Last week I had my first experience with a GPS based guidance system, the NeverLost from Hertz. I was struck by some of the good UI choices and want to highlight them here. For a more in-depth analysis, see this evaluation. I will limit this entry to my current blog topic of usability vs. learnability  as it applies to the disengaged user.

The designers of NeverLost certainly had to design for both usability and learnability. Few will have the time or inclination to read the manual once they get to their rental car, and many will be first time or occasional users. More importantly though, the NeverLost must be easy to use. I'm sure the designers of NeverLost understood the possibility of their device contributing to an accident, and let that be the overriding concern in all of their choices. The NeverLost is a good example of designing for disengaged users, that is, users that are doing something else while using your software. The principles of the NeverLost design have wide applicability (e.g. software for call centers or traders).

Here are some of the basic principles they have adhered to:

1. Constraining choices rather than interrupting on errors: A good choice in any application, the NeverLost takes it to an extreme. Typing is hard (you choose letters by navigating over a pick-list), so the search interface only shows letters that will actually return a result. So, for example, if you pick the letters “H-O-L-I-D” in the Yellow Pages search, only the letter “A” appears on the pick list, because the “Holiday Inn” is the only item in their list that starts with those letters. Searches never fail to return a result, and you can correct errors immediately. Of course, a keyboard or a touchscreen might have made this easier, but they would also add to the cost.
2. Speech rather than text: Essential for this kind of system since you really should not be looking at it while driving.
3. The display is easy to glance at: While driving, the display is simply the major routes with the current one highlighted and the car showing the driving direction.  It's very easy to see that you are on the right track.  All other information is via speech.
4. Anticipation rather than waiting for input:  We made two deviations from the directions. The first was missing our exit, and the NeverLost immediately calculated a new route and let us know what it was doing. The second was getting off at an exit to make a rest-stop—in that case, the NeverLost let us get back to the route ourselves without recalculating. Both choices were exactly what we wanted, so we didn't need to fiddle with the interface.
5. Mistakes are easy to correct: The NeverLost keeps a list of the destinations you have input and lets you easily go back to them. A Cancel button lets you back out of all choices. Having these features lets the NeverLost stay out of your way most of the time, because it knows that you can revert to an old state when you need to.

All of these are good ideas for any interface, and if you imagine that your user is distracted, your user interfaces will be even easier to use when they aren't. Of course, a speech interface is not right for all applications, but sounds usually are, and can help to alert users when their attention is required.
When conducting usability tests, keep in mind the environment your users are in, and try to match it. For example, don't gather users to your pristine environment. Go to them if you want to see how they really use your software and the distractions they encounter.