Thanks for the feedback!
AutoDrive naturally supports gates and any vehicle can pass through them seamlessly (gates open automatically as the vehicle approaches).
However, pathfinding calculates movement using an expanded collision safety buffer around the vehicle to guarantee it won't clip into obstacles or slide into water. If a gate opening or passage next to water/cliffs is too narrow for this safety margin, the pathfinder will consider it impassable. Expanding the chokepoint or placing landfill (as you discovered) gives the vehicle enough clearance to safely plot a path.
