Crash Severity Analysis at Roundabouts: A case study in Quebec, Canada

Numerous studies both in North America and abroad have studied the roundabout as a means to improve the capacity and safety of intersections. The focus of these studies has been to reduce both the number and the severity of accidents, and have generally come to agreeable results. Nevertheless, few studies have dealt with the identification of accident factors within roundabouts. The varied physical and environmental characteristics of Quebec’s roundabouts and the behavioral traits of its drivers requires further study in order to help reduce the occurrence and severity of accidents. This paper presents a crash severity analysis to identify the effects of different contributing factors on injury severity outcomes. To achieve this, an ordered logit modeling technique was used for the analysis. Moreover, an important effort was made to integrate an accident database containing accident records from the year 2000 to 2011. Among other results, it was found that factors such as a larger number of involved vehicles, accidents occurring within the intersection, vehicle rollovers, the involvement of buses, accidents occurring in the dark on unlit roads and snow conditions led to increased injury severity within roundabouts, whereas factors associated to accidents involving only cars, animal strikes and snow-covered roadways were found to be associated with less severe injuries. Burns, Miranda-Moreno, Saunier, Ismail 3 INTRODUCTION The occurrence of road vehicle accidents is well documented with respect to the adverse economic and emotional effects they inflict on society (1). Because of this, traffic engineers and policy makers have shown considerable interest in finding ways to identify critical factors of both accident occurrence and their severity outcomes (2) (3). International experience with roundabouts tends to indicate improved safety and performance when compared to traditional signand signal-controlled intersections. The inherent safety benefits of a roundabout are often attributed to the fundamental design features which lead to lower travel speeds, the elimination of head-on and right-angle conflict areas and the need for pedestrians to only cross one vehicle movement at a time (4) (5) (6). However, largely due to the relative scarcity of roundabouts within the North American road network compared to typical intersection controls, few studies have investigated the severity of accidents occurring in or around roundabouts. Most of the previous work on roundabouts has been related to accident frequency or occurrence (7). Roundabouts first started appearing in the Province of Quebec in 1998, and have been the subject of increasing popularity. Because of this, Quebec now has the highest number of roundabouts in Canada (8). Consequently, there are concerns as to the applicability of roundabouts in terms of both safety and performance due to the particular weather and driver behavioral conditions that exist within the province (9). It is therefore suggested that detailed safety studies be conducted in order to better understand which factors increase or decrease the frequency and severity of accidents. This could help identify countermeasures or actions that decrease accident consequences in roundabouts. Changes to the established design guides may be required in order to better suit local conditions. In order to improve the safety of all road users within a roundabout, planners and engineers must be able to modify the physical environment in such a way as to reduce the dangers within each facility. Factors relating to vehicle, driver, roadway design and weather conditions need to be studied to better understand how each characteristic affects the injury severity outcome of a given roundabout. In this way, designers would be able to estimate the safety benefits of a specific design element change by holding all other factors constant. This paper aims to investigate a number of factors which may be associated with the injury severity outcomes of accidents occurring in and around roundabouts. Accordingly, the most significant factors which affect the severity of accident injury outcomes were identified using an ordered logit regression model. The studied variables pertain to roadway, environmental, vehicle and human behavior characteristics that are thought to impact injury outcomes (10) (11). This research is part of a larger effort to measure the safety of roundabouts in Quebec, through the analysis of crashes, surrogate events (conflicts) using video data collection and road user surveys. It is based on a dataset containing all recorded motor-vehicle accidents within the Province of Quebec for the period of 2000 to 2011, from which the accidents occurring within proximity of a roundabout were extracted. The data was provided from the Quebec Ministry of Transportation (MTQ) and Quebec’s Automotive Insurance Board (Société de l’assurance automobile du Québec (SAAQ)). Each record contains information on the time, date and location of the accident, as well as characteristics of the roadway, the environment, the vehicle and the driver. The injury severity levels of all individuals involved in a given accident are also included in the dataset. Burns, Miranda-Moreno, Saunier, Ismail 4 LITERATURE REVIEW Over the last few decades, there has been extensive literature on injury severity analysis in road safety based on historical accident records. An important part of this literature deals with the identification of accident factors that are associated to injury severity outcomes at various levels. Among these factors, one can mention road geometry, signs and traffic control characteristics at the accident location, traffic conditions at the moment of the accident, and vehicle and passenger attributes including driver, weather, visibility, surface conditions, etc. As a unit of analysis, severity studies commonly use the individual (passenger and driver), vehicle or accident level. The analysis level selected is often dependent on data constraints (12). In this important literature, some studies have also looked at accident occurrence outcomes classified by injury severity types. Also, to model injury severity outcomes, many statistical methods have been proposed including traditional ordered response models to take into account the inherent ordering of the reported injury severity levels (13). Other models such as probit, multinomial, mixed logit and latent class model have been used (14) (15) (16). For a comprehensive literature review of the statistical methods used in this topic, one can refer to (1). Many injury severity studies have also identified the contributing factors of accidents at different location types such as rural roads, signalized urban intersections or special facilities such as highway railway crossings and freeway ramps (2) (17) (18). In this rich literature dealing with a variety of issues, various reports have investigated the safety of roundabouts. In this type of intersection, a particular aspect that has attracted a lot of attention is the effectiveness of roundabouts when compared to either signor signal-controlled intersections. Many before-after studies have investigated this important question in Europe and North America. The effects of converting intersections to roundabouts have been documented in various literature-review studies. For instance, one can refer to the work of Elvik (2003) that carried on a meta-analysis of studies reported outside the United States. This study concluded that roundabouts are associated with a 30% to 50% reduction in the number of injury accidents and fatal accidents are reduced by 50% to 70% (19). The changes on property damage crashes are highly uncertain, and an increase often can occur in some conditions (e.g., three-leg intersections). Some North American studies of roundabout accident frequency have been documented, and have noted several safety benefits. For instance, Retting et al. (2001) studied the benefits associated with roundabouts. Using a dataset with 17 converted roundabouts on high-speed rural roadways, this research conducted a crash injury frequency analysis. Among the main findings, they show that the average injury crash frequency (before and after the transformation) was reduced by 84%, average injury crash rate was reduced by 89%, angle crashes were reduced by 86%, and fatal crashes were reduced by 100% (4). An important NCHRP report, entitled Roundabouts: An Informational Guide, has also made an important effort to summarize this literature (6). An overall conclusion of the international literature is that important benefits can be expected from the conversion of traditional intersections to roundabouts, with some exceptions for pedestrian and bicycle accident risk, which is less clear. Accident frequency modeling (safety performance functions) has also been used to investigate the link between geometry and traffic conditions and crash occurrence (20). Despite this rich literature covering several issues in roundabout safety, very few studies have investigated which factors have the highest influence on the injury severity outcomes at roundabouts. Perhaps the only study looking at injury severities at the vehicle and crash level is the recent study of Daniels et al. (2011). They investigated the factors associated to severity of crashes or injury outcomes at roundabouts using injury crash records on roundabouts in Burns, Miranda-Moreno, Saunier, Ismail 5 Flanders-Belgium. Logistic regression was used for this purpose to represent the two grouped outcomes (fatal and serious injuries as one category, and minor or property damage only in the second category). Among other results, they found that crash severity is strongly dependent of the involved types of road users. In particular, vulnerable users (pedestrians, bicyclists, moped riders and motorcyclists) have a higher probability of getting seriously injured in a roundabout crash. Bicyclists represent almost half of all those killed or seriously injured in multiple-vehicle collisions at the investigated roundabouts. As in other facilities, the effects of age, geometry and light conditions are less substantially correlated to the injury severity (20). No studies of this type have been reported in North America, in particular in Canada. This can be associated to the lack of data and spatial location of crashes. This paper aims at investigating potential factors that have a large influence on the injury severity outcomes of accidents at roundabouts. To our knowledge, this is the first severity study of this type in Canada and the Unites States.