehicles on the streets. This project is to look for few accidental prone spots in our locality and analyze the causes of accidents and try to provide viable solutions for improving the safety in the accidental zones. In this study, the accident zones were taken and a particular spot was selected and suitable measures to control the accidents were suggested.
... Kennedy [10] ise çalışmasında Almanya, Hollanda ve Fransa gibi ülkelerde dönel kavşakların kullanılmasının öncelikli nedeninin iyi güvenlik kayıtları olduğunu belirtmiştir. Ayrıca uluslararası literatürde, öncesinde geleneksel kavşak olarak kullanılan kavşakların dönel kavşağa dönüştürüldükten sonraki güvenlik performansları karşılaştırılmış ve dönel kavşağa dönüştürülen kavşaklardaki kaza oranlarında önemli bir azalma olduğu belirtilmiştir [11,21,[34][35][36][37][38][39][40][41][42][43][44][45]. Buna ilave olarak diğer araştırmacılar, çalışmalarında inceledikleri mevcut sinyalize olarak işleyen kavşakların dönel kavşaklara çevrilmesi durumunda trafik kaza sıklığı ile beraber şiddetinin de azaltılabileceğini göstermiştir [35,[46][47][48][49][50][51]. ...
Ulaşım sistemi içerisindeki trafik akımının güvenli ve etkin biçimde işlemesi üzerinde büyük rolü bulunan kavşaklar, faaliyet alanı, geometrik özellikleri ve denetim şekillerine göre farklı kategorilere ayrılmaktadır. Kontrollü eş düzey kavşak kategorisindeki dönel kavşaklar, trafik akımının merkezi ada etrafında yönlendirildiği ve ada içerisindeki araçların ilk geçiş hakkına sahip olduğu kavşak tipidir. Dönel kavşakların her bir geometrik elemanı kavşağın işleyiş, kapasite ve güvenliği üzerinde rol oynamaktadır. İyi tasarlanmış bir dönel kavşak, araçların dolaşım yolu boyunca hızlarının sabit kalmasını sağlayarak kavşak güvenliğinin optimize edilmesine yardımcı olur. Bu makalede, dönel kavşakların geometrik elemanlarının kavşak güvenliği üzerine etkilerinin incelendiği çalışmalar derlenmiştir. Dönel kavşakların her bir geometrik elemanın literatürde yer alan çalışmalar ışığında kavşak güvenliğiyle olan ilişkileri ve bazı ülkelerin standart/kılavuzlarında yer alan tasarım kriterleri incelenmiştir. Bu çalışmayla amaçlanan, yapılmış çalışmalar doğrultusunda, dönel kavşak geometrik elemanlarının tasarımlarının kavşak güvenliği açısından hayati önem taşıdığını ortaya koymaktır. Ülkemizde bu konuyla ilgili olarak daha önce yapılmış detaylı çalışmalara rastlanılmaması çalışmamızın motivasyon kaynağı olmuştur. Ülkemizdeki dönel kavşakların hangi standart/kılavuzlara bağlı olarak yapıldığı ile ilgili bilgi yetersizliği olduğu da görülmektedir. Çalışma sonucunda dönel kavşaklardaki güvenliğin sağlanabilmesi için geometrik elemanların birbirleriyle etkileşim içinde ve uygun fiziki boyutlarda tasarlanması gerektiği belirtilmiştir. Ayrıca trafik güvenliği bakımından ülkemizde bu konuyla ilgili ivedilikle çalışmalara başlanılması ve ülkemiz koşullarına özgü tasarım standart/kılavuzu hazırlanması gerektiği ifade edilmiştir.
... Several studies specifically examined the relationship between geometric elements and safety benefits in roundabout. Elvik (2017) studied the safety effects of converting intersections in roundabouts by a meta-analysis. the results of this study show that converting intersections in roundabouts reduces the fatal accidents of about 65% and the injury accidents of about 40%. ...
