The hardest part about completing the Outlaw campaign and challenges is finding the police. You can drive around town for lengthy periods at time without running into the cops. There is one area at the South Port that always has a cop that you can find. Link down below that shows you where to find them.
Once you get rank 2 i believe, then you'll start seeing road blocks down some roads. The ones needed for this achievement are easy to break through, but you need to in the city and go down side streets in order to find them in my experience. They don't appear on the freeway from what i've seen.
In the slim chance there is no cop when you arrive, just leave the area for a few mins and come back. It's possible the cop was chasing someone else, but it will respawn shortly. Here is a video that shows you exactly where it's at.