Police in Tennessee are using a range-finding ultrasound unit to measure the distance between their bicycle and passing cars

Police on bikes are using sophisticated technology to gather evidence of drivers who pass too close.

A range-finding ultrasound unit measures the space between the bicycle and overtaking vehicle.

If it detects that the gap is too small, a computer on the handlebar sounds a buzzer and displays the measurement in inches.

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At the same time a Go-Pro video camera on the handlebar automatically records the near-miss.

Drivers are then pulled over, shown the video and told to mend their ways. They are warned that if they don’t, they’ll face a day in court and a fine.

It’s an initiative of officers in Chattanooga, Tennessee, where the law states cars must not get closer than 36 inches (91cm).

Trucks must be even more considerate – if they pass within 72 inches they could be up before the judge.

The state law was passed in 2007 and similar regulations are in place in 23 other US states.

Yet it is only this year that equipment suitable for a bike, and which is able to gather evidence acceptable by the courts, has been available.


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When a small electronics company, Codaxus, came up with the system, a local community group raised $1,400 to buy it for Chattanooga Police Officer Rob Simmons.

Now, as he makes his usual patrols by bike, woe betide any driver who cuts him up. He used to have to make judgement calls himself about how near they came to him but the new equipment means drivers can no longer argue the toss.

The Bicyclist and Safe Monitoring Applied Radar Technology (BSMART) unit gets their measure to within one inch, even if they are passing at speed.

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“The mission is to improve the safety of the most vulnerable road users by improving relationships between motorists and cyclists, to help motorists understand the risks for bicycles,” says Simmons’ boss, Police Chief Fred Fletcher.

The police are keen to stress they don’t want to penalise drivers – they want to educate them.

“Most of the things motorists are doing is from lack of understanding, rather than malicious effort. Most people will act more safely if they understand the issues and risks,” says Fletcher.

The Tennessee law that must be the envy of road safety campaigners in the UK where there is no distance specified.

The Highway Code Rule 163 says only that drivers must give cyclists “at least as much room as you would when overtaking a car”.

Max Glaskin is an award-winning freelance journalist who tweets about cycling and science as @CyclingScience1 and is the author of Cycling Science (published by Frances Lincoln UK, Chicago University Press USA, and seven other languages).

  • Ross

    gardening more dangerous than cycling ! What ? . what stats are you using. seasonally adjusted ones I think ? its like a lie recipe “season to taste” my lawnmower hiring me will not take off my arm , that is becuse i have total control of it

  • Jason C.

    I’ve noticed that the bicycle haters (mentally ill in many cases) start fights with bike riders and when the bike rider flips off the assailant, that assailant uses that as an excuse when speaking to the cops. I’ve read that in the paper many times. The “rude gesture” is reported by the cops as playing a part of the entire matter which is absurd. That gesture, called free speech in court cases, doesn’t justify any assault whatsoever.

  • Jason C.

    Exactly. Laws are meaningless unless the cops enforce it.
    A friend of my sister was actually knocked down by a passing car. She was uninjured as was her bike. But the driver and his passenger stopped, looked back at her, and then fled. Witnesses got the plate number and phoned police. The cop that arrived was, by all accounts, NOT interested in putting out any effort. The plate was tracked and the owner identified living about 200 miles distant. The last I knew, the cops were simply sending a letter to the owner …. She didn’t have a helmet cam.

    The helmet cams are starting to come in real handy. I’ve seen more than a few drivers arrested over the last year for intentional buzzing of bike riders.

  • Jason C.

    Doesn’t matter. Lots of people pass through red lights … no problems. Until finally something happens. Like a crash. Like an injury. Like a death.

    If its illegal, its illegal. Nobody cares about your personal experiences.

  • John Moran

    Right, we have the 36″ rule in CA for several months but the local sheriff’s just ignore it and anything else that has to do with cycling. I often see people driving in the bike lane, and have been buzzed by cars when a sheriff is behind me. I’ve given the cars a fist or finger and the sheriff did nothing!

  • John Moran

    In CA we have the 36″ rule but I would like to see them also start the 72″ rule for trucks. I’ve been buzzed by 18 wheelers and something that big just about sucks you into them.

  • Totally Agree.

  • ridein

    Any law on the books doesn’t really mean much unless it is enforced. This is another tool to educate the public. Glad it is in the hand of a cop on a bike.

  • Pedal Er

    The points you make state the case for a one size fits all approach. 1.5m as in some other European countries would be good. All too often car drivers squeeze past cyclists.

  • Roger

    What is safe very much depends on the circumstances: absolute and relative speeds, road condition, weather, how experienced the cyclists is, curbs, size and shape of the vehicle. I’ve been passed with way less than 90 cm clearance loads of times and it was no problem st all.