I've seen Smarts with reversing sensors, which has always amused me as the back window is about 10 inches from your head & the back of the car is practically flat!
I've got parking sensors on my LWB work van & to be honest they get ignored, there's so much much they can miss. :amuse:
Curiously, Smart's aren't always that easy to see out of the back of, thanks to the all-enveloping seat pan which is also a head rest. This means you've more or less got to lean out of the seat to look round. If you've got a stiff neck like me, you tend to use the mirrors more than you should instead, leaving you parked about four feet away from the car behind - still at least it's better than backing into them! Being quite high off the ground, the front of the car behind soon disappears below your window line too. I guess it's not the size that matters, but how easily you can see out.
Having wider track at the back compared to the front doesn't help with the distance from the kerb either. Whenever my wife asks me 'how's that for distance?', I usually reply 'that's OK I'll walk to the kerb", or "don't worry, I'll get the gangplank". I shouldn't encourage her really - last time she went closer than normal I ended up buying a new alloy.
My old Citroen C4 VTS had all-round sensors, but the front ones could be fooled by clouds of hot exhaust coming out of the backs of buses, being splattered with rain from oncoming traffic and all manner of things that aren't parking hazards, even steeper-than-usual speed ramps. I once picked it up unexpectedly early from an off-airport car-park to find it really hemmed in. Every single sensor was flashing on the 'map' of the car on the dashboard and sounding off accordingly, all at the 'very close' setting. In the end, I had to get the manual out to find out how to turn them off, as I could see me losing my rag and damaging the car.
Gadgets that shriek 'Bad driver'? What about those springy probes that make grating noises against the kerb to fend off damaging your alloys? Why not go the whole hog and have Deeley Boppers fitted all around your car at bumper level? Come to think of it...........a cheap alternative to parking sensors!