Autopia

What could be more utopian than automobiles! In Disneyland, circa 1955! For the uninitiated, the Autopia ride at Disneyland consisted of very slow, miniature cars that you got to drive around on a closed track. To keep you from driving off into the Pirates of the Caribbean, your wheels straddled a curb running down the middle of the track. I remember riding this back in the ‘60’s. Of course as a little kid, I hadn't figured out that after turning, you had to straighten the steering wheel back if you wanted to go straight again. Hence, I went the entire ride grinding along the curb, one side or the other. This tabletop version is much more fun than that.

This version uses a serpentine track that captures a long string of 6mm balls - Airsoft BB's, to be specific. At one end of the circuit, the balls wrap around the dial, which essentially is the inner race of a ball bearing. Rotating the dial while pressing down gently forces the balls to recirculate through the entire track circuit.

You will need about a 150 BB's. 

Printing
I printed this in PLA with 0.2mm layers. It is important that the groove in the track is completely smooth. I recommend painting the seams for the inner and outer sides of the groove as shown below. This will make them easier to sand smooth if necessary.

Print one track, one dial, and several cars and pickup trucks, and one tab per vehicle. (Note: if you have any concerns about whether your printer will print the track groove smooth enough, I recommend first printing the TrackTest section so you can tweak any parameters as needed before printing the entire track.)

After printing, shove a ball on the track and roll it along the entire length. It shouldn't hang up anywhere. If it does. Push a small wad of sand paper or steel wool into the track groove and work it back & forth until the ball runs smoothly everywhere. Make sure you've wiped out any dust and debris from the groove before assembly.

Assembly
Pour a bunch of the balls into the well where the dial will go. Tease these balls into the groove on either side until the entire track (except for where the dial goes) is filled. Next, insert the dial into the well and align the little notch in the perimeter of the dial with the notch in the edge of the track. When the notches are aligned, there is enough room to shove a ball through and into the track groove around the dial. Add enough balls to that the track is almost full, say, 2 or 3 balls shy of full. This will leave enough room to insert the tabs of several cars between the balls. Try operating the dial (pressing down slightly as you turn it) to make sure you can drive the entire circuit of balls.

For the cars and pickups, insert a tab into the bottom of each vehicle, pressing all the way in. It's a snug fit. The tab should stick out 6mm from the bottom of each vehicle. Update: I've added a little ridge to the bottom of the tab to help keep the cars from vibrating out of place. The ridge should face the back of the vehicle as shown:

At this point, you can load up your track with several vehicles – just wedge the tab between any of the balls. Don't add so many cars that the balls get tight in the track. And now you are ready to Autopia!