Flex time can help, but I personally just end up working late anyway. I'd suggest:

1) Setting up an arrangement with a car-owning coworker that you pay him/her to use the car, say $10 "for gas" per errand or something--some amount so that they're delighted to loan it to you that is obviously more than the cost of gas.

2) Get a group of people to collectively buy a (not government owned) vehicle. Something small, used and cheap. Store at employer's location, assuming they're OK with it (no skin off their back to park an extra car generally). You could have a employee club of sorts that had dues going towards upkeep. There are plenty of clubs that exist at employers with their support but are not affiliated or financially supported/related to the employer. All the employer does is (usually) provide a meeting space.