There are several exits in the I-93 tunnel that either take you back to the surface or into another tunnel leading to the airport. As I drove into the tunnel my car radio went silent and an information broadcast from the OCC took over. A crystal clear voice said that exit 22 was under construction and that drivers should take exit 23 instead. The entire FM and AM spectrum was overridden with the announcement. What was really impressive, though, is that the normal radio program came back once I had passed the dysfunctional exit 22. I was still driving in the tunnel, but since the broadcast was no longer relevant to me, the OCC returned the control of my radio.
Adapting Jakob Nielsen's usability heuristics to what happened, you could say the OCC did a good job in several areas:
- Simple and natural dialogue. Dialogue should not contain information that is irrelevant. Every extra unit of information competes with relevant units of information and diminishes their relative importance. All information should appear in a natural and logical order. When a driver passes the exit, the message is no longer relevant to them so don't broadcast it.
- Prevent errors. Even better than good error messages is a careful design that prevents a problem from occurring in the first place. The message prevented motorists from trying to take the exit, which in turn prevented delays and kept the traffic flowing.
- User control and freedom. Users often choose system functions by mistake and will need a clearly marked "emergency exit" to leave the unwanted state without having to go through an extended dialogue. No pun intended, in this case the "emergency exit" was exit 23, the secondary route recommended in the broadcast.