HO or 1/100 scale cars work well in 15mm range, and if you need a few cars for your game then this is the route one needs to go, You can get a very large number of HO Scale cars on Ebay, from 50 to 100 cars and vans from $15.00 to $20.00 more then enough to block your roads with traffic jams.
I found that Match Box Cars and Hot Wheels Cars were just to big, the Trucks seem okay and work well.
Plus at that cost you can simulate that some are damaged, in one form or another being plastic with some heat you can create the damage.
I bought a group of 50 for just under $15, and plan to buy more. You can see comparison pictures below with figures.
Plus nothing gets your pulse going like seeing your "Heroes" trying to get down a street crowded with cars slowing down their escape! That's right "Surviving the Concrete and Steel Jungle!"
Flash might a little bright OOPS!