I've actually used these IR ranging sensors before. They have circuitry built-in, so the interface is really simple (the drawbacks are that the output is non-linear with respect to distance, and the time between outputs is kind of slow ~50ms): http://www.acroname.com/robotics/info/articles/sharp/sharp.h...
He uses a lot of LEDs. If I were to design something like this, I'd just use a speaker from a PC case and have it beep different tones depending on the distance to the object.
Also sonar is a lot better choice for this application.
This title is very misleading--it's not actually radar at all. The described system uses a very short range and simplistic IR transceiver.
$30 can buy a cheap bluetooth webcam that could do the same job (given that you have a ... laptop on your car), but cool nonetheless.
How would that give you the range to an arbitrary object - like a white wall?
good point but in how many real world situations is this going to be an issue? for situations like measuring distance to a car while parking, a camera works well.