In my neighborhood, a lot of neighbors put up signs saying it's okay to take fruit, or post on the neighborhood Buy Nothing group. Sometimes they even pick a basket and put a sign out telling people to take some of that. Generally I leave stuff alone otherwise. Something that's actually a weed here, like fennel growing at random through someone's back fence, is different, though typically I just take a tiny snip to sniff, or a few seeds to chew on. My husband likes to pick cornelian cherries in certain public parks (he makes jelly out of them).

There are things I don't pick by public roads (especially stuff like berries) because they're too close to all the car exhaust and I figure the ground is somewhat polluted. I am always kind of surprised at people planting vegetables in their parking strips (though raised beds on a non-busy street are probably okay).