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Visibility and audibility of safety-critical functions

This pattern serves as a guideline for the display of safety-relevant functions.


Information considered safety-relevant is not displayed, or is covered by other user interface components, or an audible alarm cannot be heard, or can be mixed up with similar alarms.


This pattern adds an additional design constraint. A possible consequence is to avoid dialogs cover up safety-relevant parts of the interface.


The solution describes guidelines for the design of user interface components and audible signals.

  • Identify the safety-relevant parts of the interface. In the case of the voice communications terminal, these are the call queue, the role bar, and the status bar.
  • Run the application in exclusive full-screen mode so no other application can be on top of it. If other functionality is required it must be integrated into the full-screen application.
  • Open dialogs only in fixed size on fixed positions and ensure they do not obscure any safety-relevant parts of the interface.
  • Display all types of notifications in designated areas. Do not use modal dialogs.
  • User interface components must be designed such that it is not possible for the user to cover up safety-relevant parts of the interface with the own hand.
  • Audible alarms must be designed such they fit consistently into the entire scheme of audio signals used in the control room. Avoid to use the same signal for different types of alarms.

Safety impact

Using this pattern consistently reduces the probability of errors caused by misreading screen displays or even missing important events.

Usability impact

This pattern increases avoids misinterpretations when reading the screen, and therefore, reduces error probability.


Has been used in several European railways voice communication devices.

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