At the hour of the race, a green light signifies the beginning of the relatively slow formation lap during which all cars parade around the course doing a final tire warmup and system checks.[18] The cars then return to their assigned grid spot for the standing race start. The starting light system, which consists of five pairs of lights mounted above the start/finish line, then lights up each pair at one-second intervals. Once all five pairs are illuminated, after a random length of time (one to nine seconds), the red lights are turned off by the race director, at which point the race starts.[19]