Evaluation of the Impact of Travel Time Reliability on Urban Expressway Traffic Safety

In urban areas, toll expressways have been constructed to provide motorists with safe, efficient, and reliable trips. Numerous studies have investigated the relationship between safety and efficiency. However, there has been only a limited focus on how traffic safety and reliability are related. In this study, multiple travel time reliability indicators were developed on the basis of an automatic vehicle identification system installed on the expressway of interest. The effects of travel time reliability on crash frequency were evaluated within the Bayesian hierarchical Poisson lognormal framework. It was confirmed that the reduction of travel time reliability caused by late arrivals would lead to more crashes. Nevertheless, the effects of travel time reliability were more significant on multivehicle crashes than on single-vehicle crashes. In addition, the necessity of using direct reliability measurement in safety analysis was assessed by calibrating models with only traditional traffic flow variables and structural equation models in which the effects of travel time reliability were modeled by latent variables. Comparison results showed that to understand better the effects of travel time reliability on safety, inclusion of direct reliability measurement would be more efficient. In conclusion, this study suggests that improvement of travel time reliability by providing estimated travel times to drivers in real-time might not only be beneficial to better travel experience, but might also improve safety on urban expressways.