Jul 25 2007
Flashing Yellow Arrows
Usability is a major factor in the design of any object — physical or virtual — with which a human interacts. Sometimes bad usability is just an annoyance: when it’s hard to mark junk mail in Outlook a user might be annoyed and waste some time, but they aren’t put in danger.
However, bad usability in other forms of design can actually create serious risk for the end user. On his blog User Centered, Eddie Lopez discuses a relatively new traffic innovation appearing in various cities across the United States: Flashing Yellow Arrows.
Flashing Yellow Arrows
Your correspondent first came across a Flashing Yellow Arrow in a city outside of Los Angeles called Pasadena, where it was installed in an intersection along with all new traffic controls. Among the passengers of your correspondent’s car a heated discussion ensued with your correspondent defending an alternative interpretation of the light than the other two travelers.
As it turns out your correspondent was completely wrong: a flashing yellow arrow (in the United States) indicates that a driver may take a left turn, but only after yielding on oncoming traffic.
So what is the usability flaw in this design? Ambiguity in the status indicator. Traffic engineers have to design for instant state indicator recognition. Doubt, hesitation and untimely driver confusion are the source of a good deal of traffic accidents.
Eddie makes a great point when he says “You don’t see a lot of innovation taking place in traffic signal design, that’s probably for a good reason.” This is on purpose, as non-intuitive and untested innovation in traffic engineering will lead to human casualties.
Design Flaw
So what are the major flaws with this design? The first problem is already clear: that the meaning of the indicator isn’t clear to most drivers. The second big issue is this indicator is redundant. When a driver is shown a solid green light (and not a green arrow), the driver already knows they have to yield to oncoming traffic before making a left turn.
Does adding this secondary indicator make the yield message any more clear? Not really. Take a look around when you drive: there isn’t a lot of overlap in traffic signs.
This is on purpose: clear, intuitive design shown at just the correct level to be seen by the driver, but not so much that would produce sign noise, and cause drivers to ignore important indicators.
Returning to the same intersection three months later your correspondent felt vindicated: city engineers had attached a sign to explain what the flashing yellow arrow meant.
The traffic engineers are demonstrating missing features axiom 2 (credit for this axiom goes to Don Norman): When a control requires an explanation to be understood, you need to redesign the control.
Well said- nice job.
(I do like “Abusability” as well…classic)
Thanks!
There is an entire school of traffic engineering and urban design that argues “doubt, hesitation and driver confusion” are, actually, the most reliable source of road safety. See Hans Monderman and Ben Hamilton-Baillie.
I’m startled at the thought of a traffic engineer actually advocating “driver confusion” but I’ll look up these sources.
I’ve read up on Hans Monderman and Ben Hamilton-Baillie. Interesting ideas in traffic design. However, to say they advocate driver confusion is wrong.
What they advocate is reducing the sense of ownership that traditional road design tends to extend to cars. By removing the traditional components of how a road is built a driver feels less “at home” on the road.
This really means people drive with more caution and awareness, which tends to mean they drive slower.
It’s an interesting idea but it has limited applications. Perhaps in dense downtowns this could work. But you still need highways and critical arterials.
I might be concerned about the lifespan of this approach. It relies too much on being different than what exists, as opposed to standing on it’s own merits.
Once a driver gets used to the new road, over time, will they begin to speed up and use less caution? Will comfort return the sense of driver ownership that this new approach attempts to quell? Only time will tell.
The purpose of flashing yellow arrows is to eliminate the danger called “lag trap” or “yellow trap.” see this website:
http://geocities.com/midimagic@sbcglobal.net/lagtrpqz.htm
The flashing yellow arrow provides a needed display which is unavailable without it. This is a display that allows the left turn to continue to be permissively made through gaps in incoming traffic while the straight-ahead light for the same approach is red.
With the flashing yellow arrow, a cure for lag trap is possible. This cure does not rely on changing the order the signal displays its green lights in to an order that prevents progression of green lights along the length of a street.
Your software is removing the “@” sign which is a necessary part of my website address.
The link loads OK for me…?