Okay, so let's see if I'm understanding this correctly.
1.) Drivers tend to park under vegetation if they're given the chance. Shade is good for both keeping the temperature of the car down and for the finish. Bird crap sucks but most folks think that it's worth the risk.
2.) Parking lots are notorious sources of stormwater runoff. Really toxic runoff, at that. They also concentrate pollution by putting all those starting, stopping, and idling vehicles in one place.
3.) All that bare space and all those vehicles make parking lots, especially big ones, prime offenders in the heat island effect.
4.) A lot of species, most notably small animals like squirrels and some birds, will cross a small gap just fine. Put ten feet of asphalt between two vegetated areas and everything from racoons to frogs will make the leap. But put a large parking lot in place, a vast, unbroken vista of bare space several hundred feet across, and many species won't cross. General rule - the larger the unbroken area without living things, the more you isolate the species on both sides. Possibly with an impact reaching for miles, especially at breeding times.
5.) Vegetation acts as a windbreak.
6.) People just plain old like vegetation. It's purty and makes people feel good.
So, all of these would tend to take us somewhere that's pretty much a no-brainer. It takes no great insight to conclude that planted areas are healthier than bare cement, asphalt, or even pavers. The interesting question becomes, how can we add vegetation without significantly reducing available space for vehicles and at a low cost?
Seems to me that one good answer would be to mount raised vertical trellises between parking spots, trellises that start about eight feet above the ground (so as to not impede visibility) and, possibly, with some arched over occasional spots, creating a "skin" of vegetation that is low cost to install and low-maintenence once it's up and running.
A few decisions would be key to pulling this off effectively. Most of these will be familiar to folks who know about projects that I've done and related deocuments I've created.
Instead of starting out with transplanting relatively mature plants, accept the uncomfortable approach of using multiple small starts and allowing them a few years to establish. To buy time in terms of preventing a bare appearance in early years and display the utility of the system, also place quick-growing, low maintenence annuals like morning glories in useful spots. And then (I suspect that a lot of you will hate this) leave the dead, brown remnants of the ivy behind at the end of every growing season to help build the mass of vegetation and speed the creation of a suspended ecosystem up there.
Plants are started in small (about 5 inches across) containers, possibly blow-molded plastic, into which are put starts that have been grown in plain old jumbo styrofoam cups chosen to just fit in the planters. These are dropped in just as is, turning the styrofoam into permanent thermal insulation and a bit of a way to stretch the room for roots at nominal weight and cost. If funding allows, eventually these plants can be transplanted into larger containers but even then the styrofoam can be left pretty much as is, since roots will tend to go right through it anyway.
These containers are mounted about ten feet up.
Trellises are built of gridwall that has been heavily painted with waterproof paint like Rustoleum (as in four or five coats on top of two or three coats of primer) and ideally purchased at surplus. Frames are made of plumbing pipe, in some places bent with a standard pipe bender into arches and into goose necks for light fixtures and then, again, painted heavily with waterproof paint. These pipes are placed into holes that are about five inches in diameter larger than the pipe and about four feet deep in the asphalt and whatever subsurface is below it. Concrete is then poured into each hole and the pipe is just dropped in and supported with a temporary frame until the concrete dries. We're going for low-cost, low-skill, low complexity. Not to mention that since most of this is plain old steel and concrete, it's comparatively non-toxic and recyclable/reusable if need be.
Ideally this time is also taken to reduce the number of those awful sodium lights that so ugly up our commercial spaces, replacing them with smaller, evenly distributed LED fixtures mounted to the same frames as the trellises and powered by photovoltaics, small wind turbines, or other power generated right there at that facility.
I know that none of this exactly sounds like rocket science but I've never seen it done yet and it seems to me like a viable, low-cost, low-risk way to ameliorate a lot of what is worst about those commercial parking lots that so demoralize so many of us. There are hundreds of possible variations on this. In a perfect world, I would love to team up a few classes of industrial design students with a few classes of landscape architecture students and advised by physical plant managers of a few local malls, hospitals, etc.
But for now, I ask that you just take a few minutes and visualize this as described here. Just a typical commercial parking lot with one of these trellises for every twenty parking spaces, ideally staggered a bit as seen from above. Could the cost be gotten down to two hundred bucks per trellis, installed? I suspect that it could. Could the marginal cost to site maintenance be gotten down to fifteen minutes per month per trellis? I suspect so and that lot of it would come down to providing the crew with the right rolling ladders and the right battery-powered tools. Is there, perhaps a better idea that you would prefer to see done?
Think about it.