The myth of ‘low cognitive load = low risk’: Quantifying interactive effects of phone use modes and cognitive demand on driving safety

Although many studies have investigated the safety of hands-free and handheld phone calls, the levels under different cognitive demands—particularly their interaction with call modes—remain unclear. Using a driving simulator, this study tested 45 drivers (37 valid) in rear-end scenarios under various call modes and cognitive demands (baseline, hands-free/handheld with low/high cognitive load). Behavioral variables were analyzed across conditions, and K-means clustering classified risk levels. A binary Logit model identified factors affecting rear-end risk, and three car-following models (FVD, Gipps, IDM) were calibrated. Results show that drivers compensate for handheld with high cognitive load by increasing forward spacing (FBD increased 34% versus baseline). Handheld with low cognitive load raised rear-end risk by 5.4%, whereas hands-free and handheld with high cognitive load showed no significant effect, highlighting the overlooked risk of low-load handheld use. FVD outperformed IDM/Gipps in simulating phone-distracted driving. Furthermore, the calibrated parameters exhibit trend variations that correlate with changes in the behavioral variable FBD. These findings enhance our understanding of the combined impact of phone use mode and cognitive load on driving safety, thereby providing practical implications for the formulation of relevant safety policies.