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Effect of Traffic Roundabouts on Accident Rate and Severity in Arizona
Abstract
Although roundabouts have been used at many locations around the world, the safety of roundabouts under different conditions has not been fully understood. In this study, 17 roundabouts in five cities in Arizona were evaluated, out of which eleven are single-lane and six are double-lane. Most of the intersections of single-lane roundabouts were controlled by 2-way stop signs before roundabout conversion, while most of the intersections of double-lane roundabouts were controlled by traffic signals. Accident data were collected and broken down into 5 categories: damage without injury, minor injury, non-incapacitating injury, incapacitating injury, and fatality. Equal number of years were used before and after the roundabout conversion at each location. The average rates of accidents, damages without injury, injuries and fatalities per year and per million vehicles were evaluated. It was found that single-lane roundabouts reduced the accident rate, whereas double-lane roundabouts increased the accident rate. The results also showed that both single- and double-lane roundabout conversions reduced the severity levels of accidents. Considering both accident rate and severity level, warrants need to be developed for roundabout conversion and number of roundabout lanes under different traffic conditions.
... Thanks to the advancements in sensor technology, more and more data can be acquired and used for this goal too. Road infrastructure s, such as roundabouts, present a high risk of incidents due to the high amount of vehicles crossing it while crossing other vehicles' trajectory (numerous studies have been conducted on different roundabouts as in Mamlouk and Souliman (2019) and Zubaidi et al. (2021)). In this context, different researchers focused on developing algorithms that could estimate a risk level in function of the traffic, drivers' behaviour, and roundabout design. ...
... It increases vehicle safety. A conflict point occurs when the course of two moving vehicles or a line of moving vehicles and pedestrians diverge, merge, or cross (Mamlouk & Souliman, 2019). Figure 4 and Figure 5 show that conflict points with the central island was 10 and without the central island, i.e., replacing right turning movements to median crossovers was 6, which indicates 40% reduction in conflict point. ...
Traffic congestion poses a significant challenge to mixed traffic flow in intersections within developing countries. This study focuses on the bustling Chittagong export processing zone (CEPZ) intersection in Chittagong City, Bangladesh. Before, it had a central island, but recently, it has been removed, and a median has been constructed to divide the opposing direction of traffic. This unsignalized intersection operates as a two-way stop-controlled (TWSC) intersection. The Highway Capacity Manual (HCM 2000) and gap acceptance method were employed to evaluate its performance. Traffic volume and pedestrian counts during peak and off-peak hours were recorded, allowing for capacity calculations across different traffic movements on approach roads. The volume-capacity ratio was determined, and control delay per vehicle was computed using the HCM guidelines. These findings established the level of service (LOS), indicating severe congestion (LOS F) for vehicles and pedestrians during peak hours.
... The number of available lanes is critical. Mamlouk and Souliman [37] indicated that single-lane roundabouts decreased the overall rate of accidents by 18%, while double-lane roundabouts increased the accident rate by 62%. The damage rate increased by 2% and 60% for single-lane and double-lane roundabouts, respectively. ...
Transforming intersections into roundabouts has shown that a sufficient degree of road safety and traffic capacity can be achieved. Because of the lower speeds at the area of a roundabout, drivers tend to become more easily adaptive to any kind of conflict with the surrounding environment. Despite the contribution to safety, the design elements of roundabouts are not uniformly fixed on a worldwide scale because of different traffic volumes, vehicle dimensions, drivers’ attitude, etc. The present study provides a brief overview of the contribution of roundabouts to road safety and the interactions between safety and the design elements of roundabouts. In addition, discussion points about current challenges and prospects are elaborated, including findings from the environmental assessment of roundabouts; their use and performance on the era of autonomous vehicles that will dominate in the near future; as well as the role and importance of simulation studies towards the improvement of the design and operation of roundabouts in favor of safer vehicle movement. The criticality of roundabouts, in terms of their geometric design as well as the provided road safety, lies upon the fact that roundabouts are currently used for the conventional vehicle fleet, which will be gradually replaced by new vehicle technologies. Such an action will directly impact the criteria for road network design and/or redesign, thereby continuously fostering new research initiatives.
... Based on three models of roundabouts, Zubaidi et al. [7] studied various factors impacting driver injury severity and their variations. Mamlouk and Souliman [8] investigated the crash rates and severity 2 of 32 of injuries in single-lane and two-lane roundabouts, finding that the former had lower crash rates. By building a safety performance function, Novák et al. [9] analyzed the impact of inlet design characteristics on injury severity. ...
