We use asphalt roofing paper (underlayment). The roll size from Home Depot (about $38) is so big you can do all the roads and parking lots on your layout, your friend's layout, and several others. You can cut it using the "score and snap" method or scissors. Here's what it looks like:
All my friends that have used any type of foam product ended up with divots where their vehicle sit. How is the surface texture Matt?