  The reliable detection of speed of moving vehicles is considered key to
traffic law enforcement in most countries, and is seen by many as an important
tool to reduce the number of traffic accidents and fatalities. Many automatic
systems and different methods are employed in different countries, but as a
rule they tend to be expensive and/or labor intensive, often employing outdated
technology due to the long development time. Here we describe a speed detection
system that relies on simple everyday equipment - a laptop and a consumer web
camera. Our method is based on tracking the license plates of cars, which gives
the relative movement of the cars in the image. This image displacement is
translated to actual motion by using the method of projection to a reference
plane, where the reference plane is the road itself. However, since license
plates do not touch the road, we must compensate for the entailed distortion in
speed measurement. We show how to compute the compensation factor using
knowledge of the license plate standard dimensions. Consequently our system
computes the true speed of moving vehicles fast and accurately. We show
promising results on videos obtained in a number of scenes and with different
car models.
