I discovered that the drone can be used to probe for breaks in the invisible barriers that keep cars out of forbidden areas; just fly the drone against one of the barriers and if it breaks through you’ve found a spot. It might take a while to probe all possible spots but it’s faster than ramming cars at potential hotspots.
I was able to get on the far-west side of the map; I made it fairly far up the river and almost to the mouth on the other end.
I’d love to hear any off map tips if anyone else has any.