Earlier quoted context omitted.
In my view the wreck isn’t made worse so much by absolute magnitude of one momentum vector as by the magnitude of the difference in momentum vectors.
You have to consider that after a wreck, both cars have to come to a stop one way or the other. Imagine being rear ended while you are stopped at a stop sign. The difference in momentum is small and there is no significant damage. Now imagine going 80mph down the highway and someone going 81mph hits the back of your car. Still a very small difference in momentum, but your car is very likely to lose control and roll o…
Conveniently, your delta-v with this massive body you don't want to run into is also the one conventionally reported for your speed.