How can a human-powered generator be used to run a slot car racing set?

Click on the YouTube video below to see how a DC pedal power generator & DC hand crank generator dynamo were used to teach K through 8th grade kids about alternative energy. Each child learned that to make their car race successfully, he/she would need to pedal at a speed where approximately 6 volts DC was being generated. If they pedaled too fast, the cars would get too much voltage and fly off the track. If they pedaled too slow, the cars would not move. This track was sold by Artin. Now it has been discontinued, but you can buy them via eBay. There are also many other brands that are equivalent like Hot Wheels, Carrera, and Mattel.