Roadside trailheads not so safe
I actually surprised some guys breaking into cars at the 19 Mile Brook trail head, though, alas, I did not realize it until they were gone. There were two guys in a van, they had pulled off the road, stopping parallel to the road blocking the view from the road, and the guy in the passenger seat had jumped out, opened the van door, smashed a window in a car, and would presumably have tossed everything in the car into the van, had I not entered the parking lot at that point. I was so focused on getting to the car and getting my plastic boots off, (this was in March) that I did not notice anything was wrong until I saw the broken glass five minutes later.
I would say that the parking lots on the main roads are the MOST likely to be risky - breaking into a car there is a 15 second operation. I filed a police report, and that seemed to be their opinion as well.