Lets be honest, the difference between 700mph and 80mph head across the median on is negligible. Either way you are almost certainly dead or crippled. Past a certain speed the only real difference is how large of an area your body is spread out over; dead is dead.
of course, once it hits you it doesn't make much difference. However, the faster and/or more massive an object is, the bigger is the radius of damage that it can do, hence the more likely is for you to be affected by it.
For example, a sufficiently fast and massive object can derail and hit a large building killing many, while a 80Mph SUV hitting a building wall will still, most likely, not be able to bring down the whole building.