It is now known that roundabouts offer high levels of safety, but different researches on this type of intersection show that several factors influence the driver’s behavior, causing occasionally wrong driving behaviors that can degenerate into accidents. Understanding driver’s preferences is an important goal in order to encourage an efficient and safe road design. This study aims to understand user’s preferences on the geometrical-functional characteristics of roundabouts, and to associate these preferences with specific driver features. For this purpose, a stated preference survey was carried out. The declared preferences were collected through face-to-face interviews. Collected data were then processed by applying the correspondence analysis (CA). This research revealed that drivers’ preferences are different depending on the type of driving behavior exhibited by drivers and, only in a less obvious manner, according to gender. Drivers with exemplary driving behavior prefer design elements that ensure a greater safety. Men with wrong driving behavior prefer roundabout configurations that are less safe but that ensure fast trajectories. Drivers with acceptable driving behavior expressed preferences that are specifically geared towards specific geometric/functional configurations of the roundabouts. The results of this work represent practical implications for a correct and safe design of road roundabouts.
... Roundabouts are typical elements of European roads that have spread around the world (Geoghegan 2011;Mapbox 2010). The gradual transformation of crossroads to roundabouts has been occurring since their appearance more than 100 years ago (Waddell 1997) because they offer a series of advantages: They reduce the speed of cars due to their characteristic circular shape (Chen et al. 2013), they reduce the number and severity of collisions (Elvik 2017; Washington State Department of Transportation 2018), they have fewer points of conflict and possible collisions (Gross et al. 2013), and they help to regulate the flow without the need for traffic lights. ...
Objective: In this work, a roundabout and a turbo roundabout model are compared and previous modeling with continuous Petri nets and safety are analyzed through indicators of complexity. Petri nets are a graphic and mathematical representation that allow a faithful modeling of urban systems.
Method: The methodology has been designed for the transformation of a real system to small subgraphs that represent the maneuvers in roundabouts, approximated as roads and lanes of incorporation. Places within the roundabout have been located and defined as continuous places from their influence and visibility toward adjacent conditions. The transitions have been modeled by time and inhibitory arcs, which represent priorities and areas where drivers must pay attention. The created networks represent a faithful model of vehicle flow trajectories in the roundabouts.
Results: The methodology is applied to the same real road intersection. The case study is a recent transformation from roundabout to turbo roundabout. The roundabout network complexity is corroborated by a greater number of entries and exits that lead to each roundabout place (reflected in the maneuvers that can be performed) and a greater number of inhibiting arcs. In most of the turbo roundabout places, the driver’s only option is reduced to occupying next place. The possibility of choosing between several places supposes a greater trajectory intersection and an increased time for decision making. The only situation where the complexity is the same between both systems is when a vehicle accesses the inner lane of the roundabout from the left lane on a single-lane road. The main maneuvers causing accidents have been modeled and their solution in a turbo roundabout is presented.
Conclusions: The reduced complexity of the turbo roundabout is due to the strict limitations in lane changes, turning turbo roundabouts into a safer model: A lower number of possible movements that can be performed by drivers and a smaller number of trajectories with collision risk. Petri nets have proven to be perfectly applicable to the representation of traffic circular systems (such as roundabouts and turbo roundabouts) and to measure the complexity and security of the system.
... Studies demonstrate that roundabouts can reduce delays, queue lengths, congestion, and degree of saturation, with lower power and maintenance costs (Burdett, Bill, and Noyce 2017). The geometric features of roundabouts, which reduce the number of conflict points, have been shown to reduce fatal crashes and injuries when converted from other intersection control types (Elvik 2003;Elvik 2017). Although the reduction in crashes is a positive step toward safer intersections, there is still room for improvement and an increased need for detailed analysis of their safety performance, as more roundabouts will be constructed in the future. ...
This work contributes to study the application of extreme value theory (EVT) in road safety analysis, estimating the risk of being involved in an entering–circulating collision in single-lane roundabouts. Detailed trajectory data of the vehicles were derived from a driving simulator experiment, and the time-to-collision (TTC) was used as a surrogate measure of safety. Three EVT approaches were applied, tested and compared: (1) the Generalized Extreme Value distribution used in the block maxima (BM) approach, (2) the Generalized Pareto Distribution used in the peak-over-threshold approach (POT), with negated-TTC (nTTC), and (3) shifted-reciprocal-TTC (srTTC). Case-study results analysis showed that BM and POT with shifted-reciprocal-TTC confidence intervals included the number of observed crashes, while POT with negated-TTC did not include it. According to these findings, both BM and POT-with-shifted-reciprocal-TTC appear promising and deserve further attention in order to develop effective ready-to-practice crash prediction models, useful in intersection design and operational analysis.
... The wide application of meta-analytical approaches in lots of research fields (e.g. [31]) encouraged the Authors to perform a meta-analysis through the random effect model. ...
