A method for quantifying risks imposed on cyclists while sharing road with motor vehicles

By Saumya Jain

Keeping vehicle occupants and pedestrians safe via engineering standards and street warrants is common practice around the world. But in spite of the growing level of support for bicycling for both commuting and recreation, bike facility design standards are rarely backed by empirical data and are often inconsistent between different cities and states. Most national guides outline desired standards without empirically calculating related risk factors. A recent study, conducted at the University of Waterloo, presents a methodology that can potentially be used by city planners for predicting the probability of unsafe interactions between bicyclists and motor vehicles based on passing events on 4-lane urban arterials with no on-street bike lanes.

The study revolves around ‘critical passing distance’ which is defined as the minimum distance between a car and a bicyclist that can be perceived as being safe during overtaking. An instrumented bicycle with ultrasonic sensors was used to collect data for 5,227 passing events on different road categories in Kitchener-Waterloo area in southern Ontario. It was found that the number of unsafe passing events were much higher on streets without on-street bike facilities compared to streets with bike facilities, especially when MVs were restricted from making lane changes due to adjacent traffic. It was also observed that the MV-bicyclist interaction was significantly affected by road segment characteristics like vehicle volumes, speed limits, and upstream traffic signal configurations.

Based on the study results, the researchers proposed linear models to estimate the probability of unsafe overtaking events as a function of AADT and section length. The input parameters for the proposed methodology are bicycle demand, section length, AADT, speed limit, and upstream traffic signal configurations. The methodology estimates the number of unsafe passing events per bike trip per hour and provides a basis for quantifying the risks imposed on cyclists while sharing the road with motorized vehicles. Although the researchers say the study is not yet sufficient for defining street design warrants, it is a step forward in empirically understanding bicyclist safety and related levels of traffic stress. With a more in-depth investigation and analysis, this study can be used by city planners to define acceptable safety standards for on-street bicyclists.

Saumya Jain is a Senior Associate at SSTI.