That's what I do as well, sometimes with a friendly head nod. It can be taken as me acknowledging their presence and the fact that I'm alert - even if I want to believe I won't get mugged by being pleasant!

There’s a big difference between quick eye contact affirming recognition of the other parties and sizing someone up for robbery. You may make eye contact with them but it’s brief and you move on and don’t remember the person. People sizing someone up stay fixated on the target.