It is very common to compare mean values of driving performance indicators (PI) like mean speed, the standard deviation of lateral position, time headway, mean glance duration, and many more, in order to investigate possible differences between different treatment groups. Just like all the PIs mentioned here, most of them describe aspects of the control level of driving behaviour according to Michon’s control hierarchy. When means differ significantly between treatment groups, this is often interpreted in relation to traffic safety gains or losses.

In this paper we are going to discuss possible pitfalls with the use and interpretation of such performance indicators, based on examples from studies carried out in driving simulator and in the field. Finally, we suggest a number of possible solutions to avoid some of the issues discussed here.