I normally warn people (or I would if it were legal, officer) with a repeated downward wave of the right arm and hand, rather like the old 'slowing down' hand signal, if you remember hand signals back in the day. I have often benefitted from oncoming lorry drivers doing this. (That is to say that I would have benefiitted if I had been speeding, which of course I never do.)
@Iceman - People have probably thought I’ve been calling them dickheads for the last 35 odd years then!!!
The warning I've seen most of the time is frantic flashing of lights to oncoming traffic. Doesn't tell you there's police specifically but it's enough to suggest that proceeding with caution would be prudent.
Helpful but not encyclopaedic in terms of descriptions. What for example does it mean when a rider heading towards you takes their hands of the handlebars, stretches their arms out to a 90 degree angle and then starts flapping them like a duck. The facial expression of the rider was a massive grin. I ask as that is what happened last Sunday near to the Washington Destination Triumph garage and I have no idea what message the rider was trying to convey. They seemed a jolly type albeit a bit strange.
We do a palm down on the top of the helmet to warn other bikes of police presence, @DCS900. Palm down and an obvious sort of "pat, pat, pat". And we had to do that about half a dozen times on our ride this past Sunday as a matter of fact. The cops were out in force which is fairly rare lately. As for an acknowledgement when passing other riders, we typically give a left-handed wave the the hand down around hip or thigh level or a tilt of the head if passing when you happen to have the clutch pulled in at the moment. I've never really thought about how the wave would work if we were riding on the other side of the road until I read this thread!
Yes i do the old style slow down hand wave. Don't even know if young ones even remember hand signals. Joe.
Yes, I always do the tap on the head for Police vehicle ahead, it represents a flashing light on a police car.