1960 Daytona 500
While Aryton Senna's crash changed Formula One racing, a crash involving a whopping 37 drivers at the 1960 Daytona 500 changed NASCAR [source: Joslin]. Considered the largest crash in NASCAR history, the accident happened in part because Daytona is a much bigger and faster track than many NASCAR drivers were used to racing on up to that point. Any time you have increased speed, you have an increased crash risk, as the drivers found out first-hand in this collision.
NASCAR had evolved from bootleggers racing souped-up cars on backroads and dirt tracks, but the crash at the 1960 Daytona 500 showed that if the sport was going to be popular with a broad audience, they'd need to improve safety. Although, as this crash shows, safety can sometimes owe an awful lot to luck. Of the 37 drivers involved in the crash, none were fatally hurt. In fact, most injuries were minor [source: Joslin]. Not all drivers are so lucky.