[MTBikerTim]

The problem is how do you survive an acident at those sorts of speeds. There will always be accidents and the forces at those speeds even with a glancing blow to the wall are enough to rip the car to peices. Say you loose only a third of your speed in a glancing hit on the wall, that's still 100mph when you are doing 300mph. Even with perfect safety features you are still loosing 100mph in an instant.