Perceived mental workload, trust, and acceptance resulting from exposure to advisory and incentive based intelligent speed adaptation systems

A 2010 field operational test completed in the United States (US) used an advisory level Intelligent Speed Adaptation (ISA) system coupled with a modest cash incentive to reduce speeding. Each participant drove an instrumented vehicle for a four week period in a naturalistic setting, with the beginning week and final week being baseline periods. The ISA system and incentive were activated for some participants, depending on assignment to experimental conditions, during the middle two weeks of the trials. Driving with the systems, particularly the incentive component, led to a significant reduction in the percentage of time speeding over the posted limit (these results are reported elsewhere). At the end of each week of driving, participants provided ratings of perceived mental workload and completed a “Trust and Acceptance” rating scale after experiencing the incentive and speed warning systems. This paper documents the results of the workload and trust data. As expected, the incentive condition was associated with increased mental demand, temporal demand, frustration, and effort. Unexpectedly, the speed warning did not reduce mental workload of drivers in the incentive condition compared with the incentive only condition. Also counter to our predictions, drivers who experienced the warning without the incentive did not indicate increased mental demand or temporal demand. Trust and acceptance ratings were generally positive for both systems, although the auditory component of the warning was rated unfavorably. Participants who experienced the incentive system rated the speed warning system as less trustworthy than participants who did not experience the incentive, and this finding may partially explain the lack of a reduction in mental workload for participants in the incentive + warning compared with the incentive only condition.