By collecting hidden keys, you activate Rush 2's 
secret cars for each track. Every four keys you 
collect, you will activate one car. The vehicles 
include:

Taxi 
Hot Rod 
Formula 1 
Prototype 
Mountain Dew Racer  
You need all 12 keys to unlock the Prototype. Collect 
all four Mountain Dew cans and you'll unlock the 
Mountain Dew Racer. 
Submitted by Duros Bith