| Year |
Drivers1 |
| Ages 16–20 |
Ages 21 and older |
| 2004 |
22.5 |
24.8 |
| 2003 |
23.2 |
24.7 |
| 2002 |
22.8 |
25.8 |
| 2001 |
22.9 |
25.7 |
| 2000 |
23.4 |
25.9 |
| 1999 |
21.5 |
24.7 |
| 1998 |
21.9 |
24.7 |
| 1997 |
21.5 |
24.8 |
| 1996 |
22.2 |
26.4 |
| 1995 |
20.6 |
27 |
| 1994 |
23.3 |
27.3 |
| 1993 |
24 |
28.5 |
| 1992 |
26.9 |
30.1 |
| 1991 |
29.4 |
31.7 |
| 1990 |
32.1 |
33.2 |
| 1989 |
29.5 |
31.9 |
| 1988 |
32.1 |
33.2 |
| 1987 |
32.6 |
34 |
| 1986 |
36.5 |
35.6 |
| 1985 |
33.9 |
34.6 |
| 1984 |
39.1 |
37.3 |
| 1983 |
41.6 |
38.7 |
| 1982 |
43.7 |
39.9 |
1 Drivers whose age is unknown are excluded.