Cyrus Farivar:

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.