Ack, I think I explained badly. The BRIDGE square is impassable. Like, there's sort of a flashing black square (on the z-level above) where you'd intuitively expect to see just bridge. And you cannot walk on that square on the upper z-level, where you'd expect to be able to walk since there's a bridge there. You CAN still walk on the ramp on the z-level below, although you can't use it to move up if the bridge is raised (which is correct, I imagine).
Think of it like this: If you have a bridge over a channel, you can walk on the bridge over the channel. If you have a bridge over ground, you can walk on the bridge over the ground. If you have a bridge over a downward ramp, you CANNOT walk on the bridge over the ramp; when the bridge is lowered, it's like there's a wall on that square and some funny graphical effect. What I'd expect to see is no special graphics (the bridge is down, you can't see what's under it) and be able to walk over it like a bridge over a channel or over ground.
So in the picture above, when I built a bridge on the top z-level, I still couldn't move between the left and right sides of the bridge. When I had miners remove the upward ramps on z-1 (without even touching the bridge), then I could travel over the bridge normally. This was all tested in adventurer mode to make sure it wasn't just weird pathing stuff; the bridge squares with ramps below them simply couldn't be stepped on.
I hope that makes the problem more clear.
[ June 04, 2008: Message edited by: AlmostEverywhere ]