  In the field of autonomous driving research, the use of immersive virtual
reality (VR) techniques is widespread to enable a variety of studies under safe
and controlled conditions. However, this methodology is only valid and
consistent if the conduct of participants in the simulated setting mirrors
their actions in an actual environment. In this paper, we present a first and
innovative approach to evaluating what we term the behavioural gap, a concept
that captures the disparity in a participant's conduct when engaging in a VR
experiment compared to an equivalent real-world situation. To this end, we
developed a digital twin of a pre-existed crosswalk and carried out a field
experiment (N=18) to investigate pedestrian-autonomous vehicle interaction in
both real and simulated driving conditions. In the experiment, the pedestrian
attempts to cross the road in the presence of different driving styles and an
external Human-Machine Interface (eHMI). By combining survey-based and
behavioural analysis methodologies, we develop a quantitative approach to
empirically assess the behavioural gap, as a mechanism to validate data
obtained from real subjects interacting in a simulated VR-based environment.
Results show that participants are more cautious and curious in VR, affecting
their speed and decisions, and that VR interfaces significantly influence their
actions.