The paper introduces a methodological approach based on genetic algorithms to calibrate microscopic traffic simulation models. The specific objective is to test an automated procedure utilizing genetic algorithms for assigning the most appropriate values to driver and vehicle parameters in AIMSUN. The genetic algorithm tool in MATLAB ® and AIMSUN micro-simulation software were used. A subroutine in Python implemented the automatic interaction of AIMSUN with MATLAB ®. Focus was made on two roundabouts selected as case studies. Empirical capacity functions based on summary random-effects estimates of critical headway and follow up headway derived from meta-analysis were used as reference for calibration purposes. Objective functions were defined and the difference between the empirical capacity functions and simulated data were minimized. Some model parameters in AIMSUN, which can significantly affect the simulation outputs, were selected. A better match to the empirical capacity functions was reached with the genetic algorithm-based approach compared with that obtained using the default parameters of AIMSUN. Overall, GA performs well and can be recommended for calibrating microscopic simulation models and solving further traffic management applications that practioners usually face using traffic microsimulation in their professional activities.
... A Czech before-after study (12) identified approximately 50% reduction of both total and injury crashes. And recent meta-regression analysis (17), based on 44 international studies, concluded that converting intersections to roundabouts is associated with a reduction of fatal and injury crashes of about 65% and 40%, respectively. ...
Roundabouts are considered the safest intersection design; however, the safety effect may not be satisfactory at each specific roundabout. This is true especially in countries where roundabout design is a relatively new concept, such as in the Czech Republic. Specifically, most Czech roundabout crashes were found to occur on entries. This motivated the presented study to investigate how entry design parameters influence safety on Czech roundabouts and, if possible, use the findings to update current Czech roundabout design guidelines. To this end, the study comprised three analyses: crash-based safety performance functions, speed analysis, and finally safety performance functions which incorporated speed. All three analyses proved that entry design parameters have a statistically significant influence on safety, in terms of crash frequency, severity and speeds. Given the study objective, this fact should be considered in Czech roundabout design guidelines.
... The results of the analysis indicated that increasing the circulatory roadway width, exit angle, angle to the next leg, and splitter island width is associated with reduced crash rates at roundabout approaches. Elvik [4] proposed a meta-analysis of the road safety effects of converting junctions to roundabouts. Based on a meta-regression analysis, converting junctions to roundabouts was associated with a reduction of fatal accidents of about 65% and a reduction of injury accidents of about 40%. ...
The aim of this paper is to identify the roundabout geometric characteristics affecting the safety perception while the typical maneuvers (entry, circulation, exit) are being carried out. The tool used was an on-line questionnaire, filled out by about 1.650 respondents. Four different dimensionality reduction methods (Cluster Analysis, Correspondence Analysis, Exploratory Factor Analysis and Confirmatory Factor Analysis) were used to analyze the data collected from the survey, in order to examine the key factors affecting the safety perception during the typical roundabout maneuvers. The considerations arising from the final model are the following: 1) the respondents' opinions regarding the safety perception of maneuvers are not preconceived ideas, but they originate from specific safety perceptions due to roundabout geometric configurations; 2) the users prefer definitely single lane roundabouts; this is an important confirmation of most results in the literature; 3) it was quantified the extent of the relationship between the safety perception of the typical roundabout maneuvers and the following aspects: a) maneuver type, b) geometric characteristics of the roundabouts design elements. This is the innovative aspect of the present research whose results have implications regarding theory, infrastructure and the application of new safety technologies. © 2017 International Association of Traffic and Safety Sciences
... According to 2 Wireless Communications and Mobile Computing World Health Organization (WHO), 1.25 million people die every year and 50 million injuries occur due to the road accidents [4]. Likewise, [5,6] present records and statistics on the topic of accidents, fatalities, and injuries. Therefore, safety is one of the prime objectives of VANETs. ...
