Bicycles. If the roads are screwed then the roads are screwed. There's almost no place around houston even where I could travel a considerable distance without roads. I'd run into rivers or large forests or other barriers that simple wouldn't be passable in a vechicle. Bicycles, trailer for the kids and a bit of gear. I know I can comfortably do 50 miles a day with this type of setup and likely much more if under duress. If the main roads are screwed then people will start hitting the side roads, bottlenecks will be at bridges, if you can cross the bridge due to traffic then you're stuck, this seems to be the case almost everywhere I've been in north america, doesn't really matter what vechicle you drive.