Yeah, I know. You might not think so... right? But there it is. That's why a head-on between two cars going 50mph is probably a fatal for both cars' occupants. Because it's like hitting a fixed object at 100mph. And yes I can now see how the BeamNG Team does a bang up job of simulating this very thing. Fun to play it on the computer, but wouldn't want to be in one.

I believe the speeds are additive, for each of the cars. That's why I said 100. And if a car going 50mph, rear ends a car going 40mph (both traveling the same direction), then the speeds are subtractive, and it's like hitting a fixed object at 10mph. I've never heard of "car going 50 hits car going 50 head-on equals impact speed of 50". Please explain your logic. If you're correct (and I'm not saying you're not) then I've learned something new tonight that disproves everything I thought I knew about physics and impact speed, since I was a kid. Damn!

If a car going 50mph, rear ends a car going 40mph (both traveling the same direction), then it's like hitting a fixed object at 5mph. The logic is momentum transfer. When hitting a fixed object, like those in crash tests, the object is stationary before and after the crash. When two identical cars hit each other at 50mph, they becomes 0mph after the hit. In the 50mph rear ends 40mph, the speed change is 5mph and the damage is like hitting a fixed object at 5mph A possible(but unlikely IRL) exception is when the collision is elastic. In this condition after the 50mph head-on collision, the vehicles "bounce back" to -50mph, in this case, is like hitting a fixed object at 100mph

Damn! So I have had it wrong all these years. That sucks. My parents and a couple of teachers along the way lied to me. Even what I thought was the 10mph subtractive rule, was wrong. I'm going to have to read up on this a little bit more. I'm hoping the BeamNG Devs are already aware of this fact. I'm sure they are! Thanks for clearing this up. As I say, a new one on me.

I mean, the devs are obviously aware, this is like high school level physics, at least where I live. And there have been tons of tests if this works in BeamNG the same way as irl, and it obviously does, WhyBeAre made an entire video about this. He had no idea why this happens but some people in the comments knew and explained it.

Hmmm.... well still amazing to me. I'm the ripe old age of 54, and this is the first I've heard that you don't simply add or subtract the speeds of the two cars to get the "like hitting a brick wall" speed. As I said, I learn something new everyday.

The key point is to understand that a wall like the one used in the Mythbusters video is very rigid, so nearly all the energy is dissipated by the car hitting the wall. --> In this case the energy dissipated by the car is 0.5*m*v^2, m being the mass of the car and v being its speed. But when two identical cars of mass m and speed v collide in opposite direction, the total energy to dissipate is now the double than the previous case: m*v^2. However there are now two cars to dissipate the energy so the energy dissipated by one of these two cars is still 0.5*m*v^2. Consequently, hitting a very rigid wall at 100 km/h is equivalent to a frontal collision between two identical cars going at 100 km/h. However a collision between two identical cars, with one of the car at rest (0 km/h) and the other going at 100 km/h is equivalent to a collision between two cars going a 50 km/h each because the total kinetic energy of the two cars is the same and is dissipated by the two cars. The only tricky point in this case is that in real world, the car at rest will slip on its tires, which will dissipate some energy by friction (heat).