Another more reliable but by no means easy way of doing it is to use lasers. This would increase the range, but you'd have to be careful about directions, focusing etc.
Basically what you need to do is, reliably, transmit some form of signal out the front of your car on the bumper (ultra sonic, sonar, light, whatever) and then time how long it takes to receive the signal again. Poor mans radar basically! You'd have to do some very impressive tests and calibrations however and have alot of complex brains in your circuits!
You would need to take into account your speed, the weather conditions, road surface. You'd also have to find a way of smoothly applying the brakes! Not an easy project!!! But it is possible. The US military tried employing similar techniques back in the early 90's to make automated vehicles for the battle field. And Mercedes uses automatic cruise control on their more hi end models. You could try contacting Mrecedes HQ technical department or have a snout about on the web, see if you can find out more about it, may help.