Near the beginning, Joanna skids to a quick stop onto the shoulder to investigate the odd music. The shoulder is a full shoulder, completely paved and her truck fits on it completely. She gets out of the truck and walks backward and is always on a solid surface. However, when she awakens, the shoulder is noticeably narrower. Only the drivers side tires are actually on pavement. The rest is just dirt.
When Joanna's truck goes off the road near the end of the film, the engine revs as if the drive wheels have left the road, but her pickup is a rear-wheel drive vehicle and the rear wheels were still on the bank.