Roundabout crashes are common worldwide but have received little attention. In particular, the investigation remains relatively understudied in distinct collision modes of roundabout crashes, including angle and non-angle crashes. This study investigates differences in factors affecting injury severity between angle and non-angle crashes and how these differences change over time. Random parameter logit models with heterogeneity in mean and variance were estimated using 2012–2019 Florida crash data. Variables considered for the modeling include temporal characteristics, environmental characteristics, road characteristics, spatial characteristics, vehicle characteristics, crash characteristics, and human characteristics. In addition, likelihood ratio tests were used to investigate the temporal instabilities of the models and differences in collision modes. The results showed that only a few variables demonstrated temporally stable effects for angle collisions (weekend and dark-lighted) and non-angle collisions (paved shoulders and cyclists), respectively. Unique influencing factors of injury severity were identified for different collision modes, such as dark-lighted, straight alignment, merge, lane departure, the disabled driver for angle collisions, and clear, fog, commercial vehicles, and aggressive driving for non-angle collisions. The results of the out-of-sample prediction simulations further demonstrate the difference in injury severity over time for angle and non-angle collisions. Overall results suggest that countermeasures can be implemented to reduce the injury severity of angle crashes/non-angle crashes based on the significant variables identified. This study may be used to improve roundabout safety by considering different collision modes.
... Crashes happening in roundabouts heighten the likelihood of resulting in slight injuries by 19.6%, and simultaneously decrease the chances of crashes resulting in serious injuries and fatalities by 17.9% and 1.7%, respectively. (Mamlouk and Souliman, 2019) study demonstrated that roundabouts decrease the number of conflict points at intersections and lower the severity of pedestrian injuries. ...
Pedestrian safety is a critical issue in the United Kingdom (UK) as pedestrians are the most vulnerable road users. Despite numerous studies on pedestrian-vehicle crashes globally, limited research has been conducted to explore the factors contributing to such incidents in the UK, especially on rural roads. Therefore, this study aimed to investigate the severity of pedestrian injuries sustained on rural roads in the UK, including crashes at intersections and non-intersections. We utilized the STATS19 dataset, which provided comprehensive road safety data from 2015 to 2019. To overcome the challenges posed by heterogeneity in the data, we employed a Latent Class Analysis to identify homogeneous clusters of crashes. Additionally, we utilized the Ordered Probit model to identify contributing factors within each cluster. Our findings revealed that various factors had distinct effects on the severity of pedestrian injuries at intersections and non-intersections. Several parameters like the pedestrian location in footway and one-way roads are only statistically significant in the intersection section. Certain factors such as the day of the week, the pedestrian's location in a refuge, and minor roads (class B roads) were found to be significant only in the non-intersection section. Parameters including pedestrians aged over 65 years and under 15 years, drivers under 25 years, male drivers and pedestrians, darkness, heavy vehicles, speed limits exceeding 96 km/h (60 mph), major roads (class A roads), and single carriageway roads are significant in both sections. The study proposes various measures to mitigate the severity of pedestrian-vehicle crashes, such as improving lighting conditions, enhancing pedestrian infrastructure, reducing speed limits in crash-prone areas, and promoting education and awareness among pedestrians and drivers. The findings and suggested measures could help policymakers and practitioners develop effective strategies and interventions to reduce the severity of these incidents and enhance pedestrian safety.
... The study found an 8.8% total collision reduction on double-lane roundabouts and an insignificant collision increase on single-lane roundabouts. Mamlouk and Souliman [5] studied 17 roundabouts in Arizona using collision rate analysis. The results showed a reduction in the number of total collisions on the single-lane but opposite effects on double-lane roundabouts. ...
... Results of each investigated configuration showed there is a major difference in both the combination and variables included in each model and the magnitude of impact of those variables. Mamlouk and Souliman (2019) investigated the effect of traffic roundabouts on accident rate and severity in Arizona State. It was found that the accident rate was less in single-lane roundabouts compared to double-lane roundabouts. ...
A roundabout may not provide an acceptable level of control and can be confusing to inexperienced drivers. Therefore, the purpose of this study is to identify the contributing factors that lead to specific driver injury severity by utilizing a random parameter binary probit model sustained by different experiences of motor drivers at 4-legs roundabouts in South Australia. Four models were estimated based on seven years of crash data (2012-2018), considering different types of motorist-driving license: learner, provisional, full, and for all datasets, including unknown licensures. The model estimates variables have been categorized into a driver, crash, temporal , spatial, vehicle, roadway characteristics, and vehicle movements. The results showed there are differences between resulting crash-injury severities when driver experience has been observed. Besides, several parameters were found to be random and normally distributed: safety equipment, crash type (rear-end crash), number of involved vehicles, weekdays indicator, stats area (crash occurred within metropolitan), vehicle type (passenger car), and posted speed limit (more than 50 km/hr.). In addition, the log-likelihood and the transferability test indicated that the data should be separated and analyzed according to the driver's license. Findings can help authorities to improve driver safety considering the influence of the driver experience.
