Hi:
I have a neighbor who wants to park his car in the garage and have it stop with all four wheels on 12 sq. in. pads. He says the wheels pull the paint off of the floor if he doesn't park on the pads. I have a spare Uno and would use that as the intelligence. I need some ideas of how he might be alerted as he approaches the pads and then alerted to stop when he is on the pads. Any thoughts?
Bob
An angled mirror facing ~45 degrees down at the back of the garage.
Hi,
Sonar unit on back wall and a beeper or flashing light.
Measures the distance between wall and the back of the vehicle.
Make it so it only turns On when the garage door is up, so its automatic.
Nice kit, but uses a Micromite,
Tom... 
A small sticker near the top of the windshield and a tennis ball hanging by a string that touches that spot on the windshield when the car is in the right position. Low-tech but accurate and low-energy.
Or a piece of 2x4 on the floor, stop when the wheel hits it or rides up on it.
Get some 100 x 50cm door mats. You'd have to be drunk to miss those.
There are also laser beam gizmos that point down from the ceiling. Class 1 laser so perfectly safe, unless you get the real cheapies then you have no idea what surplus laser they used, wear sunglasses and look straight ahead just in case.
Thanks to all who answered. Neighbor is trying mirror tip today!
I'd go for the tennis ball / larger mats combo.