I am a case in point...
I have probably the worst knee west of the Mississippi. My first total knee replacement went south when they put in a Cobalt Chromium knee that I was deathly allergic to. They cut my leg off the second time and put in a ceramic knee that, while it functions, it doesn't work well.
For me to even get out of my car I have to be able to swing the door all the way open. This enables me to swing my right leg out. Very few conventional parking spaces provide enough space to do so. Once I am out I can walk without an apparent limp. I also have serious nerve damage in my right foot and every step is painful.
I'm not looking for sympathy, you can get used to just about anything, but the point is at first glance I do not appear to have a physical handicap. If there are spaces that provide enough clearance I will use them instead of handicapped place but if I come back to my car and someone has parked close I now have a problem just getting back into my car.
Believe me, I would be more than happy to trade my handicapped plates for a fully functioning knee and the elimination of the nerve pain.
:C