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Consistency and Death

Posted 2012-03-24 Tags: None

This post is not about taxes.

There are a few key concepts that people rely on with a user interface and one of them is consistency. Consistency is one of the most important parts of a UI, and trumps practically any other; we want things to behave in a way that we are used to. This makes it easier for us to use things. Doors, for instance, have the same general UI in most cases; there is a handle, some hinges, and a big solid part that you can't walk through until the handles and hinges have been dealt with. This is pretty standard, and we understand it and deal with it, even though it's not the most user friendly way of dealing with changing state from room to room. A door is a pretty ho-hum kind of example though, especially when I have already insinuated that death is on the line in the title.

Let's talk about traffic lights.

Traffic lights work in a fairly consistent fashion here in Ontario. I would say that 99% of the traffic I encounter on a daily basis follows these patterns and it's important to note that this is not an optimal traffic pattern, but it is certainly sufficient, and one we are all used to. There are corresponding pedestrian signals; when one has a red light, one will expect pedestrians to pass perpendicular to your direction of travel, and while green, parallel. Here's a bit of a chart.

My traffic signal

Pedestrians perpendicular to me



Advanced Green (left turn only)

Don't Walk


Don't Walk


Get ready to Walk.

Here's where I encountered a problem.

While driving the other day on the Hanlon in Guelph, I was waiting to turn left at an intersection that has recently undergone construction. There are now two left turn lanes, and the flow is generally better. However, I almost saw a pedestrian die, because the UI changed dramatically from the norm. When you are waiting to turn left, the pattern that you experience is like this: Red -> Green -> Advanced Green (left turn only - traffic parallel but opposite gets an amber) -> Amber. This is a simple change, but it made a big difference to the guy who almost got creamed by the Peterbilt that was roaring down the expressway. He was watching our signals, because he understood that there is a correspondence between those signals and his signals. He was watching the lights of the traffic coming towards me, and when it turned red, he made the assumption that he had a walk sign, because in the table above, that's how it goes. That's what he's used to seeing.

However, the traffic my way did not have a red light; we had an advanced green, and the huge truck narrowly missed this fellow. There was much squawking of horns, and the guy ended up jumping out of the way; the truck squealed around to the right, and almost went off the road. It was a horrendous mess, and it almost ended up with an innocent guy, with a fair understanding of patterns, getting dead.

The lesson that I took away is this: if something is consistently done in a particular way, even if you think that you might have a better way to do it, consider long and hard all the ramifications of any changes you might make.

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