Even when the land is perfectly flat with the bridge I will sometimes has a problem with cars hitting the land when driving off the bridge. The train does not have this problem.
I don't know if this is what you are talking about, but I've seen when an anchor is not right on the edge of a drop and the bridge sags after the simulation starts, that the edge of the drop ends up poking through the deck. When a vehicle runs across it, it bumps into this land. I normally build a little ramp over this section. (It hugely increases stress but gets you past the level.)
Which level are you having this problem on?
Josiah