Road Rules are unlocked after winning one race eventTo enter into the Road Rule mode you need to press
on Dpad, the road sign at the top should change from a green background to a
Red one.
The shortest road in the game is
Watt St, the time you are aiming to beat is the
OFFLINE time which is 5.8 seconds.
You dont need a fast car but you DO need a full bar of boost and a nice run up.
Watt Street is located south of the beginning Junkyard.
You can press the 
button to bring up the map and change the Display to Road Rules to see the names of roads.Here's a picture.
Note beat 9 more to unlock
Here's a video demonstration of Watt St