Social engineering and usability: a post about toilets.

I’ve been away from this blog for far too long again, I know–sadly I have had things happening at work that have demanded my attention more urgently than this blog.  Now I’m back, and I am going to write a post about something I never thought I would see on this blog: toilets, specifically the dual flush ones.

The first dual flush system was designed in Australia in 1980, and modern ones are estimated to save households up to 67% of their annual water usage–a lot in a country that suffers chronic drought, but favoured by environmentalists everywhere.

Early dual flush system by necessity had to clearly mark which button gave a whole flush, and which a half flush, meaning buttons usually looked something like this:

The images on the buttons clearly dwmonstrate the concept of “full” versus “half”.  It might not be obvious to unfamiliar users exactly what this does, but once explained is likely to be relatively obvious (apparently, too, it is relatively easy to train new users–even children–to use “one button for pee-pee and one for poo-poo“).  The particular model displayed might have a minor design flaw, though.  The half flush button, the one an average user is likely to use most often is on the left as you face the toilet–further from the dominant hand of approixmately 90% of the population.  While this is a tiny inconvenience, it may affect behaviour in some cases, meaning the full flush may be used slightly more. Conversely in much design left is less and right is more–consider the volume knob on your car stereo, or the speedometer on your car, for example, so maybe this left-to-right design has consistency advantages that outweigh the convenience issue–experimentation would be the only way to know which side induced the “best” behaviour.

More recently, though, manufacturers have been moving toward circular designs, presumably in response to the fashion of the day.  Unfortunately, not all these circular designs are clear, as evidenced by the signage in this picture.  This design really does have a design failing; while the designers have kept to the left-half right-full convention, the half flush button (again, the one likely to be used most often) is a smaller target, on the left, and therefore harder to hit.  On the positive side, for at least some of these toilets, hitting both buttons (which is possible in this style of design) triggers the half flush, rather than the full flush (as scientifically tested by an army surgeon).  So again, this toilet may be discouraging “good” behaviour by its design, in the first instance because it is marginally confusing, and in the second because the correct usage of the toilet is discouraged by its design.

Recently, though, I was in a pub and I saw a dual flush toilet that is entirely based on the principle of encouraging “good” behaviour (in this case only using the double flush when you need to).  Because I have not been able to find a picture of this design (and strangely enough, I don’t routinely have a camera with me in the toilet), you can see an approximation of the design at left.  This design is quite clever, in that most of the time it does not require users to understand the concept of the half flush–provided they hit anywhere on the button but the small square, a half flush is what they are getting.  This may be particularly beneficial in a pub, where judgement may be impaired, and the amount of water per flush is probably the last thing on patrons’ minds.  Basically, this design is a clever little piece of social engineering, because unless you really want a full flush (enough to hit the relatively small, off-centre target that is the full flush button), all you get is a half flush.  There is a downside to this, though–it could make life harder for users with fine motor control impairments, for example tremors, Parkinsonism (or even drunkenness, ironically).  The fact that this design relies on the small square being pushed as well as the big circle (as opposed to instead of it) makes this somewhat less problematic, but users having to flush multiple times when they need a full flush (because they have poor aim) may cancel out the general benefit.

This flush design highlights the tension that can sometimes arise between what’s best for the group, and what’s best for the individual in interface design.  As to whether people do have trouble with the full flush, or whether this design really does save water, I don’t know, but it made for interesting thinking on a Friday night at the pub.  What do you think?  Would you put this flush in your house?  Is there anywhere it shouldn’t go?