Go to a train station and spawn a car, then drive the car on the tracks in the direction of an island you don't have unlocked.
You will get 6-stars.
If you keep driving on the tracks, eventually you will get in the underground subway tunnel. Go a little further, and stay away from any stations above or below ground.
Turn on your brights using "x" then sit there
Make sure you keep a lookout for any cops that may come down.
In my 5-minute journey I only encountered 1 cop which I ran over, then waited for the remainder of the time.
Just use the right stick to keep lookout in all directions as you wait.