Visual Aids for Cycle Training

I sometimes shout "slowing" in these circumstances (or use club hand gestures if there's a somebody drafting) but the guy was not even close when I looked back before the junction.

Those sort of stuff I'd advised not to do, while you have good intention of doing that, the last thing you want is to people to end up automatically assuming that the next person will do the same thing and end up rear-ending them because they did not do the hand signal/shout a warning.

My only advice is that he should simply give a bit more braking room and always be prepared to stop at all time, rather than expecting someone to warn him.