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.
Shown with Rebel Minis Civilian 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!
City Traffic.
Traffic Jam.
Hard to get across the street during regular traffic problems now add zombies!
Easy to break down to three sections.
Body.
Glass
Chassis
You can see from the break down of the vehicle that one can make wrecks fairly easy. remove the glass and file down the tires and some paint you can have burned out vehicle
I hope this helps those still wondering if they should or not get a set or more, and help set the idea for some interesting tables as well, I would highly suggest getting some. I looking at getting more, Mini Vans and some other Vehicles.