I've never parked in a disabled bay, but I do feel there are far too many of them provided. Especially at supermarkets, I never seem them more than 20% full.
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You may be going at the wrong times, or be very lucky in your area, our local Morrisons has about 12-15 and they're almost always full, our local Tesco's has about 30 and they're often full.
Mind you those are in car parks that are about 300 and 600+ spaces respectively, and thus the disabled spots are roughly in line with the percentage of the population that would likely be eligible for them.
What thoroughly annoys me, to the point of my wanting to get out of the car and remonstrate with somone, is the fact that in our town centre there are about 7 disabled spots (spread out nicely), but two of them are near a taxi rank.
The taxi drivers have taken to parking their people carriers behind the disabled spots thus blocking them, and I've seen people sat in their car waiting for 5+ minutes before the taxi driver notices that there is in fact someone in the car they've blocked in.
I would love to know what the licensing authority would make of it (I suspect it's against the rules completely, but probably never enforced).
I actually miss the days when the local parking was enforced by the police, and the local traffic wardens (one of whom run the local Beavers) patrolled the centre every day, as the parking bays and loading bays were respected.
Now it's like spotting a tiger in paris when the traffic enforcement are around, it's so rare you can mark it on your calender, and the panic that ensures is pretty good fun to watch (on one occasion I saw someone run past and almost dive into his car, it confused me until I realised he'd parked in the lading bay, and the traffic warden was slowly walking towards it making notes of almost every car).