Bonus Boards
Get all 4 bonus characters, then press Up, Up, Down, Down, <, >, <, >, B, A at the board select screen.
(Thanks to DANMONEY@webtv.net)

Bonus Characters,br. Bonus characters will be received after completing the following:
Earn an A ranking on each level and track on Street Work Mode.
Earn an S ranking on each level and track on Street Work Mode.
Complete each level and track in Time Attack Mode within the time allotted.
Earn a Perfect ranking on each level and track in Coin Mode.
(Thanks to DANMONEY@webtv.net)