How do you keep track of the positions of race cars?

The position of the cars on the track a compiled by a computer using signals from sensors embedded in the track. Each car has a transponder on board that acts like your toll pass sensor when you travel through a toll booth. Each has an individual signifier that registers it's point on the track. As a back up or in questions of where the cars were when a caution came out, video replay is also used. As far as you as a fan following the race, there are usually display screens or pylons with the the car number and position posted where you can see them.