real race track look

Grey wet-and-dry sandpaper is great to replicate surfaced roads. No need to paint, just weathering and tire marks needed!

Really? Would have never thought about that. What / how would you do tire marks?

The best way to simulate asphalt is roofing felt (tar paper). Just weather and paint stripes on it. The color is the same as asphalt. You can pick up yards of it at any construction site. It is much cheeper than sand paper (free).

Never thought of that Berny...! I'll have to try.

As to making marks on the sandpaper, the easiest is to rub black or really dark grey pastel chalk onto the sand paper...

Aaron,
I'd use 340 to 600 grit wet and dry sandpaper for your diorama base. You'll probably need to paint the sandpaper a slightly lighter color, because if you'll have a real close look, the asphalt isn't black at all - it's varying shades of gray - some lighter, some darker.
Tire skid marks can be replicated with black pastel chalks. They'll stand out more on a dark gray background.
Ripple strips and lane markers can be airbrushed using home made stencils.
Cheers
LeeTree

After using the chalk for the skid marks, do you spray a clear coat over it or leave it as is?

No need, Dave... Blow a little to remove the excess. The rest will be there forever (unless you put water on it)!

thanks, for tire marks i am looking more for making the track look like the tires have been wearing rubber off. some race tracks you can see where the cars run because there is a significant rubber buildup on the track. i would also like to give the top portion of the diorama the "marbles" look. Which is small pieces of rubber that kind of collect around the non raced portions of race tracks. Thanks for the tips everyone!