Industrial Internet of Things (IIoT) is the other name of industrial Internet. It integrates a variety of existing industrial automation technologies with computing, machine learning, and communication technologies. Vehicular ad hoc network, an application of IIoT, is a self-organized network of vehicles which tends to provide improved road safety, diminished traffic congestion, and ultimate comfort to the travellers. In VANETs, vehicles exchange data with each other directly or through roadside units (RSUs). Data dissemination in VANETs experiences numerous challenging issues including broadcast storm, network partitions, intermittent connectivity between vehicles, and limited bandwidth. In literature, various data dissemination schemes are proposed. However, most of these schemes are designed for either urban or highway VANET scenarios and evaluated under sparse or dense traffic conditions. Moreover, these schemes do not effectively overcome the aforementioned issues simultaneously. In this paper, we present a new data dissemination protocol for VANETs, which disseminates the emergency messages in different scenarios under varying traffic conditions. During dense traffic conditions, DDP4V employs the segmentation of transmission region of a vehicle in order to select the most appropriate next forwarding vehicle (NFV). Accordingly, it divides the transmission region of a vehicle in three distinct segments and selects vehicle(s) inside the highest priority segment to forward the message to all neighbour vehicles, whereas it also uses implicit acknowledgements for guaranteed message delivery during sparse traffic Conditions. Simulation results show that DDP4V protocol outperforms the other existing related protocols in terms of coverage, network overhead, collision, and end-to-end delay.
... Based on the wide application of quantitative methods to summarize the results of several empirical studies in lots of research fields (see e.g. [61] ), the mean values of the critical headways and the follow-up headways, previously estimated at observation sites characterised by similar layouts, were collected in order to evaluate their mean effect or the effect size, evaluate the dispersion in these effects and then compute a summary effect for each parameter. Table 2 shows the results of the meta-analysis of effect sizes through the random effect model and the main statistics summarised for the purpose of this study. ...
Calculation of passenger car equivalents for heavy vehicles represents the starting point for the operational analysis of road facilities and other traffic management applications. This paper introduces a criterion to find the passenger car equivalents that reflect traffic conditions at double-lane roundabouts, where the capacity is typically estimated for each entry lane. Based on the equivalence defined by the proportion of capacity used by vehicles of different classes, the criterion implies a comparison between the capacity that would occur with a traffic demand of passenger cars only and the capacity reached beginning from a demand with a certain percentage of heavy vehicles. A preliminary activity consisted of the comparison of the empirical capacity functions based on a meta-analytical estimation of critical and follow up headways, and simulation output data derived for a double-lane roundabout built in AIMSUN. The formulation of the calibration process as an optimisation problem enabled to minimize an objective function using the genetic algorithm tool in MATLAB®. A subroutine in Python implemented the automatic interaction with AIMSUN. Differently from methods that propose constant values for the passenger car equivalents, the results highlighted that the passenger car equivalents at double-lane roundabouts increased when the circulating flow increased, while a higher effect was expected when the traffic streams included a higher number of heavy vehicles.
Autonomous shuttles have been used as end-mile solutions for smart mobility in smart cities. The urban driving conditions of smart cities with many other actors sharing the road and the presence of intersections have posed challenges to the use of autonomous shuttles. Round intersections are more challenging because it is more difficult to perceive the other vehicles in and near the intersection. Thus, this paper focuses on the decision-making of autonomous vehicles for handling round intersections. The round intersection is introduced first, followed by introductions of the Markov Decision Process (MDP), the Partially Observable Markov Decision Process (POMDP) and the Object-Oriented Partially Observable Markov Decision Process (OOPOMDP), which are used for decision-making with uncertain knowledge of the motion of the other vehicles. The Partially Observable Monte-Carlo Planning (POMCP) algorithm is used as the solution method and OOPOMDP is applied to the decision-making of autonomous vehicles in round intersections. Decision-making is formulated first as a POMDP problem, and the penalty function is formulated and set accordingly. This is followed by an improvement in decision-making with policy prediction. Augmented objective state and policy-based state transition are introduced, and simulations are used to demonstrate the effectiveness of the proposed method for collision-free handling of round intersections by the ego vehicle.
Intersections are over-represented in crash statistics internationally, suggesting that treatments targeting intersections could substantially improve road safety. Aligning with a Safe Systems approach, several innovative intersection configurations have been proposed. We evaluated the effectiveness of five designs on reducing speed and modifying conflict angles using a driving simulator. The designs were 1) cut-through, 2) modified restricted squircle, 3) unrestricted squircle, 4) raised approach, and 5) raised intersection. For the higher speed green light condition, speed reductions for vehicles travelling straight through the intersection were seen for all five innovative designs on approach to the intersection and at critical conflict points within the intersection. Within the intersection specifically, speed reductions ranged from 1) 7–8 km/h for the modified restricted squircle, 2) 12.5–16.5 km/h for the unrestricted squircle, raised approach, and raised intersection, and 3) 15–19 km/h for the cut-through. In contrast, speeds for drivers turning right were either not substantially reduced (modified restricted squircle, unrestricted squircle, and raised intersection) or were increased (raised approach and cut-through). Beneficial modification of conflict angles were also observed at conflict points for three of the five designs: cut-through, and two variations of the squircle design. Innovative intersection designs may be effective in reducing intersection crash risk by reducing speeds and conflict angles.
