I'm not quite sure what sorts of edges you are talking about...

Look at this vintage photo. Notice the concrete arch the train is passing over? The sheer face you see in the picture is only about 1/3 of the total vertical drop.

Years ago I had enough courage to bicycle in the two foot wide clear area between the railroad track ballast rocks and the lip of the arch. I've since come to my senses and cross over to the uphill side of the tracks. (It's also been years since I've biked there 'cause "they've" gotten real prissy about bikes on the tracks.)

I think the point here is that if one has the confidence that one won't wreck/crash/fall-over that one can ride in very scary places.