I think its a catch 22; you want to safely stop a run away car, but don't want to destroy a car thats only a few feet in...I say, build the back of them up and slope down back towards the track and level out maybe 50-100' from the end of the asphalt (not sure of the grade). This way, the casual visitor just needs to be dusted off, but the runaway will dig in and stop. The driver will probably hurt for a while afterwards, but compared to the alternative.
It was a shame what happened to Annie. I'm glad she was able to slide it to a stop