The ability to assess the current attentional state of the driver is important for many aspects of driving, not least in the field of automation. Knowledge about the driver’s attentional state is necessary for the assessment of the effects of additional tasks on attention, and for the transfer of control between vehicle and driver. Therefore, different methods that can be used to assess attention, were evaluated theoretically and then empirically in a controlled field study and in the laboratory.

Six driving instructors participated in all experimental conditions of the study, delivering within-subjects data for all tested methods. Additional participants were recruited for some of the conditions. The test route consisted of 14 km of motorway with low to moderate traffic, which was driven three times per participant per condition. The on-road conditions were: baseline, driving with eye tracking and self-paced visual occlusion, and driving while thinking aloud. The laboratory conditions were: Describing how attention should be distributed on a motorway, giving a written percentage distribution for a motorway situation, and thinking aloud while watching a video from the baseline drive. For the analysis the on-road data were split into manoeuvres. Attention was distributed differently depending on manoeuvre type, which was evident from both eye tracking, occlusion, the think aloud protocol and the lab-based methods, therefore it is recommended to consider the type of manoeuvre when making attention assessments. The visual occlusion method is a valuable tool to assess spare visual capacity. Especially in combination with eye tracking, and in comparison with “baseline” driving it shows which glances are experienced as containing necessary information, and which glances are “spare” glances. The think aloud method is a meaningful tool to approach the driver’s actual mental representation of the situation at hand. However, this method should be used with caution, as talking about one’s attentional distribution in fact changes one’s glance behaviour in comparison to baseline driving. Expert judgements in the laboratory did not turn out to be a reliable and useful method for the assessment of drivers’ attentional distribution in traffic. This may be due to difficulties in verbally accessing procedural knowledge.

For successful attention assessment in a dynamic traffic situation it is important to have access to information about the manoeuvres made by the driver in relation to other vehicles on the road. Also, knowledge about the road layout, speed limit etc. should be incorporated into the assessment. All this requires a rather advanced instrumentation of the experimental vehicle. In addition, data reduction, analysis and interpretation are demanding. To summarise, driver attention assessment in real traffic is a complex task, but a triangulation of visual occlusion, eye tracking and thinking aloud is a promising combination of methods to come further on the way.