The Design of Everyday Things: Turn Signals

I’ll start this post off with a plug – read Donald Norman’s book ‘The Design of Everyday Things’. It is about the psychology and physiology that underlies interaction with things you use every day; why some work well (are usable), and others do not.

This post was inspired by Norman’s excellent book, as well as the long overdue arrival of hot weather here in Toronto and the string of dry days that has allowed my motorcycle to re-emerge.

Photo by Motorcycle Cruise

For those of you who are unfamiliar with a motorcycle’s interface: you steer with handlebars, left hand works the clutch, right hand works the throttle and main brake. Typically, secondary functionality is assigned to the left hand – turn signals, horn, highbeams etc. Normally, turn signals are a switch that can be pushed to the right to signal right, pushed to the left to signal left and clicked to cancel. All of this can be done with your left thumb.

Why does this make sense? Well, except for when you are operating the clutch, your left hand is free to move about and operate those controls. The same cannot be said for the right hand – you need to keep the throttle at a constant position in order to maintain a constant speed. More importantly, you really want to be ready to operate the brakes on short notice, so you always want that hand ready to get off the throttle and grab the brake. You don’t want your right hand to be preoccupied with operating secondary functions when a Mac truck pulls out in front of you.

My bike, a BMW, implements a different model. A left turn signal is initiated by the left hand and a right turn signal is initiated by the right hand. And the signals are canceled by an entirely separate button….located on the right side!

Photo by Peter Boden

What’s wrong with this?

1. It’s inconsistent with virtually every other motorcycle. Any user coming from another brand will need to relearn how to use the interface.
2. It doesn’t consider the physiology of the user. Although there is some sense in having left things controlled by the left hand and vice versa, each side of the body is controlled by a single hemisphere of the brain so more brain needs to get involved to signal an upcoming turn. Getting more brain involved at highway speeds can be dangerous since it takes it away from being aware of the road. Furthermore, the right hand needs to be considerably more dexterous in order to maintain a consistent throttle position and maneuver appropriately to operate the signal/cancel buttons.
3. It’s dangerous. I need to move my hand away from the main brake lever to signal a turn. If it takes even half a second longer to get back on the brake, it can mean an extra 14 meters traveled at highway speed. That’s most of the way through the Mac truck that just pulled out!

So why did they do it this way? The only thing I can think of is the adage that ‘Interface is Brand’. They’ve been doing it this way for a long time, and it has become a recognized element of the marque.

What can we learn?

1. Never discount the impact of the familiar. You may have come up with a theoretically better approach, but if it differs from the familiar it will have acceptance challenges.
2. Always consider human physiology. For computer interfaces, this includes visual perception, hand/eye coordination, motor memory, and short term memory capacity.
3. Always consider the interaction you are designing in the context of the user’s task. Nothing is accomplished in isolation from other tasks, so you need to be sure that your design cooperates with everything else your user needs to be doing.

The design of everyday objects isn’t limited to motorcycles! I’d love to hear any thoughts you may have about the interaction design of things you use everyday.

UPDATE: Aug 14, 2009 – There is an interesting and active discussion on this posting going at at YCombinator. Check it out!