If you ever feel like parking outside of designated spaces, this is the test you should use. Walk around your car and ask yourself, "can a person in a wheelchair go from this side of my car to the other without being too inconvenienced?"

Be particularly attentive to rough ground (cobblestone), curbs, puddles, lamp-posts or other obstacles.

If your parking passes that test, it generally also accommodates the blind, bicyclists, children, etc.

It's not hard to find spaces that pass the test -- which is also why it upsets me so when people neglect to.

But be aware of the garage door that doesn't look like a garage door. Once in SF I left my car for about 20 minutes in Haight-Ashbury on a curb that didn't seem to have any particular reason to be yellow. I returned to find that what had seemed on first inspection to be a wall was in fact a very subtle gate that slid sideways, and a few people standing around were angry with me. (When I returned to the spot weeks later, there was a new sign.)