The relationship between road safety and congestion on motorways

A literature review of potential effects


Marchesini, P.; Weijermars, W.



Mobility has been increasing significantly in the last few decades and will continue to increase. On road stretches which have insufficient capacity, traffic becomes congested. Traffic congestion has a negative impact on the economy and on the quality of people’s lives. Road users experience delay and stress, and environmental pollution increases. The effects of traffic congestion on traffic safety, however, are less obvious. This literature review investigates the relationship between congestion and safety at road sections of the main road network (mainly motorways) and specifically looks at unstable and congested traffic conditions. The hypotheses that are explored are the following: 1. Traffic congestion levels have been increasing significantly in the last few decades, and this trend seems to continue. The general perception is that crash frequency increases with increasing congestion levels. Nevertheless, it is expected that severe crashes will not increase under these traffic conditions. 2. Crash frequency, severity and type are expected to be affected by the changing flow conditions that occur when traffic starts to become congested. 3. Once traffic is congested, fewer (serious) crashes are expected to occur within the queue. In contrast, at the tail of the queue more severe rear-end crashes are to be expected. 4. It is expected that the effects of congestion on safety depend on the extent to which drivers are surprised by the congestion. This may, in its turn, depend on the type of congestion, the location of the queue and the use of variable message signs. Two types of studies are discussed; 1) studies that use aggregated data and compare various locations and/or different time periods (e.g. peak and non-peak) and 2) studies that are based on short observation periods and analyse which traffic conditions result in crashes. With regard to the first hypothesis, results of different studies are not consistent. Some studies find that high volume to capacity (V/C) ratios result in higher crash rates but less severe crashes. Another study finds that crash rates decrease at high density levels, and one of the studies does not find any relationship between congestion and crash frequency nor between congestion and crash severity. None of these studies explicitly provide information about the influence of congestion on crash rate. The results regarding the second hypothesis seem to be more consistent. The studies suggest that crash likelihood increases as speed variability increases (a typical indicator for unstable traffic conditions). Also large speed differences between lanes and density variability seem to increase crash likelihood. Crash severity seems to decrease with increasing volumes (or V/C ratios). Golob, Recker & Pavlis (2008) report that crash severity does not seem to change during the transition from free flow to congested conditions, yet decreases once traffic is congested. Furthermore, the studies seem to show consistent results with regard to crash type. Rear-end crashes are more likely to occur during unstable conditions. The third hypothesis could not be fully proved, due to the scarcity of the literature on this topic. Nevertheless, the studies which rely on disaggregated data provide some insight in this respect. The study by Golob, Recker & Pavlis (2008) (also based on disaggregated data) reports some findings that can be related to the situation within the queue. It is observed that once traffic is congested, crash severity is greatly reduced when all lanes present similar flow conditions. The authors find that under these conditions of uniformity, crashes with (fixed) objects are more likely to occur. When only the left and interior lanes are congested, rear-end and side-impact crashes are more likely. The fourth hypothesis about the effects on safety of structural and incidental congestion could not be investigated on the basis of the available literature. In summary, the literature discussed provides insight into the relation between unstable traffic conditions and crash frequency, severity and type. Moreover, it offers a better understanding of rear-end crash occurrence at the tail of the queue. Although the available literature does not provide a complete picture of the relationship between congestion and safety, it does present an overview of the current status of research on this subject. However, several questions are still to be answered. As increasing congestion seems to pose a significant problem to traffic safety, there is need for further research. Therefore, we recommend investigating the relationship between congestion and traffic safety on motorways in more detail, by linking data on traffic volumes with congestion and crash data.

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SWOV, Leidschendam