When you plan on getting the "Walk Free" achievement go to the main bridge toward the center of the map.
The bridge is above an island.
Travel to the center of the bridge were you can go underneath onto the train tracks
There is a passage way were you can drive (remember: you can only do this when you don’t have access to the bridge).
Once you get to the center you will automatically get six stars.
Don’t worry about the cops because there above you and can't shoot down on you and the helicopter is too high to take a shot.
If you stay all the way to your left were the fenced wall is, all you have to do is crouch against it for a few minutes until you get it.