Sunday, December 13, 2009

Violating the Rule of Least Surprise

When you mention user interfaces, people usually think of a graphical interface with mouse and keyboard or a touchscreen. But user interfaces do not exist solely in the world of computer displays. In the world of embedded systems, a user interface can take on any number of variations. One guiding principle that particularly applies in this domain is the "Rule of Least Surprise." An example I use is that if you put someone behind the steering wheel of a vehicle of any type, they would expect that turning the wheel to the left directs the vehicle in that direction. To do anything else, would surprise even a three year old who has ridden a tricycle.

Even though I love my current car, it has two areas which violate this rule. As shown in the picture shown here (source), the instrument panel is in the center -- not directly in front of the driver. Both interface surprises stem from this placement.

The first violation is that when the lights are on at night, there is no light at all directly visible in the direct field of vision of the driver. It is very easy to forget to turn the lights on at night when in a well-lit area because you have to look to the middle to realize that the instruments are unlit. This has actually resulted in me getting pulled over by a friendly police officer to remind me to turn my lights on.

The second violation is when you turn on the turn signal to turn left. The blinking light indicating you have done so is in the right hand side of the driver's field of vision. In other vehicles, this would be in the left hand side
of your field of vision.

Neither of these is a major issue and I have grown accustomed to both. But both show how humans have expectations about how common devices operate. When your clever design changes this, users are surprised. In the end there may be a benefit, but often it is a difference just to be different. Is there a surprise in a system you are familiar with? Is it an improvement or just a difference? Share.


  1. Wow! Never knew people could design such systems. You mean there is *no* intrument cluster in front of the driver at all? OMG. What advantages / Benefits you see from this? I seriously see this as as stupid design.

  2. When you get used to it, it is nice at night to have no light at all directly in front of the driver. But it definitely takes getting used to. Sometimes I still get a quick panic at night that the lights are off. :)

  3. @Aanjhan, this is the case with the Tata Nano as well.