Depends also on the curvature of the path. For example a 3-wide path with a series of sharp bends will actually have an optimal path that is more elongated than might seem practical. So if you have two sharp turns, down, left down, the shortest path actually shifts across down the middle of the pathway. It's one of those oddities that WILL cause injuries if you send a mine cart down the middle of the path. If you have to have it, build several pits. Three tile long pot holes with one floor over the top. The floor can be marked for high traffic, and dwarves should use those to cross while the mine carts just bounce underneath and pop up on the other side. Me, I just don't bother to run mine carts through busy hallways.
You might also alleviate hauling issues using minecarts as well.