After some experimenting Mike Chambers has made a speed detector using an Arduino board and custom software written for Adobe Air. The hardware uses two laser pointers aimed at matching photoresistors. The Arduino then measures the time between the first and second beam being broken by a moving object, using this data to calculate the speed of the object. You can see this in action:
Kudos to Mike for releasing all the design information as well as the code for both the Arduino and Air on his github page. You too can build the 'Accelerate' project with one of our Arduino-compatible boards and associated hardware. For more information on the project visit Mike's website.