Florida handed out around 500 tickets last year to drivers who veered too close to cyclists when passing. But only eight drivers were actually found guilty.

Like 26 other states, Florida has a law requiring cars to give bikes at least a three-foot gap of space. The problem is that it’s fairly impossible for a cop to judge exactly how far away a driver is and issue a ticket (or win a case if they do). So a new gadget uses sonar to measure.

With the device attached to a handlebar, a bike cop gets a ping if a driver gets inside the three-foot zone. One of the inventors compares it to a radar gun that an officer would use to see if someone’s speeding. “I think it’s a fine enough line that you really do need some technological proof,” says Christopher Stanton, co-founder of Codaxus, the Austin-based engineering firm that created C3FT (the name spells out “see three feet”).

The engineers first started working on the project after realizing that officers in Austin were struggling to enforce the three-foot law in Texas. They also wanted to collect statistics on how many drivers were obeying the new law.

“If you have a thousand cars drive by you a yard away, and one car drive by at two feet, the only thing you go home and talk about is that one person who nudged you,” Stanton says. “So we wanted to have that actual statistic about the real driver behavior profile.”

They built a working prototype. Then, when an Austin police officer named happened to move to Chattanooga, Tennessee, to become chief of police, they had their first customer. The head of the local bike patrol, Robert Simmons, had been looking for something like this for years.

“Officer Simmons is just a guy with a bunch of drive and ambition,” says Mark Przybysz of Friends of Outdoor Chattanooga, a nonprofit that does bike advocacy work in the city. “He had a friend get hit back in ’09 and since then wondered what he could do. He called multiple police departments around the country and everyone said it was impossible, but he kept digging.” When he finally learned of the new gadget, the Chattanooga police department didn’t have the budget for a prototype, but Friends of Outdoor Chattanooga was able to sponsor it.