How wide should the tracks be?
 

The Xmods site uses a lot of 5' PVC. I was wondering what you guys think minimum, and average track width should be. Like, a 6" width track wouldn't be any fun, and neither would a 10' wide track.

Reason is, I'm thinking about building a track using dense thick foam, which won't be so hard on the car as PVC. I will be using a dremel and making the pieces interlock so I won't need massive 10'x20' sheets of foam.

If it turns out, I'll post pics.

It depends on how many cars you'll be running and how technical the track is. A narrow track with a lot of turns might be fun for two car but could be a log jam for more.
My lanes (rcp-track wide L) are 2 foot 9 inches. That gives plenty of room to experiment with different lines and have room to pass and avoid wrecks.
If your not fully committed to doing a diy track you should look into an rcp-track. What type of surface are you going to use the track on? The grip surface on the rcp-track is fantastic!

the offroad tiles are nice too;)