Honestly I don’t really like this idea, it would make getting around annoying and you’d encounter hostile vehicles out of nowhere and being shot at when you’re just walking around an unpopulated road. It’d be especially annoying if you had no gun to fire back at them 
I made a suggestion like this a while back about rare-ish “convoys” as a mini bossfight/random encounter thing, containing decent loot which spawn and despawn, and they only shoot at you if you shoot them first. I think something like that would be a better way to go, otherwise it’d just be annoying.