In making driver workload assessments, it is important to evaluate the driver's level of brain activity because the operation of a motor vehicle presumably involves higher-order brain functions. Driving on narrow roads in particular probably imposes a load on the driver's brain functions because of the need to be cognizant of the tight space and to pay close attention to the surroundings. Test vehicles were fitted with a functional near-infrared spectroscopy (fNIRS) system for measuring bloodstream concentrations at 32 locations in the frontal lobe of the participating drivers in order to evaluate their levels of mental activity while driving on narrow roads. The results revealed significant increases in cerebral blood flow corresponding to the perceived workload. This suggests that increases in cerebral blood flow can be used as an effective index for estimating mental workloads. Additionally, a comparison made of narrow road driving using right- and left-hand-drive vehicles revealed different tendencies due to differences in the direction of the subjects' attention. If greater attention was allocated in the leftward direction, the right lateral portion of the frontal lobe was predominately activated, while the left lateral portion was activated when greater attention was allocated in the rightward direction. This finding suggests that fNIRS measurement of frontal lobe activation might be used as an index of the driver's allocation of attention in the right and left directions, in addition to its use in estimating mental workloads.