... However, the effects of property-damage-only (no injury) crashes are highly ambiguous (Daniels et al., 2008). (Souliman, 2016) examines the outcomes of single-lane and double-lane roundabouts in the State of Arizona. The author indicated that single-lane roundabouts decreased the overall rate of accidents by 18%, while double-lane roundabouts increased the accident rate by 62% and Damage rates increased by 2% and 60% for single-lane and double-lane roundabouts. ...
Intersections present a significant safety concern, as such in an effort to reduce the more serious injuries occurring at or near intersections, many jurisdictions have turned to implementing roundabouts. Despite the advantages that roundabouts provide, crashes still occur, and less severe crashes are on the rise. The study presented in this paper investigates a crash-based analysis to better understand the factors that may influence less severe crashes to those of more severe crashes given various roundabout configurations and crash types. Using Oregon’s crash database from 2011 to 2015 a series of log likelihood ratio tests were conducted to validate that four separate random parameters binary probit models by configuration type were warranted. The outcome of each tested configuration (full, three & four leg, four leg, and three leg models) shows a major difference in both the combination and variables included in each model and the magnitude of impact of those variables. These differences illustrate that various roundabout configurations (full, three & four leg, four leg, and three leg models) do in fact have different factors highlighting the need to examined crashes at roundabouts by configuration type. Variables related to driver error, weather, alcohol use, barrier conditions, vehicle movement, location of crash, and restraint use were found as key differences between the various tested configurations.
Traditional crash frequency models cannot estimate crash frequency for individual traffic movements at an intersection, which precludes the safety evaluation of individual movements and identification of hazardous ones. This paper proposes a movement-based (MB) model that estimates crash frequency for individual movements as well as for the entire intersection. A base model using the safety performance function form in the Highway Safety Manual was also developed for comparison against the MB model. This study used crashes collected for five to eight years at 41 signalized intersections in North Carolina for the model estimation and validation (21 intersections for the estimation and 20 intersections for the validation). The models were validated using cumulative residual plots, test set validation, and in a case study. The test set validation showed that the MB model yielded slight improvements in estimations compared to the base model (1.17%−5.83% reductions in mean absolute error and 3.32%−6.64% reductions in root-mean-square error). The case study showed the MB model correctly identified hazardous traffic movements that had demonstrable safety problems based on observed and estimated crash frequencies. The MB model will enable engineers to identify hazardous movements and approaches to implement safety improvement countermeasures at the deserving locations and movements.
Determining the numerical value of the safety effectiveness of countermeasures or the Crash Modification factor (CMF) is one of the issues that has received considerable attention in recent years in the world. In this regard, several studies have been carried out to determine the CMF for converting intersection to roundabout in different countries. The results of these studies have led to a reduction of about 35 to 70 percent of crashes at the intersections. Since there is no serious study regarding determining CMF of constructing a roundabout and the percentage of crash reduction due to this countermeasure in Iran, the researchers are seeking to determine the CMF and reduction percentage of the crashes due to this measure in Iran. For this purpose, Salman Farsi roundabout of Ilam city was evaluated. AADT rates were obtained from the available traffic counts and crash data from the police. For determining CMF, empirical Bayes method was considered. The reason for using empirical Bayes method is its high efficiency in removing the phenomenon of regression to mean and other time-alteration factors and the high precision of this method is due to the use of SPF in order to reasonably estimate the frequency of crashes in the period prior to countermeasure. During studying, a SPF from AASHTO 2010 was used. Using this tool and predicting crashes before and after the construction of the roundabout, as well as calculating the frequency of expected crashes before and after it, the effectiveness of safety was calculated. The results for crash frequency is as crash CMF and crash frequency reduction percentage indicates a CMF of 0.68, implying a 32 percent reduction in crash. The statistical significance evaluation of this reduction shows that the 32 percent reduction of crashes after constructing roundabout is significant at 90 percent confidence level.
Introduction:
This study develops an injury severity model that demonstrates level of pedestrians' injury severity during pedestrian-vehicle collisions, specifically those involving distracted driving.
Method:
It uses data from a police-reported collision database that contains pedestrian-vehicle collision information in Nova Scotia, Canada. A latent segmentation-based ordered logit (LSOL) model is developed in this paper that comprehensively examines the influence of built environment characteristics on pedestrian injury severity. It estimates a latent segment allocation model within LSOL modeling framework to capture unobserved heterogeneity across pedestrians. Two latent segments, high- and low-risk segments, are identified probabilistically based on pedestrian characteristics and action, driver action, and collision attributes.
