It's ironic that in a sport that's all about speed, sometimes the rules require that speed be limited. Since faster speeds allow drivers less time to prevent crashes and enable more severe and deadly crashes, limiting speed to keep the sport viable makes sense.
At the 1987 Winston 500 in Talladega, Alabama, Bobby Allison was going 211 mph (340 kph) during qualifying laps and averaging 208 mph (335 kph) during the race [source: Owens]. It was all going great until his tire blew. That sent his car into the air and the safety fence before it returned to the track and was hit by other cars.
While there were no fatalities, NASCAR quickly realized that when cars are going that fast, fans need more than fences to protect them. At those speeds, the risk of the cars heading skyward is just too high. So regulations were changed and carburetor restrictor plates, which reduce engine power and speed, were made mandatory for all NASCAR cars [source: Owens].