Is it true that in a head-on collision the actual force of impact includes both cars’ speeds combined?

Yes. Two cars colliding head-on, each moving 40 mph, is equal to the force of one car hitting a stationary object at 80 mph. However, some accounting must be made for the fact that the combined collapsibility of both cars makes the collision generally less severe than would be a single car hitting a rigid, unmoving concrete barricade. This fact makes head-on collisions the second worst type of crashes on the road.