There's plenty of them as you progress through the game, but sometimes a bit randomized, which I like quite frankly. And whether vehicles are even at them or not can be random. Usually fast traveling back and forth can make vehicles spawn if there's none there though.
I don't use truck turrets anymore though, they leave you too exposed. By the time the transition animation from driver's seat to turret is complete, you can take a lot of damage from enemies behind you. I just toss a rock through the windshield as soon as I get in and ready my M79, which takes out enemy vehicles a lot faster.
Between that and Autodrive, you're good to go.