The systems I heard of used radar, though laser has certainly been experimented with by now. Actually machine vision is the hot item and all the autonomous vehicles are trying to use it as the most critical sensor. There was recently a challenge to design an autonomous vehicle to do a significant cross-country drive by itself. I don't think any vehicles finished.
Distance is not very useful. Distance and velocity are. For a system to be useful, really distance, velocity, and direction and needed.
For example, if you're going forward and somebody changes lanes to get 20 ft in front of you, you don't want to hit the brakes like you would if a building was 20 ft in front of you. So you need to know how fast it's closing.
And you may be travelling down a nondivided road, someone on a side road pulls in front of you to get on the lanes going the other direction in a smooth motion. This is alarming because a system may detect that there is suddenly an obstacle in front of you, and you're approaching it at the speedometer speed. But in fact it is not standing still, it has horizontal motion and will be out of the way by the time you cross paths.
The geometry is also important. Going through a turn, you may register telephone poles on the side approaching rapidly. Since you're turning this shouldn't be a situation that requires braking. In fact you may be approaching a turn and the steering wheel is straight while the system registers the pole/tree approaching at a speed that would require braking if you don't turn. It doesn't understand your intent to turn.