Do these use radar or...

[Abe Froman]

...is there some type of undetectable distance measuring device on the inside?

http://www.kxan.com/global/story.asp?s=6470415

[allyoops]

It uses laser, but not in the same way as a lidar gun so jammers and detectors are not effective.

They're called Poliscan Speed and it's made by Vitronic in Germany and imported into the USA by Nestor.

watch the video on the importers site - really good explanation.

http://www.nestor.com/Speed_lo.wmv

This is one of those systems that you need to use stored GPS locations to warn of the known locations where this trap is set up.

Al