A distracted driver behavior typology: The role of regulatory environments

Distracted driving is a recognized pedestrian safety risk, yet behavioral heterogeneity within distraction periods remains poorly characterized. We analyze 39,271 one-second observations from 720 naturalistic crosswalk traversals (162 drivers, SHRP2 NDS, 2008-2013) using a Hidden Markov Model (HMM) and Gaussian Mixture Model (GMM) pipeline. Five kinematic states form a near-sequential deceleration cascade, with nondriving-related task (NDRT) engagement concentrated in low-demand states. Monte Carlo cross-validation identified two driver profiles distinguished not by distraction frequency, which differed only negligibly once signal-phase exposure was accounted for, but by regulatory responsiveness: the degree to which approach speed responds to traffic control. The Regulated Tasker (27.7 km/h aggregate approach speed) adjusted speed with the regulatory environment; the Unregulated Tasker (43.9 km/h) responded about a third less strongly. A mixed-effects model on all 162 drivers confirmed that drivers differ significantly in regulatory responsiveness (p=0.024) and that task-active periods were slower, not faster, within driver. An event-triggered analysis of 395 NDRT onsets showed that speed change temporally preceded task engagement, consistent with demand-drop task timing; task-active seconds disrupted the orderly deceleration cascade and shifted initial deceleration onset 102.8 m closer to the crosswalk. The infrastructure-resistant subtype (Unregulated Tasker) is the safety-critical finding: its risk appears less amenable to signal-based control, suggesting physical speed management as a candidate countermeasure to be tested.