I have noticed lately here in Southern California(Ventura County) that a lot of us are forgetting to give a heads up as we pass other riders. This can be quite startling and dangerous. I was passed twice yesterday as I rode on Foothill between Santa Paula and Ventura by medium sized groups of riders(8-12). To be riding along and suddenly have someone a few inches from your left flank can be alarming and then, before you know it, surrounded by riders and having no wiggle room. As they passed , I mentioned that a little warning would be nice, I was not acknowledged. I was taught in my cycling class in college in the seventies that this was not only courteous but necessary for everyone's safety. Think about it, Joe

I have noticed it around here, too. Maybe it's because I am older now and getting passed more often.
Maybe not.

i would go further and say that i don't think cyclists here in SoCal ever learned that little bit of courtesy protocol in the first place. riders around here NEVER call out that they are approaching (no 'on your left', 'rider back', nothing...). it's even worse on trails while hiking with riders on mountain bikes - no heads-up alert of any kind. this is standard practice anywhere else i've ridden..... i generally have a less-than-polite response to these folks. why is it so lost of these folks to exhibit even the most modicum of courtesy on roads and trails ?

WELL STATED! Joe

I always give a quick ding from my bell when coming up behind another rider. I don't want to try to pass them without them knowing I'm back there, because I don't want them swerving into me.