The act of looking back is often read by other drivers as a desire to turn - so well in fact that one often does not need to hand signal to negotiate a turn. You may not have the intention to turn when you look back, but others can not read your mind.

For about a year I was very frustrated by this as often when I looked back to find if I could prepare for a turn that there would be a motorist present who would slow thinking I wanted to turn then, but it would have been better to merge behind them. Confusion resulted. Timings got messed up. I resorted to hand waving motorist by me and other non-standard hand signals to help reduce the ambiguity.

This was the primary reason I bought a mirror, so I could asses traffic conditions before looking back and/or signaling. It solved all that problem and more.

Al