Here in the city there are enough bikes around that vocalizing ("passing on your left" or just a "hello") is a safety issue.
If the bike you are passing doesn't know you are there, they could kill you.
Imagine happy little Knott tootling along on her single speed mixte. She spots her prey ahead, a slightly tired roadie in full spandexed glory on a "crabon" infused plastic bike. As she silently stalks him, her front wheel overlaps his rear wheel, then gets to his cranks... then he swerves to his left to a avoid a patch of gravel and knocks into Knott's front wheel throwing her into the traffic. Mayhem ensues.
Ooopsy.
I looooooove passing racer-boys while I'm on inappropriate bikes. (passing the dude used as an example above is a highlight in my little world: going uphill on a ss mixte wearing sandals FTW!) It helps if they are tired and I'm not. And I always let them know I'm there so they don't kill me.
There have been many times I've been startled by said dudes passing quite close to me with no warning. They are fast and they are concentrating. I generally yell out "ON YOUR RIGHT, HONEY!!!!" Since I'm old enough to be their mother, I can call 'em "honey" or "sugar" or "sweetie." And since they are young enough to be my children, they usually say "sorry." And since I'm a damn good rider and have been on bikes since before they were born, I manage not to startle so bad that I swerve into them.
The big city is a dangerous place for bikes. (some chubby ol' biddy on a ss mixte might shame you!
)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson