User interface design as systems design

When designing complex systems, it is standard systems engineering practice to carefully design the interfaces between subsystems. Yet when designing human-computer systems, the interface between human and system is not usually thought through in such terms. Instead, the human is often given wide access to arbitrary parts of the system, and the result is a complex human-computer system that fails in various ways. We illustrate this argument with a case study of a public walk-up-and-use rail ticketing system. We show that the interaction imposed on the user is inappropriate to the user's task needs; we show how user interface problems arise through access to organisational conventions that are of little interest to users. Furthermore, the wide interface is beyond the resources of the rail organisation to manage. Conversely we show that an interface designed to hide irrelevant complexity (exactly as one would do approaching user interfaces as a systems engineering design problem) can have a beneficial impact on the user experience, including improving the reliability of the total system.