Microwaves reflect well off metal objects - this fact is put to good use in speed guns and speed traps...
Some cars are fitted with low power microwave emitters at the back to help drivers park safely. The reflected waves are picked up by sensors. If the time delay gets too short, a warning beep alerts the driver!
Some cars are now fitted with adaptive cruise control. This works using microwave emitters at the front. These judge the distance to the car in front and automatically slow the car down if you're too close. In the event of an emergency, the brakes are automatically applied!