What's the best way to signal right turns?

Some bicyclists signal right turns by raising their left hand to the square, others by extending their right hand.

Is one way or the other better for communicating to other cyclists and drivers? The Santa Clara Bicycle Coalition summarizes some interesting research into the matter:

What's the safest way to signal that you're turning right? According to a 1979 study by Drury & Pietraszewski, the bent-left-arm signal was correctly perceived by 65% of following drivers, but the straight-right-arm signal was perceived correctly by 78%. Not only is it safer, but the right arm signal is easier to teach children: "Point which way you're going."

The study also determined that correct perception of arm signals was reinforced by the position of the bicyclist on the road. Finally, if a bicyclist looks back, drivers interpret it to mean that the cyclist is about to do something, but they don't yet know what.

The conclusion is that we should look back at drivers to get their attention, point in the direction of our turn, and move when safe to the correct road position for that turn.

The original research is reported in Drury, C.G., and Pietraszewski (1979) "The Motorist's Perception of the Bicyclist's Hand Signals," Ergonomics, 22.9, 1045-1057.

Right now Missouri law does not allow bicyclists to signal right turns by extending the right arm. The Missouri Bicycle Federation's 2005 legislative agenda include a provision to fix that problem.