What affects passing distance between vehicles and cyclists?

bike and car
Photo: Shutterstock

Transportation infrastructure that encourages bicycling without interfering with traffic flow or exceeding budgets is a key issue for many state and local governments. Ensuring the safety of bicyclists is a central part of this challenge.

One factor that may influence a cyclist’s sense of safety is the distance maintained by passing vehicles. As part of a recent capstone project, a team of graduate students at the Humphrey School of Public Affairs partnered with Hennepin County to investigate the factors that affect vehicle passing distance (VPD) on different bicycle facilities across the county’s network. The researchers also examined vehicular encroachment—when a vehicle fails to meet the minimum three-foot VPD required by Minnesota statute. 

The team used Hennepin County’s C3FT bike-mounted radar and a GoPro video camera to capture VPD during nearly 3,000 passing events. Roads used as part of the study included the following categories of bicycle facilities: buffered bike lane (painted), protected (bollard) bike lane, no facility, shoulder, standard bike lane, and bike boulevard.

One male and one female researcher executed test rides during the evening rush hour, when vehicle traffic was typically highest. Researchers then reviewed the video footage and documented the VPD, whether or not there was a car in the adjacent lane, and the vehicle type.

Key findings include:

  • The overall encroachment rate was low. Of the total 33 encroachments measured, 64 percent occurred on the road with no bicycle facility, giving it the highest encroachment rate of about 5.7 percent. All other roads had encroachment rates of less than or near 1 percent.
  • VPD was greatest on the bollard bike lane and lowest on the roads with no facility and the standard bike lane. Protected or buffered bicycle facilities are best at reducing VPD and the rate of encroachment, but bike lanes and wide shoulders also reduce the likelihood of encroachment.
  • In general, as vehicle size increases, average VPD decreases. Buses and large trucks had the highest likelihood of encroachment.
  • VPD was smaller for the female rider, and she experienced 73 percent of all encroachments.
  • Excluding facility type, the presence of an adjacent vehicle played the largest role in reducing VPD.

According to the research team, these findings confirm that road design and traffic planning decisions affect how vehicles and cyclists interact. The team also suggests that additional research is needed to understand how VPD and encroachment are related to a feeling of safety for cyclists.

The research was conducted by graduate students Josh Pansch, Isaac Evans, and Lila Singer-Berk and advised by Professor Greg Lindsey. Study results were published in the May 2018 ITE Journal.

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