If you are talking about car parks as parking garages, then those 2-4 story garages probably have some decent retrofit possibilities. In addition to the possible conversion into complete building structures, they could also see use as urban food production and waste facilities.
Imagine a 4 story concrete garage:
That was easy.
Now think of the top level. Flat and open to the elements. Now imagine it full of raised bed planters, and cultivated in a variety of different crops. Now were not talking about plants that put down deep roots or anything, but a variety of seasonal and climate-appropriate crops will do.
Now look at the second level from the top. Imagine glass on the sides, enclosing the structure. On the east west and south sides these windows would be augmented by these,
puctured through the concrete at strategic locations. With windows and lightpipes cultivation should also be quite feasable in a good portion of this floor as well, plus its enclosed status will give it some additional use as a seasonal extender by permitting earlier starts to seedlings and later seasons beyond the first frost. The shadier north portion could be used for storage, compost generation or small livestock pens.
Materials for glass enclosure, pens and raised beds could easily be acquired locally, salvaged from other uses at low costs.
On the floor up from the bottom, the balance of livestock production would occur as well as food processing (butchery, canning, etc), vermiculture and mushroom cultivation. Waste cultivation material would be composted here too.
The bottom level would house retail (related to food production or not), food waste recovery (think received shipments of left overs from individuals and restaurants as well as spoiled unsold products from stores and possibly a small waste vegetable/animal oil recovery center for biodiesel production.
That's a start anyway. Most of those uses require little in physical infrastructure investments. While use of a carpark is certainly an odd idea, there isn't anything that stick out as physically precluding its use. It would not be a true farm of course, as grain and feed production would occur elsewhere (you need larger expanses for basic staple growth as well as chicken/rabbit feed). But it would augment the local food supply, cut down on food miles drastically and introduce some circular flow of nutrients. When partnered with a nearby farm, this structure could possible generate excess compost that could be returned to the field on the truck's return trip from hauling grain, powered by biodiesel generated right there.
Am I under the illusion that this will solve the food problem/. No.
Like I said before though, every bit will help. Including using an unneeded garage