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The crash performance of seagull intersections and intersections with left turn slip lanes
Abstract
Alternative intersection layouts may reduce traffic delays and/or improve road safety. Two alternatives are reviewed in this research: 'priority-controlled Seagull intersections' and ‘priority-controlled intersections with a Left Turn Slip Lane’. Seagull intersections are used to reduce traffic delays. Some do experience high crash rates, however. Left Turn Slip Lanes allow turning traffic to move clear of the through traffic before decelerating, thereby reducing the risk of rear-end crashes. Although there is debate about the safety problems that occur at Seagull intersections and Left Turn Slip Lanes there has been very little research to quantify the safety impact of different layouts. In this study, crash prediction models have been developed to quantify the effect of various Seagull intersection and Left Turn Slip Lane designs on the key crash types that occur at priority intersections. The analysis showed that seagulls are not safe on 4-lane roads, that roadway features like kerb-side parking and nearby intersections can increase crash rates and that left turners in LTSLs can restrict visibility and create safety problems.
Background
Road Traffic injuries (RTI) are among the top ten leading causes of death in the world resulting in 1.35 million deaths every year, about 93% of which occur in low‐ and middle‐income countries (LMICs). Despite several global resolutions to reduce traffic injuries, they have continued to grow in many countries. Many high‐income countries have successfully reduced RTI by using a public health approach and implementing evidence‐based interventions. As many LMICs develop their highway infrastructure, adopting a similar scientific approach towards road safety is crucial. The evidence also needs to be evaluated to assess external validity because measures that have worked in high‐income countries may not translate equally well to other contexts. An evidence gap map for RTI is the first step towards understanding what evidence is available, from where, and the key gaps in knowledge.
Objectives
The objective of this evidence gap map (EGM) is to identify existing evidence from all effectiveness studies and systematic reviews related to road safety interventions. In addition, the EGM identifies gaps in evidence where new primary studies and systematic reviews could add value. This will help direct future research and discussions based on systematic evidence towards the approaches and interventions which are most effective in the road safety sector. This could enable the generation of evidence for informing policy at global, regional or national levels.
Search Methods
The EGM includes systematic reviews and impact evaluations assessing the effect of interventions for RTI reported in academic databases, organization websites, and grey literature sources. The studies were searched up to December 2019.
Selection Criteria
The interventions were divided into five broad categories: (a) human factors (e.g., enforcement or road user education), (b) road design, infrastructure and traffic control, (c) legal and institutional framework, (d) post‐crash pre‐hospital care, and (e) vehicle factors (except car design for occupant protection) and protective devices. Included studies reported two primary outcomes: fatal crashes and non‐fatal injury crashes; and four intermediate outcomes: change in use of seat belts, change in use of helmets, change in speed, and change in alcohol/drug use. Studies were excluded if they did not report injury or fatality as one of the outcomes.
Data Collection and Analysis
The EGM is presented in the form of a matrix with two primary dimensions: interventions (rows) and outcomes (columns). Additional dimensions are country income groups, region, quality level for systematic reviews, type of study design used (e.g., case‐control), type of road user studied (e.g., pedestrian, cyclists), age groups, and road type. The EGM is available online where the matrix of interventions and outcomes can be filtered by one or more dimensions. The webpage includes a bibliography of the selected studies and titles and abstracts available for preview. Quality appraisal for systematic reviews was conducted using a critical appraisal tool for systematic reviews, AMSTAR 2.
Main Results
The EGM identified 1859 studies of which 322 were systematic reviews, 7 were protocol studies and 1530 were impact evaluations. Some studies included more than one intervention, outcome, study method, or study region. The studies were distributed among intervention categories as: human factors (n = 771), road design, infrastructure and traffic control (n = 661), legal and institutional framework (n = 424), post‐crash pre‐hospital care (n = 118) and vehicle factors and protective devices (n = 111). Fatal crashes as outcomes were reported in 1414 records and non‐fatal injury crashes in 1252 records. Among the four intermediate outcomes, speed was most commonly reported (n = 298) followed by alcohol (n = 206), use of seatbelts (n = 167), and use of helmets (n = 66). Ninety‐six percent of the studies were reported from high‐income countries (HIC), 4.5% from upper‐middle‐income countries, and only 1.4% from lower‐middle and low‐income countries. There were 25 systematic reviews of high quality, 4 of moderate quality, and 293 of low quality.
