For the last month now, Chattanooga Police Officer Rob Simmons has been biking around town with a new, unique ultrasonic sensor to make sure that drivers are obeying the state’s three-foot buffer law.

The law, which took effect in 2007, requires that when motorists pass a bike, they’re supposed to allow that much distance. But enforcing this law using only human eyes is pretty much pointless—it would be like trying to enforce speed laws without a radar gun. Tennessee is one of 24 states nationwide to enact such a law, but no other states have enforced the law using a device like this.

Simmons rides with the brand-new Bicyclist and Safe Monitoring Applied Radar Technology (BSMART), a handlebar-mounted device that uses an ultrasonic sensor to calculate the distance between the bike and a passing car. Then it displays how far away the nearest edge of the car was to the bike in inches.

"With the device, it’s highly accurate, to within an inch," Simmons told Ars. "No matter how fast [the car] is going by, it even catches the rear view mirror."

Simmons started Chattanooga’s Safe Bike Initiative earlier this year, which not only includes more in-person interactions, but also billboards around town telling drivers about the three-foot rule.

The 13-year veteran on the Chattanooga Police Department added that for now, he hasn’t actually issued any fines of the 17 people he's stopped. Rather, Simmons pulls people over in an attempt to teach them about the law. Sometimes, in heavy traffic, he can pull over a car even on his bike. But other times he requires a nearby fellow officer driving a patrol car.

"Our main goal is education, education, education," he said. "I’m not there to cite people. I speak to them and ask them: ‘Do you know about the distance?’ They know that there’s some law, but they don’t know the distance. I had some donations come in and I printed out some pocket-sized cycling-related law books. I usually pass them out and give them a verbal warning. Their attitude will determine whether I issue a citation. If they take it and just throw it on the seat, then I know I wasn’t effective. Then in that case I would issue a citation."

The citation for violating the city law is $120, including court fees, while a violation of the state law is over $200. But Simmons added that he’s worked with local judges to order a bike safety class rather than have people pay the fine, for now.

Enough space

The custom-made device, which sells for $1,400, was built by Codaxus, a two-man startup based in Austin, Texas. In the case of Chattanooga, a group called Friends of Outdoor Chattanooga footed the bill, who then donated it to the police department.

It’s a three-part setup, with a GoPro camera to capture the car (the camera has to be provided by the agency), the ultrasonic rangefinder aimed at ongoing traffic, and the centrally-mounted computer, which displays the distance in a simple two-digit format.

Christopher Stanton, one of Codaxus’ co-founders, who is also on the Austin Bicycling Advisory Council, told Ars that in his hometown, police simply have to learn what three feet looks like and then are able to issue tickets that way. But, the human eye isn’t perfect.

The device "provides a concrete way to have a verifiable result," he said, noting that there is only one complete device in existence right now, with another one in the process of being built.

Stanton got connected with Chattanooga, as he is friends with Police Chief Fred Fletcher. The Texan has given demonstrations to other agencies, but no others have deployed it in the field as of yet.

"[We should all] be a little calmer, and give a little extra space, so we can all get to where we’re going," Stanton said.

Other bike advocacy organizations nationwide would likely be interested, including the Oakland-based Bike East Bay, which represents the East Bay region, just across from San Francisco, California. (Full disclosure: this author is a dues-paying member of this organization.)

"The BSMART device sounds like a great product though, if it gives the police the actionable data they need to write some tickets for unsafe passing," Robert Prinz, the organization’s education director, told Ars.

"We fought hard with our partners at the state group CalBike to get the California three-feet bicyclist passing law passed last year, but it's an additional struggle now to educate people about the law, as well as encourage police to enforce it," Prinz continued. "This year Oakland Police Department has been training a lot of new bike patrol officers, which we fully support. On top of better mobility and community policing opportunities, it also gives them a greater appreciation for the challenges people face biking around the East Bay every day, as well as a greater understanding of the vehicle code as it applies to how people on bikes and in cars operate around one another."