Removing the bridge walls is removing the obstacle completely. You basically just have open terrain. There is no obstacle to navigate in this case.
My testing has highlighted the fact that the "bridge"isn't the problem. (recent beta patches for earlier CC's has dealt with the "Bridge"element as problematic)
It's the positioning of the string of two parallel impassable elements.
In theory the vehicle should pass between them... in practice it will not... from close range , from a distance, with waypoints or without.
The vehicles size shouldn't be an isuue either (but it maybe if this calculation is incorrect)