The Phantom Menace, or the Crossing that Never Was…


A false affordance causes a real hazard

The picture above is of a speed hump near where I work. As you can see from the the picture the edges of the speed hump have also been piano keyed to provide an indication to drivers. On first examination one might think that this was a reasonable visual warning to drivers who miss the associated traffic sign.

Well maybe not, notice the pedestrian fences? Now ask yourself the next logical question why were these fences put up?

The answer?

Unfortunately the piano keys also provide a false affordance because they resemble the more traditional pedestrian crossing stripes and therefore encourage people to cross with the expectation that they have right of way.

…the term affordance refers to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used. … Affordances provide strong clues to the operations of things. Plates are for pushing. Knobs are for turning. Slots are for inserting things into. Balls are for throwing or bouncing. When affordances are taken advantage of, the user knows what to do just by looking: no picture, label, or instruction needed.

Norman 1988, The Psychology of Everyday Things, 1988, p.9

This effect is so strong that even the fact that the speed hump edging is saw toothed and does not extend fully across the hump is not enough to dissuade pedestrians that ‘this is NOT a crossing…’.

My guess is that this is such a powerful false affordance because crossing safety is an ingrained element of western education, and as a result ‘cross at the crossings’ is an almost instinctual response in most of us.

There are two ways to eliminate this false affordance, in both cases using the principal of separation. The first would be to cognitively separate the hump symbol from that of the pedestrian crossing i.e. use a distinctly different shape and/or color for the hump symbol. The second would be to physically separate the symbol from the hump so that the perceived geometry of a crossing is not present.

There is a lesson here about how strong cultural cliches can be and how dangerous it can be to mess with them if you don’t know what your doing.


1. In other countries such as the UK and US speed humps are marked using diagonal stripes or triangular teeth.

2 responses to The Phantom Menace, or the Crossing that Never Was…


    I wonder how much of that false affordance is real. My guess would be that those barriers have as much do with lawsuits as it does with the affordance people actually see in the situation. Phased differently, how much of the design logic is based upon what the designers think that people think as opposed to what people actually think.


      Matthew Squair 12/08/2011 at 3:37 pm

      Good question! The traffic authorities have made the situation even more ambiguous by combining pedestrian crossings on top of a speed hump. As I’ve seen people actually walk out on these without looking I would say quite real, and much more likely for younger children. See the link for one council’s obvious concern. There’s a rule of thumb about interface design that goes, “when you have to start issuing instructions on ‘how to use’ there’s usually a deficiency in your interface design”, that’s probably an appropriate observation here. 🙂

      Y’know fundamentally I can’t believe that the Australian designers elected to use a road marking so cognitively close to that of a pedestrian crossing. Even using the diagomal white stripes used in the US or the UK’s triangles would be better. Using one symbol so close to that of another and then compounding the error by placing them together in other situations is just, er, well.. dumb.