I replaced the bumper cars from Echo, to make them work you need a fenced area and the cart runner. The cars are placed on the runner and then the sim walks to the car and gets in it. No NPC is created.
I used a normal railing fence to surround the bumper cars "ride", about 25 tiles square and then a pets room divider on one side to allow sims to walk over to the cars but prevent the cars driving all over the lot.
For those interested in reading more: http://modthesims.info/showthread.php?p=...ost3210227
It might be possible to do whatever the cars use to move around (a custom rig, perhaps?), but that seems to be uncharted territory. As far as I know, custom cars are just custom models that use the existing means of vehicular movement. To my knowledge, no one has made cars that drive outside the established "into/out of the driveway" and "straight down the road" paths.