[logfromblammo]

That is highly application dependent.

If you can set up clear distance markers 100m apart on the road itself, a regular surveillance camera set up with its line-of-sight perpendicular to the road could probably estimate speeds between 5 and 80 mph with at least 1 mph precision, and adding some image processing software could probably give you 0.1 mph precision or better.

Oblique views, curved roads, unclear markers, or even differently colored cars could produce false results, or no results at all.

It may be enough to convince your local cop shop to set up a radar trailer on your street for a few days, but you're not going to be convicting anyone of reckless driving with it.