I'm guessing Radar might be the best to use,
The problem you're likely to find here is that most of the radar modules you can afford will be Doppler units, which can measure speed but not range.
Okay, I was looking up some info on Doppler units. This seems to work similar to sonar where it sends out a wave and one is returned but then measures the Doppler effect of the wave instead of just the wave response time, therefor measures velocity instead of distance. All of the Doppler units I've looked at have all been stationary. I'm assuming this may be because of the same wind sheer effect that sonar gets? Assuming it doesn't, at speed in theory, if I was closing on a vehicle in front of me I would be getting a negative velocity reading from the Doppler, so to match speeds I would have to write the program to 0. And maybe set the distance by output power/amplification? Even if it means having a static distance.