Thanks for it as contribution! I've heard the collision between two cars with newton physics wouldn't work - your demo proves the opposite!

Your roads seem to do the job they should do, but they seemed to change the slope a bit too sharply!

Relating FPS: Did you try to switch off the stencil_shadow? Do you use 3D sounds? (They can slow down remarkably!)

Another thing is the camera: right in the beginning a street above in the way of your sight to the car! You should trace from the car to the camera to find any obstacles and then set the camera closer to the car! There are tutorials about camera codes around in the community. At least at Grimber's site you should find one.