on the big dig and witnessed them putting in double loop connecters. a single loop will tell you how many cars travel in that lane in a given day. a double loop will tell you how many and how fast. a cop just sits down the stretch with a pair of binoculars, no laser, no radar and has a 99.999% conviction rate in court. you can spot these by the two overlapping driveway crack sealer look when you're driving down the highway.