Underground shortcut:
Locate the loop that is halfway through the first lap of route 2A. There is macho dirt on the left and ramp to the right at this location. Press B(4) while on the loop. You will go underground after exiting the loop.

Hidden turbos:
Jump a ramp on any level and do a back flip in the air. Certain ramps will have a turbo that can be collected with that move.