Sensors for Moving Vehicles

[Darcy Neal]

I'm trying to figure out how to sense when there is a moving vehicle around you, while the person is actively moving, whether it may be via car, motorcycle, bicycle, etc. The closest i've found has been a sonar/IR/range sensor, but i don't know how well those would perform if the source is bouncing down the road and has alot of interference. What would be some of the different ways to sense other cars? Maybe you could sense the radio signals coming off of the vehicles... Sense the headlights at night with a camera, or maybe during the day too....
There's an interesting article I read @ http://www.sparkfun.com/news/702 that talked about wearing sensors that log your information in the case of a wreck, but i'm trying to sense a vehicle riding behind me, as a means of accident prevention.
Any ideas? This would be such a helpful tool for Motorcycle nOObs like me.

[cstratton]

If they are overtaking you, they would have a positive doppler shift. Most everything else that is behind relative to you own movement would have a negative one.

If they aren't overtaking you, do you need to detect them unless/until they accelerate and start overtaking?

This would probably require getting antenna / transducer patters somewhat directional - ie, behind only, and relatively horizontal in order to not detect the nearby ground during bouncing (though are we talking bouncing as in a pogo stick or as in a bicycle?). Some filtering to find signals that are consistently at a positive shift frequency rather than oscillating between positive and negative would help too.