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A bus trap is a metal grate placed over a ditch or depression in the road with tines spaced far enough apart that small-diameter-wheeled vehicles fall in between the tines but close enough that larger-diameter-wheeled vehicles, such as buses, may pass.
Different versions exist for restricting access. The tines cross the path of the road, not parallel with the road direction. Small-wheeled vehicles "bottom out" in between the tines, preventing the vehicle from continuing over the obstacle.
However, there are now many vehicles on sale which are wide enough to navigate bus traps effectively; many luxury cars, SUVs and performance cars with their wide bodies and thick tires can get through the bus trap without 'bottoming out'. Because of this, many cities are favouring other means of keeping cars out of bus-only areas, such as heavy fines backed up with camera enforcement, or radio-controlled gates or bollards.