Revealing mistake: Just before the truck slides off the cliff edge, you can see that the truck's cab door is open where the stunt driver jumped out.
Revealing mistake: At the reptile farm, the phone booth Mann uses to try and call for help has no phone lines attached to it, above or below ground (you can see that there are no underground wires when the truck mows the booth down). It's definitely not a wireless/radio telephone, either. There should be lines running to it, but there are none. (00:57:00)
Chosen answer: The movie opens with him in downtown Los Angeles, California, so he's probably from there or somewhere in Southern California. He's heading to Bakersfield, California, which is north, about 2 hours away. In the short story, he's heading to a client in San Francisco. However, in the film he gets off the main freeway running from LA to Bakersfield (IH-5) and takes Highway 14 towards Canyon Country.
Bishop73