Ian,
The Trafficam / Thermicam product from Flir may fit the bill and would be more reliable than the AIM-S. The cameras can do counting or operate as a standard loop or queue loop. In addition you can do some basic conditioning in the camera.

We are using them for SCOOT detection and replacement for VA loops. You need only 2 cores between the camera and the Processor card, this can be connected directly to your IP comms.

James