Affordances for Garbage Cans

Here some pictures of Japanese garbage cans, taken in the streets and subway stations of Tokyo. Intuitively, the holes on the cans are shaped and sized to let only some type of objects pass through, enforcing with physical affordances what the labels indicate. So you can easily spot round shaped holes to suggest that only bottles are allowed in that particular container, or long holes to presumably accomodate paper.

These slots not only helps people to quickly recognize the type of garbage contained in a specific trash can (it works especially well if you cannot read Japanese and would have to guess or look inside the can to know what they contain), but also it works as a simple physical constraint to enforce specific behaviors. This is a good example of Shingo's Poka-Yoke design principle, which -in the words of wikipedia- has the "purpose to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur". In sums, the old mantra of the idiot-proof interface.