When I first started walking barefoot to work, back at the end of april, I was worried about hurting my feet by stepping on glass. Several people warned me about it, but I couldn't find any good data on how likely it was. I've now been commuting barefoot most days since then, and I found a glass splinter in my foot for the first time today. (It wasn't so bad; it hurt some, I pulled it out with tweezers.) I count about forty hours of walking barefoot commuting to work daily over a little more than two months, including that I wasn't walking the whole way barefoot at first and some days I took the bus. Adding in other times I've been barefoot, on the weekends, etc, and I think it comes to about fifty hours of walking around somerville and cambridge. So: 2% chance of glass per hour walked (but wide error bars).

Note: I watch the ground pretty carefully when I walk, avoiding glass etc. If you are less careful, you'll probably step on more bad things.