I'll add a few comments to Corporate wage slave's note.
MrQ has a mild disability and is in possession of a handicapped parking placard. Things were really bad after his back surgery in 2008, but he's doing a lot better now. Lots of PT and hard work on his part.
We usually fly AA and transfer in DFW. When flying with MrQ, I particularly look for DFW connections because they have an excellent golf cart service, plus the SkyTrain for everyone else.
In DFW, I notify the gate agent upon arrival that we need a golf cart. That person will call the service and the golf cart will stop the next time they pass by. Usually there are handicapped-marked seats near the concourse in clear view of the golf cart.
If there is no gate agent, we just go to the handicapped-marked seats with the blue placard in hand.
Either way, when the golf cart approaches, MrQ sort of waves the placard as a way of catching the attention of the golf cart driver. If the cart is already full, the driver will call for another cart, if not we climb aboard. The carts can accommodate a reasonable amount of carry on luggage.
Occasionally I've had to connect at DFW without MrQ and with a personal injury of my own. (Knee surgery, sprained ankle, etc) I just flag down a golf cart and tell the driver "I have a hurt foot and need to go to gate 42. Can you take me?". I've never been refused. Maybe I just look pitiful.
That is all in DFW. LHR is a different story.
My friends Judy and Carol flew back to the US from France, spending a few days in London with friends. Carol suffered a bad knee injury just before they left London. She visited the doctor (hooray for British health service!!!) but had no handicapped designation. Apparently they did not know to call in advance and get help, besides the injury took place just a day or so before their departure. They were flying BA, connecting to AA in ORD.
Judy had a terrible time getting help for Carol in LHR. To this day she rants about how she could not get golf cart rides for Carol in LHR. Upon arrival in ORD, however, it was a different story. Carol was helped through the entire connection and they made their way to LAX.