Managing in-vehicle distractions: evidence from the psychological refractory period paradigm

Driver distraction by in-vehicle tasks has a negative impact on driving performance and crash risk. This paper describes a study investigating the effect of interacting with a surrogate in-vehicle system task -- requiring a two-choice speeded response -- in close temporal proximity to a subsequent lead vehicle braking event. The purpose of the study was to determine the 'task-free' interval required before a braking event to ensure safe braking performance. Drivers (N = 48) were split into six groups and randomly assigned an in-vehicle task defined by stimulus (three levels) and response modality (two levels). Four blocks of intermixed single- and dual-task trials were presented. The time interval between the two tasks was varied on dual-task trials. Slower braking responses on dual-task trials relative to single-task trials indicated dual-task interference. Driver braking performance demonstrated the psychological refractory period effect -- an increase in reaction time with decreasing temporal separation of the two tasks. The impact of in-vehicle task stimulus and response modality on performance is discussed in relation to predictions based on Multiple Resource Theory. This study demonstrates a fundamental human performance limitation in the real-world driving context and has implications for driver response speeds when distracted. Specifically, the presentation of an in-vehicle task in the 350 milliseconds before a braking event could have severe safety consequences. The use of the findings to manage in-vehicle stimulus presentation is discussed. Problems with implementation of the results are reported.