Hi there, I'm looking for a device that can via laser (or other
method), tell me when the beam has been interrupted (say by a car) and
send a signal via USB (or other technology such as Serial or Parallel,
Wireless, etc) to be processed by computer.  Basically I'm looking for
a method by which I can track traffic on my street by detecting
whether a car tire has cut the beam for example.

I'm a capable programmer who could build a device driver to handle
such a device, however I'm not familiar enough with the basic
electronics needed to build my own.

Any leads on such a device would be well appreciated.  Thanks!