lead is the thing that worries me the most. even though it’s no longer in the current exhaust that hits these plants, it’s still in the soil from the previous decades.
there are definitely certain plants to watch out for more than others, and i don’t know them off hand. sounds like we need a wiki page listing what plants are known to absorb what pollutants.
for me, whenever i see something along a roadside that i want, i just start looking in that same area, farther away from the road. even though roadsides make a good habitat for a lot of plants, they are not the only place to find those plants, and with some looking, i’ll usually see that there’s an even better selection somewhere else.
stalking plants is not too different from stalking animals. don’t look for a deer in the ocean, right? plants have their own islands, too. get to know the plant and what it likes: soil content (sandy, moist, disturbed, rich?), sun exposure (fully exposed, partial shade, underbrush?), competitiveness, invasiveness, etc.
and you can’t always rely on field guides’ descriptions of habitat because they often times say stupid things like “this plant enjoys roadsides” without telling you what it is about roadsides that the plant enjoys.
don’t be fooled by the plants’ healthiness equating your healthiness, either. you wouldn’t chow down on poison ivy just because you saw a horse doing it. the plants’ ability to handle lead is very different from your own body’s ability to handle lead. datura plants survive gloriously with alkaloids in them that would shut your body down–that’s their natural way, but it doesn’t jive with your body’s natural way.
also, if you’re going to be eating potentially dioxin-carrying animals from your urban environment, i would think you’d want to cut down the volume of any pollutants you let into your system.