As far as I can tell, the issue of disagreement here is the point of reference.
From the driver's perspective, when he's driving towards the brick wall at 100km/h, his speed relative to the wall is 100km/h. When he hits the wall, the force applied by the wall on the car is enough to cause the car to accelerate (negative) to 0km/h in the distance of whatever the crumple zone is.
From the driver's perspective, when he's driving towards the vehicle approaching from the opposite direction, his speed relative to the other vehicle is 200km/h, if both vehciles are doing 100km/h in directly opposite directions (in the same plane approaching each other). When the two vehicles collide, from the driver's perspective, his speed relative to the other vehicle is initally 200km/h at the beginning of the collision, and 0km/h at the end of the collision. If both vehicles are of equal weight, then the force applied by the opposing vehicle is sufficient to stop the driver's vehicle in the same distance as when he hit the wall.
So, relative to the wall, the driver was doing 100km/h and stopped in the distance of his cars crumple zone. Relative to the other vehicle, the driver was doing 200km/h and still stopped in the distance of his cars crumple zone. I would much rather hit the wall if I had to choose.
Most people are looking at this from the perspective of an observer standing on the side of the road watching a car hit the wall or two cars collide. That's fine, but in the scenario of two cars colliding head on, the forces viewed from the perspective of the observer are not the forces acting on the driver because the observer see's the first car with negative acceleration forces from 100km/h to 0km/h and the second car with negative acceleration from 100km/h to 0km/h as effectively two separate events, where as to the driver of either vehicle, the collision is very definitely a single event.
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