I've always loved slot cars. I wanted to find a way to provide a slot-car themed project for my Grade 10 and 11 Computer Science Students.
What resulted is a small slot car track with a level crossing. Both tracks approaching the intersection have light-sensors that detect when a car is approaching the intersection. Depending on the delay between the cars approaching the crossing, the software will turn the 'later' car off, allowing the first car to pass through and then turns the car back on to resume travel.
The arduino consists of code to monitor 4 Light Dependent Resistors (I built relay-based switches but you could just hook the LDRs up to the Analog side of the Arduino directly) and two MOSFET driven 9V relays that manage the power to the two tracks.
Processing handles the user interface that displays lap counts and allows the user to adjust the track-off delay for each approach based on car speed.
Worked out really well. I had two students do the project last year and they were both sucessful.
I'll be using this code and circuitry when I build my super-4 lane race track in the basement.
Here's a video: https://www.youtube.com/watch?v=QC5G4psllpY