Colour blindness affects a small but significant percentage of the population, mainly males. It is estimated that around 7 to 8 percent of men are red-green colour blind (the most common type of colour blindness), while less than one percent of women are. Strangely enough, I knew a girl in high school who was colour blind, but I digress…

Transit maps, as informational design, should pay attention to how colour blind users perceive them. Shown above are a few examples of transit maps which have been run through a Photoshop filter called Vischeck which simulates the effects of colour blindness. The left half of each image is a simulation of red-green colour blindness, while the right half is the standard map.

On a simple map with just a few lines, as shown in the Washington DC Metro, things aren’t usually a problem as the routes are easily distinguishable from each other.

The London Underground map does an excellent job of using contrast to differentiate between adjacent route lines, so usability is hardly impaired at all. Look at the northern Circle Line where pink, yellow and burgundy lines become grey (mid-dark), yellow (light) and black (dark) – all very distinct from each other.

The next step up in complexity is the Paris Metro map. Its subdued pastel tones actually hold up surprisingly well – again, by ensuring that adjacent route lines have plenty of contrast between them. Note also that the background colour doesn’t shift in tone at all, giving predictable results for the routes themselves.

Compare the official Paris Metro map to the unofficial one featured yesterday, and things are quite different. The low contrast colour palette used causes many of the routes to turn into very similar shades of yellow and blue, and the background colour shifts completely from green-black to blue-black. While it is certainly still possible to trace the routes, it’s definitely harder on the eye to do so. And as I said yesterday, the workaround solution of tiny “rune” markers on each line is way too small to be of any practicable use.

In the end, a diagrammatic map will almost always be usable by a color blind person, simply because the simplified form will make it easy to trace routes, but more care must be taken the more complex the system becomes. Labelling end points of lines with a letter or number may also help where there are many route colours. A simple plugin like Vischeck allows a designer to quickly gauge how their work may be perceived by those with colour blindness, allowing them to tweak their chosen colours for optimal usage by all.