Personally I'd probably still avoid ones from by major busy roads, as they'll still be unpleasantly polluted. Or at least give them a more thorough wash before use. I wouldn't worry so much about ones by more minor roads, or in my case the ones near the railway.

Seeing how horribly dusty the plant looks is probably the main thing I use as a guide - if it would put me off eating them right there then I don't bother picking them.