you know the formula f=ma or force = mass * acceleration ?
well say there was a car that weighed 2000kg and was accelerating at 5ms-2 or 18kmh-2
after six seconds it would be going 30ms or 108kmh
or after two seconds it would be going 10ms or 36kmh
you'd expect the car going at 108kmh would clonk you harder then the one going 36kmh wouldn't you? but their acceleration is still the same at 5ms-2 !
so the force according to the formula would be the same at 10000 newtons for both speeds even the one is obviously more than the other!
The force you are looking at is the force acting on the car to cause the accelration....this is NOT the force of impact when the car hits you (which would more closely be the Kinetic energy of the vehicle K.E. = 1/2 mv^2)