Handicapped drivers -- HOW does their disability impact how heavy they can press their foot on the gas pedal?!?

I don't understand this. Not ONCE in my driving life have I been behind a swift-moving car with a handicapped logo on the license plate. And yet, WHY? If they have the limbs (this being arms and feet) to be safely issued a license, what's the fucking problem? HOW does their medical issue affect their foot pressing harder on the gas pedal? I see no answer for this. The only possible thing I can think of is that they're already so scarred by whatever handicap they have, they desperately fear adding to their misery by being in a car crash. Which, of course, is logic that won't help them on the day that I snap and careen my car into theirs