The vehicles are static func_detail brushes and the road,walls,etc are world brushes with scrolling textures applied to create the illusion of movement.

A trigger_push on the ground pushes the player towards the back of the map into a insta-kill trigger_hurt to simulate being left behind, the bumpers also have similar trigger_hurt brush entities in front of them.

The passing cars in the opposing lanes are attached to an invisible func_tracktrain that follows an invisible track, when they reach the end of the skybox they teleport back to the begining of the track, same thing for the buildings.

wow thats really cool

That's why i like the source engine so much, illusion effects are neat.

Goldsrc (Half-life 1 engine) can also do these things, like the scrolling textures.