Authors' Conclusions
The EGM shows that the distribution of available road safety evidence is skewed across the world. A vast majority of the literature is from HICs. In contrast, only a small fraction of the literature reports on the many LMICs that are fast expanding their road infrastructure, experiencing rapid changes in traffic patterns, and witnessing growth in road injuries. This bias in literature explains why many interventions that are of high importance in the context of LMICs remain poorly studied. Besides, many interventions that have been tested only in HICs may not work equally effectively in LMICs. Another important finding was that a large majority of systematic reviews are of low quality. The scarcity of evidence on many important interventions and lack of good quality evidence‐synthesis have significant implications for future road safety research and practice in LMICs. The EGM presented here will help identify priority areas for researchers, while directing practitioners and policy makers towards proven interventions.
Providing rapid and safe traffic in citiesrequires the use of a set of planning and organizational measures.While the implementation of architectural-planning measures requires, except for significant investments, a quite long period, organizational measures can lead, albeit to a temporary but relatively rapid effect. For this purpose, an analysis of traffic flow metricson the approaches to the intersection, traffic light cycle, and the traffic phasing was carried out.Conducted research indicates that left-turn flows cause significant delays andreduce traffic safetyat the intersection.Therefore, increasing the efficiency of the crossroadsneeds redevelopment measuresby restriction of left-turn flowsfrom the main road, change of the traffic flows phasing and optimizing the traffic light cycle. In the presence of adjacent intersections, the scheme of left turn realization is developed. The simulation of intersection functioning in existing conditions and under the change of left-turn flows movement organization was carried out in the PTV Vissim software environment It is established that restriction the left-turn flows atthe controlled intersectionallows optimization the traffic light cycleand equip it so that the average delay for traffic flows will decrease in comparison with the current state
Over recent years there has been an increase in rural lifestyle subdivisions that require direct access from high-speed rural roads. Transit NZ and other road controlling authorities are concerned with the impact such subdivisions will have on the safety of the
rural road network. A significant number of rural crashes occur at intersections. The
crash prediction models developed for priority junctions in this research (between 2004
and 2006) can be used to predict the effect on intersection safety of new development in
rural areas. Making such predictions is difficult at present due to the relatively low crash
histories observed at most sites, particularly when focused on specific crash types and
road deficiencies such as poor visibility and high speeds.
As traffic volumes grow on the rural road network and four-laning becomes necessary
along many road sections, many of the priority-controlled intersections that provide local
access to communities are reaching capacity or have safety problems. Little research is
currently available in New Zealand to assess the safety implications of the various
improvement options, including grade separation, roundabout control and signalised
control. The crash prediction models developed in this research project, along with
severity ratios (e.g. fatal/serious/minor), for signalised and roundabout controlled
intersections in high-speed areas, can assist in assessing the likely impact on safety of
proposed intersection improvements
Accident Prediction Models have been developed for urban and rural intersections and links. This includes models for traffic signals, roundabout, priority and uncontrolled intersections, rural highways, motorways, urban arterials , collector and local streets. Accident (1995 to 1999) and traffic data at over 1000 sites throughout New Zealand were used to develop the models. Test statistics have been prepared showing the goodness-of-fit and confidence intervals of the models. Application of the models in economic appraisal (evaluation) has been discussed along with the changes that would need to be made to the (NZ) Project Evaluation Manual to incorporate the models.
This paper reviews model relating accidents to traffic flows, with particular emphasis on the appropriateness of the model form and the statistical technique employed for parameter estimation. The development of generalised linear models for predicting accidents at intersections in New Zealand is then described. It is shown that the new models fit the empirical data better than existing, simpler models. the use of the models for predicting intersection accidents in three networks is described.