Road posted speed limits (PSL) in Abu Dhabi (AD) were raised in mid-2018. Before raising PSL by 20 kph, AD drivers were legally allowed to drive 20 kph above PSL. Thus, there was a speed buffer within which drivers would not be fined for speeding. While it has been stated (through public media) that the elimination of this speed buffer would increase safety, this paper challenges this view. This paper describes how the argument for increased safety through increased PSL quickly breaks down. The paper bases its arguments mainly on basic physics, as well as local research data and findings. More specifically, this paper not only shows how existing road design may not safely accommodate raised PSLs (or even previously lower PSLs), but also how fatal road injuries are more likely to occur on AD roads with higher design speeds. The paper also describes how increased PSL may increase the potential for liability and greenhouse gas emissions. This paper concludes that while speed buffers should have been removed, PSL should not have been increased since higher PSL may negatively affect road safety while decreasing travel times only marginally. That is, there is not much to gain from raising PSLs, but there is potentially a lot to lose. Thus, the author urges policy makers to make decisions based on science and solid evidence. This is relevant as decisions such as increasing PSL on public roads may have a significant negative impact on public health through increased injury/mortality rates and pollution levels.
Price elasticities are widely used in transport planning, forecasting and appraisal. This paper presents the largest ever meta-analysis of price elasticities of travel demand based on 2023 elasticities drawn from 204 British studies published between 1968 and 2020. A large number of credible variations in elasticities emerge that add to the evidence base, challenge some official recommendations and are useful to practitioners. The paper also presents an original meta-analysis of the length of time until the long run is achieved, based on 386 observations obtained from 47 studies. This is an important complement to the price elasticity meta-analysis and it also challenges official recommendations. Whilst the research is based on British evidence, the insights of a methodological nature at the least are transferable to other contexts.
Properly designed roundabouts may lead to safety improvements based on both reducing approaching speeds and controlling traffic. Measurements of deflection of vehicle trajectories are commonly used to estimate roundabout speed control. One of these measurements is the deviation angle, which is mentioned in both the Italian and Swiss road standards and, in specific conditions, can be more effective than other methods.
This article presents a general mathematical formulation for linking several geometric parameters with the deviation angle in different rural and urban one-lane roundabout configurations, which is currently missing in the literature. For urban roundabouts, refuge islands for pedestrians and cyclists were considered. Based on the proposed formulation, a sensitivity analysis of the influential geometric parameters was conducted. Results suggest that an insufficient deflection of trajectories (deviation angle less than 45°) is always present for roundabouts with inscribed circle diameter less than 25 m; for urban roundabouts with refuge islands for pedestrians and cyclists having inscribed circle diameter less than 34 m and orthogonal legs; and for roundabouts with angles between opposite legs smaller than 140°. The main parameters which are responsible for a decrease in the deviation angle are: a decrease in the inscribed circle diameter; a decrease in the angle between opposite legs; and an increase in the width of the circulatory lane. Some optimized procedures for roundabout design, the generalized application of the deviation angle method and alternative speed control measures in cases of small deviation angles are discussed.
In this paper, we use estimates of the effectiveness of existing road safety technologies and interventions to estimate the reduction in road safety deaths in six Indian cities over the next decade, in order to achieve the SDG 3.6 goal of a 50% reduction by 2030. Only the existing interventions are considered and technologies that might be available in the future are not accounted for. The results show that similar policies for all cities will not produce the same results, and that achieving the SDG 3.6 goal does not automatically reduce fatality rates in cities with high fatality rates. The introduction of safer cars with currently available safety technologies, although necessary, will have much less effect than the combined effect of motorcycle safety technologies, speed control and traffic calming. This study suggests that while SDG 3.6 targets may be satisfied by 2030 with great effort in India, the presently available safety interventions may not be adequate to bring about death rates below 2.0 per hundred thousand persons. Since the situation differs significantly between cities, it will be important to evolve city-specific policies for safety intervention priorities and changes in travel behavior. The desired reduction in road traffic injuries in India will not be possible without much greater investment in road safety research and road design for safer travel.
