Super Bikes:
Goto the "Menu Screen" and enter the following:
C-Up, C-Left, C-Left, C-Right, L, R, C-Down, Z

All Tracks/Bikes:
Goto the "Main Menu" and enter the following:
C-Up, C-Left, C-Left, C-Right, L, R, C-Down, Z

Scooters:
Goto the "Menu Screen" and enter the following:
C-Down, C-Right, C-Up, C-Left, Z, Z, L, C-Left

Access Insanity Level:
Goto the "Thrash Mode" screen and enter the following:
C-Up, C-Left, C-Left, C-Right, L, R, C-Down, Z

Female Biker:
Goto the "Menu Screen" and enter the following:
C-Right, C-Left, Z, L, R, C-Up

Cop Mode:
Goto the "Main Menu" and enter the following:
Z, C-Left, C-Down, C-Left, Z, L, R, C-Down