A number of alternative intersection layouts may reduce traffic delays and/or improve road safety. Two alternatives are reviewed in this research; 'priority controlled Seagull intersections' and intersections with a Left Turn Slip Lane. Seagull intersections are used on roads to reduce traffic delays. However, some do experience high crash rates. While left turn slip lanes allow turning traffic to move clear of the through traffic before decelerating. Although there is debate about the safety problems that occur at Seagull intersections and Left Turn Slip lanes there has been very little research to quantify the safety impact of different layouts. In this study, crash prediction models have been developed to quantify the effect of various Seagull Intersection and Left Turn Slip lane designs on the key crash types at priority intersections.
The management of speed is considered an important safety issue at roundabouts. The approach speed and negotiating speed through roundabouts depends on both the geometric design of the roundabout and sight distance. In New Zealand and in the Australian States the design standards (based on Austroads) recommend long approach sight distance and provision of relatively high design speeds. This is in contrast to European roundabouts where visibility is normally restricted and the geometric design encourages slow approach and negotiation speeds. The 'flow-only' models developed in Turner (2000) have been extended to include sight distance, intersection layout and observed speed variables. Models have been produced for the major motor-vehicles only, pedestrian versus motor-vehicles and cyclists versus motor-vehicle accident types. 'Flow-only' models have also been produced for roundabouts on roads with high speed limits. The models have then been used to assess whether a move towards the European design philosophy, of lower speed and reduced sight distance, is likely to produce lower accident rates for all modes, particularly in areas that have high concentrations of pedestrians and cyclists.
Previous research on right turn against accidents at 4-arm, two-way traffic signals has identified that the daily volume of right turning and through traffic are key predictor variables for accident occurrence. The objective of this research was to consider a number of non-flow variables, and determine whether these are also key predictors for accident occurrence. The outcome is a more refined accident prediction model for this accident type. A particular interest was the impact of right turn phasing on accident occurrence.
The report gives the findings of a study of personal injury accidents at a sample of 84 four-arm roundabouts on main roads in the UK. The study includes small roundabouts and roundabouts of conventional design, in both 30-40 and 50-70 mile/h speed limit zones. Tabulations are given showing accident frequencies, severities, and rates by roundabout type. The accidents are further analyzed by type (entering-circulating, approaching, single-vehicle, etc) and by road-user involvement (cyclist, motorcyclist, pedestrian, etc). The accident frequencies by type are related to traffic flow and roundabout geometry using regression methods. Equations are developed to enable roundabout accidents to be predicted for use in design and appraisal.
The NSW Centre for Road Safety is currently conducting a study to evaluate the safety performances of various unsignalised T-junction configurations on high speed rural roads in NSW. This paper summarises the results of the Stage 1 study involving Seagull T-junctions only. The findings were that two types of crashes, Right Near and Right Through, were predominant among all the crashes, and those that result in casualties. Driver impairment, speeding behaviour and road environment conditions were a factor in a small percentage of these crashes. Of the deemed to be at-fault male drivers, almost two thirds were at and above the age of 67. However, for each of the other age groups below this, there was a higher incidence of female drivers than male drivers involved in Right Near crashes. The former is consistent with other studies on older male drivers. Consistent with the RTA's "safe system approach", key solutions include substantially reduced speeds for through traffic and removal of critical side impact crash types. This may require further investigation prior to implementation.
Testing goodness of fit for a Poisson distribution is routine when the mean is sufficiently large; the scaled deviance G or Pearson's X statistic follow approximate chi-square distributions and perform the task well. When the mean is low, typically less than one, the approximations to chi-square distributions are poor. In this paper we explore the underlying reasons for this behaviour and present a practical resolution of the problem, in both single distribution and regression contexts. An extension to negative binomial models is also given. This research is motivated by a real example drawn from road accident modelling.
Assessing the safety performance of left-turn facilities at rural T-junctions in New Zealand. University of Auckland Masters Project (788A and 788B)
- C Urlich
Urlich, C. 2014. Assessing the safety performance of left-turn
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