[zuminator]

Do you have any support for this argument at all? It seems to overlook the reality of the situation. When one car is in a collision with another, regardless of their relative velocities, when they collide, in a serious accident they will both rapidly decelerate to a speed of zero. That's what the danger is. Their absolute speed has dangers above and beyond their relative velocities -- less control of the vehicle and greater kinetic energy wrt the environment.So, a car traveling at 25 hitting a car moving at 5 may have a 20 kph/mph differential, but the danger is minimal compared to a car traveling at 95 hitting a car at 85, where both vehicles quickly go down to zero, rolling and smashing into barriers and other vehicles along the way.

[tertius]

I'm not saying that differential is all that matters. A car doing zero and one doing 20 in an accident is much better than the 80 vs. 70 example.

What I'm saying is apples to apples.

Speed limit is 60, car doing 50 + car doing 80 is a lot more dangerous than two cars doing 60. In both an accident and in actually causing one to begin with.

I agree with most of what you're saying.

[JoeAltmaier]

Right - a head-on collision between objects of equal mass and velocity is equivalent to one hitting a brick wall (or immovable object) - it loses all its kinetic energy on the spot, turning it into heat and mechanical work (e.g. crushing the car and driver). Objects of slightly different mass or velocity means the work done on each is shifted, the heavier or faster one 'winning' a bit but still. The absolute values of the velocities are hugely important.

[tertius]

Agreed regarding absolute value, my usage of differential is not comparing different scenarios but focused on one.

So 25 mph + 40 mph in a 25 mph is dangerous.

80 mph + 55 mph in a 55 mph is dangerous.

Obviously the former has much less energy, but within itself, the differential is what creates the most accidents and danger.

[petraeus]

Prime example

Quebec woman who stopped on highway for ducks, causing fatal crash, loses appeal