Combining roadside interviews and on-road observation for assessing prevalence of driver inattention

•We combined on-road observations from a driving car and roadside interviews.•Using a handheld cellphone was the most frequently observed secondary activity.•Observed prevalence of secondary activities was lower for drivers with passengers.•Interviews revealed high prevalence of cognitive distraction, e.g., daydreaming.•Based on both methods, inattention prevalence was estimated at 24% of driving time.