Causes
The 43 cars or 36 trucks in a NASCAR race often run in one to three packs, sometimes only inches apart, while traveling over 200 mph (320 km/h) in three- or (at Talladega mostly) even four- or five-abreast formation. This may be exacerbated in recent years, when restrictor plates and new safety rules have yielded cars with fewer performance variations. This is especially true at Talladega, where handling is not a major factor and packs seldom break up for long.
The close quarters allow small margin for error. One error or sudden mechanical failure is all that is needed to start a chain-reaction crash, with cars scrambling to avoid the crash often getting caught up in separate incidents. Tire smoke often reduces or eliminates visibility, and wrecked cars may partially or completely block the track. Cars well behind the accident can get caught up in the crash due to poor visibility or debris.
During long stretches of green-flag racing (particularly at Daytona), the cars typically spread out around the track. But once a caution flag comes out, the pace car picks up the leader, and the remainder of the field catches up and "packs up" behind it. When the flag returns to green, the tightly bunched, nose-to-tail lineup reduces maneuvering room during acceleration to racing speed. It is not uncommon for several smaller crashes or one big crash to occur immediately after a yellow period. This phenomenon has been referred to as "Cautions breeding cautions."
As a general rule, a "Big One" will include a minimum of seven cars, but usually has at least ten cars involved in the crash.
In the 1990s, key areas of slick and often rutted grass infields on several circuits were covered with asphalt skid pads, which scrub speed from spinning cars.
Read more about this topic: The Big One (NASCAR)