He wasn't used to it yet. Case in point: holding his lane. There hadn't been any other cars on the highway for weeks. Not even abandoned ones. Why was he holding his lane? There were rarely zombies out on the blacktop... nothing to eat, and apart from the on ramps it's fenced off. He just went around them where he saw them. When he needed gas he got off the highway and found a parked car. Quick look around to make sure there was time and then siphon away. He'd been doing it for years, long before there were zombies.