Après avoir défini brièvement dans l'introduction le contexte de la circulation routière, ce chapitre est consacré aux campagnes de communication en sécurité routière qui constituent un des principaux moyens d'action essentiellement des pouvoirs publics mais aussi des assureurs, des autorités locales, des entreprises, etc. pour promouvoir une mobilité plus sûre. Il s'agit tout d'abord de donner une définition des campagnes de communication en sécurité routière et de présenter les principaux objectifs de ces campagnes. Sur la base de résultats de projets européens et de méta-analyses quant à leur efficacité, il y a ensuite de livrer les facteurs qui augmentent les chances de succès des campagnes (type de campagnes, cadres théoriques, durée de la campagne, cadrage de l'information, etc.) avant d'exposer brièvement les différentes étapes qui guident la préparation, la mise en oeuvre d'une campagne ainsi que son évaluation et la diffusion des résultats quant à son impact ou absence d'impact. Enfin, une conclusion et des perspectives viennent clore ce chapitre.
Economic evaluations of road safety measures are only rarely published in the scholarly literature. We collected and (re-)analyzed evidence in order to conduct cost-benefit analyses (CBAs) for 29 road safety measures. The information on crash costs was based on data from a survey in European countries. We applied a systematic procedure including corrections for inflation and Purchasing Power Parity in order to express all the monetary information in the same units (EUR, 2015). Cost-benefit analyses were done for measures with favorable estimated effects on road safety and for which relevant information on costs could be found. Results were assessed in terms of benefit-to-cost ratios and net present value. In order to account for some uncertainties, we carried out sensitivity analyses based on varying assumptions for costs of measures and measure effectiveness. Moreover we defined some combinations used as best case and worst case scenarios. In the best estimate scenario, 25 measures turn out to be cost-effective. 4 measures (road lighting, automatic barriers installation, area wide traffic calming and mandatory eyesight tests) are not cost-effective according to this scenario. In total, 14 measures remain cost-effective throughout all scenarios, whereas 10 other measures switch from cost-effective in the best case scenario to not cost-effective in the worst case scenario. For three measures insufficient information is available to calculate all scenarios. Two measures (automatic barriers installation and area wide traffic calming) even in the best case do not become cost-effective. Inherent uncertainties tend to be present in the underlying data on costs of measures, effects and target groups. Results of CBAs are not necessarily generally valid or directly transferable to other settings.
Vehicular Ad Hoc Networks (VANETs) are becoming the favorable option for the Intelligent Transportation Systems (ITS). The efficient distribution of information in the form of data packets, is essential in order to support the vision of smart ITS. In VANETs, vehicles communicate with other vehicles directly or through existing infrastructure for sharing traffic information. Th data distribution is usually done using broadcast method. Data broadcast in VANETs is a challenging issue since high mobility vehicles with varying density have to share and route the safety critical information to other intended vehicles in real time. Thus, it needs to be addressed by the existing and new solutions for the same. Keeping this in mind, this study presents a detailed description of the existing data broadcast protocol for VANETs with an objective of selecting a particular scheme for a particular application with respect to its benefits over the others. This study reviews and compares numerous existing multi-hop data broadcast protocols in terms of various attributes such as data forwarding strategies, objectives, type of architecture, application scenarios, assumptions, evaluation metrics, simulation platform. Furthermore, an original taxonomy of these protocols is introduced based upon the road scenarios with critical discussion on each categorization with respect to its strengths, weaknesses and important constraints. Finally, various perspectives, challenges and shortcomings of the existing research work are discussed.
Meta-analysis collects and synthesizes results from individual studies to estimate an overall effect size. If published studies are chosen, say through a literature review, then an inherent selection bias may arise, because, for example, studies may tend to be published more readily if they are statistically significant, or deemed to be more “interesting” in terms of the impact of their outcomes. We develop a simple rank-based data augmentation technique, formalizing the use of funnel plots, to estimate and adjust for the numbers and outcomes of missing studies. Several nonparametric estimators are proposed for the number of missing studies, and their properties are developed analytically and through simulations. We apply the method to simulated and epidemiological datasets and show that it is both effective and consistent with other criteria in the literature.
