I thought Glenn did a real good job of stocking up before he left the prison. I thought that scene did a good job of showing how effective they are at scavenging and finding ways to survive.
Every damn last one of them (except the bus fodder )fled on foot and completely bypassed the six or seven running cars the governor's people drove to the freaking front gates.
Like posted above "it's like no one watches while watching"

At the time most escaped, the tank & people were still firing at the prison. Also, the zombie herd was streaming in from the broken fence area.
Tyreese and Maggie's groups maybe went out e other way, but Michonne was right there carving up the governor, and the tank was done when Rick/Carl and Daryl/Beth left (as several shots of the bad special effect smoke bomb behind the tank tell us). And we know Glen for sure went out the front.Since Daryl and Beth ended up on the tracks, maybe they went out via a alternate route (because apparently it's good to put prisons in places with lots of approaches and exit roads), but then you have to figure Rick and Michonne took the same route since they found each other.
So at a minimum, Rick, Glen, and Michonne (two thirds of whom are battered or weak from illness) all bypassed working vehicles during their escapes.
At least Tyreese asked Carol about getting her car. Here reasoning of why not to go back for it made sense, but doesn't explain why she left it in the first place.