Results:
Results suggest that higher mixed land-use, transit stop density, length of sidewalk in the collision locations, and collisions occurring near schools yield lower pedestrian injury severity. In contrast, pedestrian-vehicle collisions in arterial roads, curved roads, sloped roads, and roundabouts tend to result in severe injuries. Interactions between distracted driving type and built environment characteristics are examined in this study. For example, using a communication device while driving on straight roads increases likelihood of higher pedestrian injury severity. This study also confirms the existence of heterogeneity across latent segments. For instance, higher percentage of people commuting by walking in the collision areas yield severe pedestrian injury in high-risk segments and lower injury severity in low-risk segments. Practical applications: The findings of this study will assist transportation planners and road safety stakeholders in developing effective and prioritized policies to reduce pedestrian injury severity involving distracted driving incidents.
This study estimated potential reductions in motor vehicle crashes and injuries associated with the use of roundabouts as an alternative to signal and stop sign control at intersections in the United States.
An empiric Bayes procedure was used to estimate changes in motor vehicle crashes following conversion of 24 intersections from stop sign and traffic signal control to modern roundabouts.
There were highly significant reductions of 38% for all crash severities combined and of 76% for all injury crashes. Reductions in the numbers of fatal and incapacitating injury crashes were estimated at about 90%.
Results are consistent with numerous international studies and suggest that roundabout installation should be strongly promoted as an effective safety treatment.
Roundabouts have become popular in Australia and many countries in Europe during the past few decades. In the United States of America, however, roundabouts are just beginning to be recognized as an alternative treatment for roadway intersections. An effort was made to collect traffic and traffic crash data for existing roundabouts in the United States and to perform a statistical analysis to determine the effectiveness of roundabouts as a treatment for intersecting roadways. General information about thirteen roundabouts located in Maryland, Florida, Nevada, and California was collected and is included for readers’ use. In addition, six retrofitted roundabout sites with accident data ranging from 1 to 3 years before and after were analyzed. In all but one case, the reduction in accidents for roundabout sites was in the range of 60 to 70 percent. A chi-squared test and a normal approximation test were performed using the accident data from these six roundabout sites. Both of these tests indicated a significant difference in the reduction of frequency and mean of accidents at 95 and 99 percent confidence levels, respectively, between pre-roundabout and post-roundabout periods. Results, though limited, are encouraging and in line with findings of past European and Australian studies involving roundabouts. Additional studies on the safety performance of U.S. roundabouts should be conducted in the future when more data are available to reinforce these findings.
Modern roundabouts are designed to control traffic flow at intersections without the use of stop signs or traffic signals. U.S. experience with modern roundabouts is rather limited to date, but in recent years there has been growing interest in their potential benefits and a relatively large increase in roundabout construction. This interest has created a need for data regarding the safety effect of roundabouts. Changes in motor vehicle crashes following conversion of 23 intersections from stop sign and traffic signal control to modern roundabouts are evaluated. The settings, located in seven states, are a mix of urban, suburban, and rural environments with the urban sample consisting of both single-lane and multilane designs and the rural sample consisting of only single-lane designs. A before-after study was conducted using the empirical Bayes procedure, which accounts for regression to the mean and traffic volume changes that usually accompany conversion of intersections to roundabouts. For the 23 intersections combined, this procedure estimated highly significant reductions of 40 percent for all crash severities combined and 80 percent for all injury crashes. Reductions in the numbers of fatal and incapacitating injury crashes were estimated to be about 90 percent. In general, the results are consistent with numerous international studies and suggest that roundabout installation should be strongly promoted as an effective safety treatment for intersections. Because the empirical Bayes approach is relatively new in safety analysis, the potential of this methodology in the evaluation of safety measures is demonstrated.
Roundabouts have become popular in Australia and many countries in Europe during the past few decades. In the United States of America, however, roundabouts are just beginning to be recognized as an alternative treatment for roadway intersections. An effort was made to collect traffic and traffic crash data for existing roundabouts in the United States and to perform a statistical analysis to determine the effectiveness of roundabouts as a treatment for intersecting roadways. General information about thirteen roundabouts located in Maryland, Florida, Nevada, and California was collected and is included for readers' use. In addition, six retrofitted roundabout sites with accident data ranging from 1 to 3 years before and after were analyzed. In all but one case, the reduction in accidents for roundabout sites was in the range of 60 to 70 percent. A chi-squared test and a normal approximation test were performed using the accident data from these six roundabout sites. Both of these tests indicated a significant difference in the reduction of frequency and mean of accidents at 95 and 99 percent confidence levels, respectively, between pre-roundabout and post-roundabout periods. Results, though limited, are encouraging and in line with findings of past European and Australian studies involving roundabouts. Additional studies on the safety performance of U.S. roundabouts should be conducted in the future when more data are available to reinforce these findings.
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