This is one of those ideas that seems obvious once you see it: build a long plastic awning covering each double row of parking spaces in a shopping mall parking lot, so when people come back to their cars after shopping the interiors of their cars aren't deathly hot.
I've now seen this in parking lots as far apart as Brazil, Thailand, and South Africa. As I was driving a car in a couple of these places, I really appreciated having shade, and it might even have made me stay longer.
I can understand why places which have to plow in the winter don't do this; it is an impediment to the plows. On the other hand, I've never seen this in California, below the snow line. Why not?
And awnings could probably be designed which would allow plows in places like Texas or Georgia, where there isn't that much snow, to move ice or spread salt.
(pmat mentioned having her car get so hot in the lot at Citizens Bank Park that the glue holding the rear view mirror got unstuck, which reminded me of this.)