Oh go on then, I'll bite. I'm in that kind of mood.
Let's leave aside that supermarket speed limits are advisory.
What you are advocating then is that in order to avoid a queue at the traffic lights it would be MORE sensible to travel at an undefined higher speed in an area that is SPECIFICALLY going to packed with unpredictable kids, mothers and babies, old folks, and you and me.
I see. And what speed, then is the best compromise between reducing the queue at the lights on the one hand, and the level of damage done to a person by a ton and half of metal at the other? 10 mph? Save five seconds, three more weeks in casualty? 20 mph? Save 30 seconds, lose a few folk?
You may argue that when you are in the far corners of the supermarket carpark there is nobody about, and therefore you can do 30 or 40 for two hundred yards quite safely. I agree. But it isn't possible in a variable risk situation to legislate for all possible circumstances, and therefore a conservative limit is advised (or, on the streets, made into law). If that entails a certain amount of frustration in drivers then tough; it's the real world and the concept of speed limits at all is a given.
There could of course be a world with no limits, and each driver makes his own judgement to road conditions. If you were to hit someone running out from between two parked cars you could shrug your shoulders and say "in my judgement I was travelling safely, even though if I had been doing 15mph less that child would be alive now." And everyone would say, "yep, drivers always know best". That's not the kind of world I would want, I don't know about you.
I do get grumpy about this and I'm sorry, I don't mean to get personal with anyone. But in my view the ONLY reasons for speeding are for personal convenience or because it feels good or is fun. I don't deny the attraction of those things. But there are risks attached, which in the example above I think I would not be prepared to take.
I would be prepared to whizz along at 90 on a open clear sunny motorway, because I think I can *probably* manage that risk. Legally I would be in the wrong; Parliament has decreed the maximum speed/risk compromise to be found at 70mph.
If, however, something went wrong and I killed or crippled someone I'd have to live with that for ever.