With the Iron skull on, you should avoid letting marines drive at all costs. However, even then you can still end up having a highway mishap. In this case, a close call with a banshee bolt propelled me too close to the edge to recover. As this was just before the Scarab was due to appear, it meant going back to restart the level after getting more than two thirds of the way through.

A multitude of distant Covenant cruisers, as well as a closer group of Banshees and Phantoms, patrol the area around the portal device where Truth has parked his Forerunner Dreadnought.

A Grunt checks his instrument panel as the Chief prepares to plant a plasma sticky on the front of his ride. This was during an Iron skull run, and rather than try to board the Ghost, or use a plasma pistol to disable the vehicle then kill the pilot, I went for the safer-- and more satisfyingly destructive-- approach.