Other than the usual traffic signals and the obligatory one and two finger “salutes” for spectacular driving by other competent drivers these are the ones I know, any more? Just dropping the left hand out to the side is saying “Hi” Nodding is acknowledging the other rider. Taking the left hand and putting your palm toward the ground and pumping your hand up and down is a single to slow down. Signal used to warn bikers of danger ahead like road hazards. Taking your left hand and patting your helmet on top is saying there are cops or alternately circulating one raised finger. Clicking one finger in front of face (like taking a pic) = Speed camera ahead. If a rider ahead of you holds up 1 finger over his head, he's suggesting single file. Any more? Helmet on the ground beside the front wheel, or helmet a car length behind the bikes rear wheel on the side of the road means you've broken down and could use a hand. Gravel in the pocket to “signal you are following to closely”.... (not recommended) Left hand thumb and pinky out, index, middle, and ring fingers folded against your palm, twisting side to side = "Lets stop and get a drink" The only other hand signal i know of it the same for double file (or staggered at higher speeds) but you raise the two fingers and twist your hand for change possion in the group.