We study recently developed nonparametric methods for estimating the number of missing studies that might exist in a meta-analysis and the effect that these studies might have had on its outcome. These are simple rank-based data augmentation techniques, which formalize the use of funnel plots. We show that they provide effective and relatively powerful tests for evaluating the existence of such publication bias. After adjusting for missing studies, we find that the point estimate of the overall effect size is approximately correct and coverage of the effect size confidence intervals is substantially improved, in many cases recovering the nominal confidence levels entirely. We illustrate the trim and fill method on existing meta-analyses of studies in clinical trials and psychometrics.
Using accident data for three time periods obtained at some 900 treated sites, the reductions in total and target accidents attributable to various types of remedial treatment were estimated. The occurrence of the regression-to-mean (RTM) effect at treated sites was demonstrated and quantified. It was found that grossly exaggerated estimates of the safety benefits of remedial treatment will be obtained of RTM effects are not properly controlled. The magnitude of the RTM effect varied somewhat in site type, with a tendency for lower values at treated junctions than at other site types. The magnitude of the RTM effect also varied with accident type. For some treatment types, comparisons of before-and-after data give a very distorted picture of the effects of treatments on different types of accidents.
This study evaluated the impact of double-lane roundabout conversions on public attitudes, traffic performance, the environment, and safety at two intersections near Bellingham, Washington, and evaluated whether older drivers avoided the roundabouts by taking an alternative route. Driver support for the roundabouts increased from 34% before construction to 70% at 1 year after construction. At 1 year after construction, more than 40% of drivers did not believe the signs and pavement markings adequately conveyed information about appropriate speeds, right-of-way rules, and navigation of the roundabouts in the presence of large trucks. After other roadway changes were accounted for, substantial declines in the delays and queue lengths on minor roads, the proportion of queued vehicles, fuel consumption, and vehicle emissions were attributed to the roundabout conversions. Analyses of crash rates per million entering vehicles found that the roundabout conversions were associated with reduced rates of injury and fatal crashes combined and increased rates of property-damage-only crashes. Only the increase in the property-damage-only crash rate at one roundabout was significant. The odds that drivers age 70 years and older traveled the study corridor instead of an alternative route after the roundabout conversions were 0.32 times the odds before the conversions. These findings are generally consistent with prior research finding substantial traffic, environmental, and injury reduction benefits of single-lane roundabouts. However, the greater complexity of double-lane roundabouts may present challenges, as some confusion persisted 1 year after construction, evidence that some older drivers may have taken an alternative route to avoid them was found, and the rates of property-damage-only crashes increased.
Black spot management is a widely implemented measure to improve traffic safety. This study evaluates the safety effects of an extensive black spot programme that has been implemented in Flanders-Belgium. In total, around 800 black spots were selected, from which 134 locations, redesigned between 2004 and 2007, were included in this study. The adopted approach is an empirical Bayes before- and after study that accounts for effects of general trends and for the stochastic nature of crashes, including regression to the mean. Two different comparison groups were established. The analyses showed a decrease in the number of injury crashes of 24–27%, significant at the 1%-level. A separate analysis for crashes with serious or fatal injuries showed a decrease of 46–57%, also significant at the 1% level. The highest effects were found for the implementation of changes in the layout of priority controlled intersections and for the installation of traffic signals, which showed a decrease of respectively 42% and 35% in the number of injury crashes. Signalized intersections at which left-turn phasing was implemented resulted in a decrease of 22% in the number of injury crashes, changes in the layout leaded to a decrease of 11%. The conversion of intersections (both signalized and priority controlled) into roundabouts resulted in a decrease of 21% in injury crashes. The black spot programme generated a favourable effect on each of the road user categories (car occupants, moped riders, cyclists, motorcyclist, pedestrians and truck drivers).
Roundabouts are a form of at-grade intersection control that is used frequently around the world and is becoming popular in the United States. Roundabouts are being used to replace two-way and all-way stop-controlled intersections and traffic signals in the United States. Roundabouts have also been used recently in Vail, Colorado, to improve an existing stop-controlled freeway interchange system. In this study, five single-lane roundabouts are studied to assess their safety and operational performance. All five sites were stop-controlled before roundabouts were installed, and overall the sites experienced a reduction in accident frequencies, rates, and also control delay. Aside from a review of safety and delay data before and after installation of the roundabouts, this study includes a comparison of field-measured control delay with that predicted by SIDRA, an analytically based software package that can analyze atgrade sign- and signal-controlled intersections as well as roundabouts. This study will help agencies better understand their ability to predict delay at American roundabouts. Finally, findings are presented regarding the accuracy of the roundabout capacity model contained in the 1997 update to Chapter 10 of the Highway Capacity Manual. Because of the lack of roundabout entries that are operating at capacity in the United States, an approximation of potential capacity based on available gaps in the circulating stream was made. These findings indicate that the manual may be optimistic in its prediction of capacity for single-lane round-abouts in the United States; however, it should be noted that the lack of roundabout entries operating under capacity in the United States only allows for an approximation of field capacity to be made at this time.
