Most oval NASCAR tracks are banked at various degrees to create greater momentum and velocity through the turns. Banking on the 23 tracks that play host to NASCAR Sprint Cup series races range from a relatively steep 36 degrees at Talladega Superspeedway to a somewhat flat 12 degrees at Martinsville Speedway.
"Progressively banked" oval tracks, such as Bristol Motor Speedway, feature a racing surface that gradually increases in degrees of banking -- 24 to 30 degrees -- as the track gets closer to the outside retaining wall. This permits cars in the longer outside lanes to maintain higher speeds through the corners than those in the shorter inside lanes, resulting in more competitive racing.
Source: NASCAR