I often ride on a certain rail-trail and there is one section where it closely parallels a very active railroad mainline; one that sees about 50+ trains a day. There are several streets that cross the railroad and the path and some of the drivers there love to stop and wave the bikes across. It is doubly dangerous because the car going in the other direction probably isn't stopping (and may not even have noticed the bike), and also because the car that stops inevitably causes other cars behind him to stop on the tracks. I've never seen an accident there but it is always awkward.