The rapid development of autonomous vehicles promises to integrate them into our current transport systems in the near future. This development will lead to a completely new traffic environment in which connected and autonomous vehicles (CAVs) will need to interact with other road users. To enable this interaction, external human-machine interfaces (eHMIs) have been shown in several studies to be highly beneficial for the trust and acceptance of CAVs.
However, it is shown that harmonisation of eHMI signals is necessary, as the signals developed are extremely diverse and sometimes even contradictory. Therefore, this publication proposes clear principles for the design of eHMI signals. Important factors are taken into account, such as the way a CAV needs to communicate with other road users in different traffic situations. The authors present a total of 17 principles to ensure the safe and efficient use of eHMIs in a systematic and application-oriented way.