Defend your vehicle is a good idea imo, because it provides the illusion of an open world for players to explore when in reality players won't want to take much risk and the GM saying 'are you sure? You don't think your car can run for more than 100 miles before your out of gas' and also the fact that stocking up on too much gas creates a serious fire hazard. Also, the safest area from zombies is always the open wilderness, right? So, there'll be nomadic civilizations. If you want to stage it right when the apocalypse is happening (rather than post apocalypse), then there's the possibility for even more hostile humans (especially the military), and the players can just coincidentally be on a road trip. Or something more humorous. There's no way that any group of players should be able to survive an extended stay in a city (the zombies will knock down any barricades, good forts are likely well defended by evil humans, there's probably zombies already inside unclaimed forts, etc.).
Breaking into a decent fort that can be defended forever (at least until zombies evolve the ability to digest metal and/or spit acid), could be a misadventure - eventually, the players will die when they run out of food. There's no way to sneak out of a city without a car, and if you get a car, you'll probably die when the zombies hear and swarm you (and if they fail to do so, they'll swarm you at the next intersection).
So yeah, if the GM wants to pit the players in a certain direction (and they're in the wilderness, where it's technically possible to travel anywhere; especially if you've given them a map), have them purchase reports from rangers (convenient npcs that scout out certain areas and then sell what they see in the form of reports which could be videos, audio or just written word if possible... the 'rangers' are the most trusted company of these npcs - yeah, it's straight from fall out 3). The reports will simply say that the best chance of getting gas is in 'X region'. If the GM doesn't want the players to have much gas, they can just say that a lot of it might have been stolen since the report was scribed (scribes are rangers that compose reports). If the GM wants to deny players some information, make certain reports more expensive than others so that players can't afford X right away, or that X report is new information waiting to test the market, or that it hasn't even been scribed yet until a certain point in the campaign.
If the GM wants to stop the players from stealing reports - they could make the rangers tricky (a stolen audio tape that demonstrates itself is actually a blank copy). Also, rangers are often heavily armed and alert. Players aren't the hot shot heros - they're just trying to survive and (if they're good aligned or good-ish) help npcs that are weaker. If they're evil, players will obviously make more enemies (some groups note every person, and some ranger reports contain social contact and personality/biographical info; note that all reports are pretty much biased in some way since each is written by a different scribe - only the most valuable reports are edited by other scribes). A reputation system helps too. Also, the world is virtually unknown, so if the players are stronger than every npc they know, they'll eventually find an npc that's stronger than them and that's never heard of the players.
There's also accessibility - one ranger group or camp will have different reports to sell than another group or camp. Every now and then, players might reach a caravan, which sells reports in addition to other stuff. Reports aren't always correct though - players need to gather information in order to confirm reliability (or check it out themselves). Some reports aren't adventures at all - just copies of lost written word/audio/video that rangers have discovered or the equivalent of newspapers. If players feel like they need to revisit a ranger camp or re-find a ranger group, the GM can always kill off that camp/group or pretend that they've moved without a trace and skillfully concealed their tracks (as all rangers are capable of doing); in the name of preventing redundant sessions of play - reports about the rangers are generally free too (made for delivery to other rangers; rangers don't really share information otherwise, because it devalues their own reports), so players will most likely hear bad news about the ranger group or camp that they were going to re-visit before they go to the trouble of traveling too far.
Another fun possibility is the players making their own maps. The GM simply describes stuff. The GM can also have maps available, but he won't show them to the players until they purchase them from a carto-ranger.