I don't know if these cars can be made safe or not. I'm sure Dawg and a few others here seem to think they would be safe. That's fine. They can be the crash dummies BTW, your dads air tools work somewhere around 120psi. These cars work with 4500psi. That's more than 37 times more power. Or 37 times more presure.
My thoughts are that compressing air is not efficient, and the efficiency goes down as the pressure goes up--at least with current technologies. Anyone who has run a compressor has seen this, as it will build to 80 pounds really quickly, but then 80 to 100 slows down, and 100 to 120 takes longer than going from 0 to 100. However, I would not be concerned with pressures being dangerous. The tanks will obviously be designed to take the pressure with some sort of relief valve in place if pressure gets too high. Think of the pressures in a normal gasoline engine--way up there as well. In fact, the same type of argument you're making was made when cars transformed from steam technology to internal combustion technology. People were afraid of gas engines and the companies making steam cars called them "explosion cars" Hmm, why does that sound familiar? ;-)