The car is put on a hoist which is set up with a computerised Laser alignment machine. Laser cameras are attached to the wheels. The car is moved backwards and forwards and then the steering from side to side. This allows the computer to work out where the wheels are pointing and to provide the above image so that they can be correctly aligned.
Ideally the yellow arrows will point to the centre black mark in the middle or the green bar. As you can see from the above screen shot the tow in on this car was well out of alignment. We always try to get the alignment as precise as possible which does take a little more time. Not all workshops put this effort in and one of the big tyre discount companies just leaves it if it is anywhere in the green bar.
Wheel Alignment Laser Camera mounted on wheel