Virtual/mixed reality 3D models of real-world environments can be used to run behavioural and other experiments with real human subjects, replacing the traditional approach where studies are conducted in physical environments. Use of the virtual/mixed reality environments can minimize problems related to feasibility, experimental control, ethics and cost, but care must be taken to ensure that the environments are immersive and create “suspension of disbelief”. In this position paper the issues involved are discussed and illustrated by a 3D virtual model of an urban environment that is being used to study the role of fear in pedestrian navigation.