I don't know about other States, but in Wisconsin, we get disabled parking permits that we hang on the rear-view mirror.
Whenever someone drives me in his car, I take the permit with me, so he can park in the disabled spot without worrying. If your grandmother has one of those, ask her for it when you drive her.
In Wisconsin the permit also allows me to park on a metered street without putting any money into the meter. This is very nice, because when I cannot walk well, I can just drive the car to the next place and park near it without having to insert a coin into three-four meters (where a regular person would just park at one place and walk to the rest).
Before I got the permit, I felt like a prisoner. I could not drive to Wal*Mart or the grocery store, except late in the evening when few cars were parked there. That simply because I was unable to walk the distance. Now, I can go anywhere at any time, and it is truly great!
eric+ I'm sorry to hear about the dirty looks.
A disability is not always visible. For example, people with serious asthma can get a permit simply because walking too much may trigger an asthma attack. Parking within a short distance of wherever they go prevents that from happening, so they appear perfectly healthy even if they are not.
Everyone, please, do not give dirty looks to people who you think should not be parking in disabled spots - you never know what disability that person may have.
It is, by the way, a real pain to be navigating through a grocery store in a wheelchair and people not noticing you, shoving their shopping carts out of their way, right into yours! So, if you can walk, please do not leave your shopping carts in places that can block a wheelchair.
I personally hate asking people to move their cart, so often end up waiting for several minutes before they unblock my way.