it doesn't work like that
the closing speed would be 220 mph (150 + 70) (not 230 lol). if both vehicles weigh the same, the kinetic energy within the faster car will make the slower car [on impact] decelerate and start traveling backwards.
if 2 vehicles weighing the same hit head on at 70 mph each, the closing speed will be 140 mph, but on impact they will both decelerate at the same rate, it would be like hitting a wall at 70.
but, if a car was traveling at 50 mph and hit a 40 tonne lorry coming the other way at 50 mph, it would be a closing speed of 100mph, and it would be like the car hitting a wall at nearly 100 mph, as the lorry would stop the car and shove it backwards instantly
lol, it's surprising how much I ramble on when I'm drunk.......