The rocks are in the same frame of references the passengers. The only difference is they're not seat belted in so there is room for some small differences in acceleration.

If your car is doing 50 the rocks are doing 50 and then do have a fair amount of inertia compared to the outside world, but everything in the car has that same inertia. That two-ton automobile is the actual danger.

Absolute worst case those rocks aren't seeing any more than a couple of G, let's say you max out at around 3G which would be a little bit intense for car related escapades. The rock hitting you in the backseat would be like maybe it falling a dozen feet and landing on you absolute worst case.

Now, if you didn't have a windscreen, and you hit a stone wall that stopped you immediately The rocks would have a 50 mph variance to the wall. But, honestly, 50 mph on a rock is not really significant. Baseball pitchers double that.

You could try to argue that hitting another oncoming car would give them a 100 mph variance. But at that point the rocks aren't the problem the hundred mile an hour variance between the two multi-ton cars is more than enough.

The old wise tale that putting things on your dash turn them into dangerous high speed projectiles is false.

I think if you were looking for a solid argument of them being a bad idea, they will easily roll off your dash, get under your feet and would have a much better chance of interfering with your pedals.