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Enforcing 3-foot passing law with the help of science

Police in Tennessee are using a new device to measure whether motorists are adhering to safe passing distances when overtaking cyclists.

The device, shown off by police on National Bike to Work Day, can measure how close a car is to a bike.

One Chattanooga local, Pearl Pangkey told  WRCBtv there's still a long way to go.

“I don't think there's enough bike lanes and I think we just have to educate the public,” she said.

“I was going really slow because there was traffic and I was peddling and I hear this really loud engine sound and I turn and there's this big black SUV right beside me. I could have reached out and touched his hood,” she added.

“Where this is a collision between a cyclist and a motorist, the cyclist is almost always going to lose in that collision. They're going to get hurt,” Chattanooga Police Officer Rob Simmons, who created the device, said.

“It's an opinion of an officer that sometimes will hold up in court and sometimes won't because judging distance from a distance is very difficult,” Simmons added.

Together with a Texas-based engineer, Simmons used radar to measure precisely to the inch how close a car is coming to a cyclist.

“[The signal will] bounce off the vehicle and then come back and I'm given a distance in inches of how far away the motorist was when they passed,” he said.

“As cyclists, we need to be predictable to the motorists and the only way to be predictable is to follow the laws that are set up. We are to act like vehicles and we have all the rights and responsibilities as vehicles,” Simmons said.

Drivers who flout the 3 foot rule are subject to a fine.

Last year we reported how painted bike lanes make no difference to the speed and closeness with which drivers pass cyclists, according to a study, but if roads don't have centre-line markings, drivers pass cyclists more slowly.

That's the conclusion of a new study by two academics from the Universities of Leeds and the West of England.

They set out to find out how much difference road markings make to the amount of space drivers give cyclists when passing, and the speed at which they pass in 20mph and 30mph zones.

Stella C. Shackel of the Institute for Transport Studies at the University of Leeds, and John Parkin, Professor of Transport Engineering at the Centre for Transport and Society, University of the West of England published their findings in the paper Influence of road markings, lane widths and driver behaviour on proximity and speed of vehicles overtaking cyclists.

Using a bike equipped with an ultrasonic distance sensor, the authors measured the passing distance of vehicles, and side-facing cameras were used to measure passing speed. Recording each pass allowed the researchers to determine the type of vehicle and so differentiate between cars, light goods vehicles, heavy goods vehicles and so on.

In September last year a law requiring drivers to stay at least three feet away from bicycles when overtaking was passed in California.


The law is designed to take the guesswork out of what a ‘safe’ passing distance is - but politicians say it’s more about creating a visual image for drivers of how far away they should be, than being something police are able to enforce.

The law has long been debated in the state, with earlier versions being vetoed by opposing politicians in 2011 and 2012. The new bill, which has been simplified from earlier incarnations, was sponsored by the city of Los Angeles, which is known for its car-centricity. 

A growing activist base has supported the bill, which was deemed necessary despite a 2010 launched a “Give Me 3″ graphic campaign encouraging drivers to create a safe distance between vehicles and cyclists.

Under the law, if traffic or roadway conditions prevent motorists from giving cyclists 3 feet of clearance, drivers must “slow to a speed that is reasonable and prudent” and only pass when the cyclist will not be endangered.

Fines run to $35 for violations, but this rises to $154 with additional fees. Drivers who collide with cyclists and injure them while violating the law will be subject to a $220 fine.

After an unpromising start, having to be bribed by her parents to learn to ride without stabilisers, Sarah became rather keener on cycling in her university years, and was eventually persuaded to upgrade to proper road cycling by the prospect of a shiny red Italian bike, which she promptly destroyed by trapping a pair of knickers in the rear derailleur. Sarah writes about about cycling every weekend on road.cc.