How much do collisions slow an object?

[Culture20]

The moving object could be a car, a rock, or in my case, a superhuman with 300 HP, 100 DR, and multiple levels of IT:DR flying at mach 2 with no morals.
When this creature decides to fly through the buildings in a city block, or say, a mountain, how far do i reduce the distance traveled? Is it just a matter of "you do 2250d collision damage this second, you're at a dead stop when you cease to obliterate things in your path, or does the first hit reduce the velocity for the next, etc. until velocity is zero (first hit is 2250d, next maybe 2249d, etc)?