Well, if you just burn it off with a decon kit, you could do a pretty solid job in an hour. That or lacquer thinner. To do it with a clay bar is going to take at least a couple of hours per car, plus you'll need patience and some serious upper body endurance. I'd do it chemically too, if I had numerous cars to do.
The decon kits use an acid/alkaline/ph balanced wash series, which is not nearly as harmful to your car as it sounds. Almost every new car bought or sold gets this process, the only downside is that this will melt through your coat of wax, and you'll need to reapply. I'd do that part myself, cause I don't know what the "Experts" use for wax, but I doubt it's Souveran

. After the chemical soak, they probably use a red (more aggressive) claybar to take the extra off (I bet they'll scratch it up here).
Personally, I would find out what the city was paying them, and if it was over 100 per car, I'd do the work myself. Here are the supplies you're going to need.
Finish Kare Decon Kit 60 bucks (
www.fk1usa.com)
Claymagic red bar 20 bucks (anywhere really)
Some sort of polish (PO85RD for me, SSR1 for the less rotary-enhanced)
LSP (red or black, go RMG than Souveran, light colors, go EX-P than FK1 Pink).
If you let them do it, you'll need to polish and wax anyways, most likely. If you are not comfortable with the decon kit, I would suggest letting them do it. Doing it yourself with a clay bar is miserable, and you're going to induce more marring than they would, if you go at it hard enough to remove all the spots.
Best of luck,
Tim