Aims: Our purpose was to explore the effect of navigation instruction on older drivers' driving performanceat left turn intersections. Left turns at intersections are particularly complex because they require manyperceptive and cognitive abilities under considerable time pressure.Methods: Fifty-four participants were recruited: 18 drivers with early-stage Alzheimer's disease (AD),18 neurologically healthy older drivers and 18 younger individuals. Various cognitive processes weremeasured, and 9 left turn maneuvers with or without navigation instruction were evaluated during anin-traffic road test. The psychomotor, planning and decision-making components involved in left turnwere also analyzed closely.Results: Only older drivers (both healthy drivers and those with AD) were negatively affected by navigationinstruction during the maneuver. The planning and decision-making components were more likely to beaffected by the navigation instruction.Conclusion: This finding highlights the importance of carefully considering the use of navigation instruc-tions when developing navigation systems. Adapting this instruction is necessary to simplify ourunderstanding of the real-world driving environment and to avoid increasing the cognitive load of olderdrivers.