Brain Activity Involved in Vehicle Velocity Changes in a Sag Vertical Curve on an Expressway: Vector-Based Functional Near-Infrared Spectroscopy Study

A sag vertical curve on an expressway, where a downgrade changes to an upgrade, often causes reduced vehicle velocity that results in traffic congestion and occasional accidents. This study conducted an experiment on an actual expressway. The experiment used functional near-infrared spectroscopy (fNIRS) to measure the impact of a sag on driver brain activity while driving. fNIRS provides real-time monitoring of localized hemoglobin concentration changes in the cerebral cortex and can detect brain activity by calculating cerebral oxygen exchange. Cluster analysis of vehicle velocity over 965 m from the sag point revealed a constant velocity group (69.7%) and a reduced velocity group (30.3%) with significant velocity reduction [–9.8 ±6.5 km/h (p = .01)] in the first 425 m. Brain activity in the constant velocity group increased significantly in the parietal association cortex (PAC) and the supplementary motor area (SMA) (p < .05). In the subsequent 450 m, vehicle slowdowns gradually disappeared, and PAC activity in the reduced velocity group began to increase followed by increased prefrontal cortical activity. These findings suggest the possibility that the presence or absence of activity in the PAC, which is responsible for visuomotor coordination, spatial perception, and attention, influences differences in vehicle velocity reduction. The simultaneous activation of the PAC and SMA may indicate that the motor-related functions were activated on the basis of the driver's perception of the road environment and vehicle speed. This possibility suggests that traffic safety measures that stimulate the PAC to better awareness may be effective in reducing slowdowns after a sag.