They fall with the hull still green. Perfectly normal. They still need to dry and cure some before you eat them. A few will fall off prematurely and will not be fully developed, but most should be fine. The hull comes off easier when it is still green, but those green hulls really stain. The dried black ones will stain too if they are wet.
A car running over them will smash the hulls but it will not come close to cracking the hard shell. It is a good way to get the hull off and get them to dry out faster.
Black walnuts contain a substance called juglone. The way I understand it, it is more concentrated in the roots but the leaves, bark, and hulls all contain it too. Juglone prevents certain plants from growing, tomatoes for example. I've read different things on how fast juglone breaks down. I've read where you can compost the leaves and the juglone will be broken down, but I've also read to not compost any product of the black walnut tree at all. I don't know which is true, but when I get black walnuts I do not add the hulls or the shells to my compost heap. And I dispose of them where they cannot leach into my garden or orchard.
Instead of using a hammer to crack them, I use a vise. I don't smash the nutmeat or my fingers nearly as much that way.