9 Aug 2020

Do bicycles slow cars down on low speed, low traffic roads?

From Sunday Morning, 2:21 pm on 9 August 2020

Frustrated with bikes slowing traffic? New research from Portland State University shows cyclists don't significantly reduce the speeds of vehicles around them, although it may feel like they do.

The researchers say the findings apply to how passenger cars are affected in urban roads without bike lanes, and that the findings apply for New Zealand roads too.

Two cyclists with protective equipment are approaching an intersection in a busy part of the city towards the setting sun.

Photo: 123RF

While the results may seem counter-intuitive, co-author of the paper Jaclyn Schaefer told RNZ's Sunday Morning it's likely any hold-up from bikes in traffic feels more significant than it actually is.  

"Human perception is a little bit fallible, and there can be a bit of a psychological effect at play when motorists are asked this question — the negative experiences, even if they're minor, do tend to stand out in our minds a little bit more.

"Based on the data that we studied, those reductions in speed were unlikely to occur."

Schaefer says the study looked at six Portland roads with speed zones less than 40 kilometres per hour, and fewer than 2000 vehicles per day. Data was collected at different times of the day, including peak traffic hours.

"There were plenty of opportunities for a motorist to overtake a cyclist without incurring long distances of slower speeds. Even if we get stuck behind a cyclist for five, 10, 15 seconds, sometimes that perception of time or impediment magnifies in our brains."

The researchers collected two sets of speed data; from cars that were driving behind other cars, and cars that were driving behind bicycles. They found: "The actual speed differences were generally in the order of 1 mph or less."

And they concluded this speed difference wouldn't be enough to cause congestion on the road.

Portland State University researcher Jaclyn Schaefer.

Portland State University researcher Jaclyn Schaefer. Photo: Supplied/ Portland State University

Earlier research has also shown we don't always make the best assessments of time when driving, Schaefer says.

"Our perception of time savings can also be skewed. There's been some research looking at some travel time differences when speed limits have been reduced; for a 40 percent reduction in the posted speed limit the travel times only increased by something like less than 25 percent.

"You'd assume there'd [be an equivalent time increase] based on physics and speed and time relationships, but there wasn't.

"The researchers concluded that things like traffic control devices, traffic signals, pedestrian crossings, other intersections with turning traffic - all of those factors really play a big role in our travel time, and they tend to affect travel times a lot more than just a few kilometres an hour difference in speed."  

Schaefer says it's worthwhile for drivers to think twice if they feel slowed down by cyclists — is it really going to have much impact on your journey?

"You might want to take a step back and re-evaluate if it's your perception, or if it's really happening," she says.

The research is part of a wider project by the university's College of Engineering to update the methods used to decide on speed zones for urban streets.

"The hope is that our study dissuades policymakers from tossing out shared roadways as a viable option because of the perception that bicyclists will impede the mobility and speed of drivers," Schaefer says.