[mmbleh]

The original comment was hasty, I admit. But there are a lot of ways to tackle these issues.

Radar clutter, attenuation, penetration, etc are all affected by wavelengths. By using multiple frequencies, you can get a better idea of what is and isn't really there. As far as interference, there are ways to filter out noise. The car presumably knows how fast it is going. Therefor, it can do doppler filtering on the received waves. IE... car knows it sent waves at 50 GHz and is traveling at 60 mph. It knows to expect a response at ~54 GHz from the front and 50 GHz from the side.

For tracking objects, you can use a pulse-doppler radar [0] to get both range and rate information.

[0] https://en.wikipedia.org/wiki/Pulse-Doppler_radar