Such has been around for years. Also used by traffic engineers to measure traffic flow.

Like Belmont says, these have been around for years. Ever wonder what those boxes along the highway in the middle of nowhere are doing?

And if you take time to read the signs along the road you will already know what radio frequency to tune in for up to the minute weather & traffic reports.

Many companies are reading traffic speed these days simply by listening to moving cell-phone signals anonymously in those areas and tracking the speeds.

I guess even Google maps can plot traffic speeds from this data already in cities where it matters. Just push the button that says 'TRAFFIC' at the top.

Andypels, you can't reliably do front-tire-then-back-tire because you need to know the exact wheel-base of the car to get the speed--an extended pick-up vs. a Geo could change your reading more than 50 percent. You use two sensors at a set-distance and they both sense the front wheels (first hit).

Hyenuf, most of the systems with a 'box' by the road are reading an actual switch closing in the 'wires' that cross the lane. These are wear-prone and easily damaged. When my company reads speeds of vehicles that are about to get a red-light violation photo, we use inductance-loops buried in the pavement. If you're going too fast to stop, we know it before you're in the intersection.