Here is a big difference between cars and bikes. Cars move into another lane when passing (OK, maybe not here in Miami) and bikes pass very closely and in the same lane. That means that if the person in front is not aware that you are coming up behind them, they could easily push you out into traffic simply by a mild swerve (missing a peice a debris, or just zoning out). Also if you check you drivers handbook, if the driver in front is unaware of you passing, you are required to give a quick sound of the horn.

I have rarely seen anyone that knows that I am there until I call out. I could have a 4x8 peice of plywood heald up over my head and they still would not see me. C'mon, how much time do you spend looking behind you when on a serious ride? The guys I pass certainly are not looking behind them.

I have also noticed that when a person catches you with their peripheral vision, without prior notice, they tend to swerve toward you, not away, pushing you into traffic.

As a matter of fact. The last time I did not call out, I had a collision. I was drafting with a guy for about six miles, we were taking turns pulling about every mile. It was my turn. He knew I was there. I started coming around just as he decided he had had enough and made a hard left. We bounced off of each other twice and thankfully no one went down.

So, for your safety and mine, I will be calling out.