In the MSA of my campus, almost everyone says salaams to each other. We have an office where most muslims hang out and a prayer area nearby. The traffic of muslims is pretty busy around that area, so almost everyone say salaams, probably because most of them know each other.
When outside in public, it depends. In muslim stores/areas we usually say salaam because I think it strengthens the bond as muslim. But in general, like in malls for example, it's a little awkward to just say salaam as that sister is doing her thing. She might not get it the first time. But in the situation that you said, if you look muslim, she will obviously notice and probably expect or prepared for a salaam.
I was wearing my kufi one day and went to Target. The cashier lady said that she liked my hat, and asked if I was muslim. I said yes and she was very happy to she's muslim too then said salaam. I think that's pretty neat.