This representative sample of what's out there may give you some idea of what's out there at various price points:
Unfortunately, you're talking several thousand dollars for an outdoor unit with 10's to 100's of meter range (as of March 2015). The chart is from a blog article I wrote on the topic. Google used a $70-80K unit on their original vehicles. The 2nd to last item on the chart above, while not as capable as that unit, is advertised as a unit for vehicle automation.
Sophisticated stereo cameras with image processing may be an alternative, and may be cheaper, but you're still talking significant $, not just a webcam and OpenCV.