Roundabouts may be new builds but often are conversions from existing intersections. When contemplating the later, there is a need to estimate the safety effects of conversions. Several studies have estimated large reductions in crashes and severity; however, these results pertain mainly to conversions from unsignalized intersections. Results for conversions from signalized intersections have been less conclusive or consistent and tend to be somewhat dated. The objective of this study was to fill this void by estimating the safety effectiveness of converting signalized intersections to roundabouts.
IntroductionHistory and DevelopmentModelling Publication BiasAdjusting for Publication BiasFormal Tests for Publication BiasApplying ‘Trim and Fill’ in PracticeLimitations and Future ResearchExamplesConclusions
References
Asymmetry in funnel plots may indicate publication bias in meta-analysis, but the shape of the plot in the absence of bias depends on the choice of axes. We evaluated standard error, precision (inverse of standard error), variance, inverse of variance, sample size and log sample size (vertical axis) and log odds ratio, log risk ratio and risk difference (horizontal axis). Standard error is likely to be the best choice for the vertical axis: the expected shape in the absence of bias corresponds to a symmetrical funnel, straight lines to indicate 95% confidence intervals can be included and the plot emphasises smaller studies which are more prone to bias. Precision or inverse of variance is useful when comparing meta-analyses of small trials with subsequent large trials. The use of sample size or log sample size is problematic because the expected shape of the plot in the absence of bias is unpredictable. We found similar evidence for asymmetry and between trial variation in a sample of 78 published meta-analyses whether odds ratios or risk ratios were used on the horizontal axis. Different conclusions were reached for risk differences and this was related to increased between-trial variation. We conclude that funnel plots of meta-analyses should generally use standard error as the measure of study size and ratio measures of treatment effect.
The trim and fill method allows estimation of an adjusted meta-analysis estimate in the presence of publication bias. To date, the performance of the trim and fill method has had little assessment. In this paper, we provide a more comprehensive examination of different versions of the trim and fill method in a number of simulated meta-analysis scenarios, comparing results with those from usual unadjusted meta-analysis models and two simple alternatives, namely use of the estimate from: (i) the largest; or (ii) the most precise study in the meta-analysis. Findings suggest a great deal of variability in the performance of the different approaches. When there is large between-study heterogeneity the trim and fill method can underestimate the true positive effect when there is no publication bias. However, when publication bias is present the trim and fill method can give estimates that are less biased than the usual meta-analysis models. Although results suggest that the use of the estimate from the largest or most precise study seems a reasonable approach in the presence of publication bias, when between-study heterogeneity exists our simulations show that these estimates are quite biased. We conclude that in the presence of publication bias use of the trim and fill method can help to reduce the bias in pooled estimates, even though the performance of this method is not ideal. However, because we do not know whether funnel plot asymmetry is truly caused by publication bias, and because there is great variability in the performance of different trim and fill estimators and models in various meta-analysis scenarios, we recommend use of the trim and fill method as a form of sensitivity analysis as intended by the authors of the method.
This study evaluates the effectiveness of the Black Spot Programs in Western Australia. Reduction in crash rate at the treated locations and the economic benefits of these treatments were assessed. The results showed that the programs have been effective overall, reducing all reported crash rate by 15%. The estimated crash cost savings were 50.8 million Australian dollars, of which 89% could be attributed to the reduction in casualty crashes. This led to net savings to the community of 40.4 million Australian dollars ($35.1 million attributable to casualty crashes) after subtracting the capital costs of treating sites, maintenance and operating costs. The benefit cost ratio across all treatment sites was 4.9. Evaluation of the treatments has identified some effective treatment types and others without any significant change in either the rate or cost of crashes. The latter could be due to insufficient number of sites that received the treatment, the post-treatment period being relatively short, or the treatments genuinely had little impact on road safety. Findings of this study provide objective information for the development of effective strategies on road safety investment.