Please add this content :D

-Tire marks for every vehicle!
-smoke where vehicles hit something
-sparks flying off your scrapping car!
-And a freemode where you can actualy steer your car and control gas and everything :)
-interchanges which you can choose which ramp you want to take and, adds a bit of exploration and maybe you could also add scenery like off and on ramps next to the highway
-deformation of vehicles<== would make your game almost unique and will attract attention and would be freaking amazing!
-day and night with headlights on cars, would see the flipping everywhere in the air and would be halarious :P
-add a driveable semi-truck which could hit a few cars and trucks before crashing
-hills, ups and downs, really tall and long bridges
-6 lane highway sections in sunken city like place and in this biome interchanges would be quite common
-tunnels in which sections could collapse and crush cars
-bridge physics, could collapse under certain amount of cars and trucks and would simply detatch as sections to avoid lag
-make replay optional to remove lag... Please!

So ya thats what would be cool to integrate at one point :)

NCouillard about Highway Crash Derby, v1.3.2