Not so long ago we (Vicomtech) did a significant effort to complete one of our best works in computer vision. The result is a system for traffic surveillance which uses a single camera, a single PC (CPU + GPU) that is able to count and classify vehicles at high speed.
You can find a summary video here:
As a result we published a number of papers, one of them in the prestigious IEEE Transactions on Intelligent Transportation Systems, and another one in the EURASIP Journal on Advances in Signal Processing.
Some of the key points of the work were the use of a multicue background substraction method that we implemented in real-time thanks to the use of CUDA, and a particle filter for the accurate estimation of the volume of vehicles.
Hope you like it!