If two cars are travelling in the opposite direction and both are doing 99 miles an hour, when they colloide head on, it is the same as if one car was travelling at 198 miles an hour at the moment of impact.
So it is only the effect and not the relative speed that changes.
Should have said, that at the moment of impact both cars are still travelling at 99 miles per hour, but the force of impact is the same as if one car hit a stationary object at 198 miles per hour.