Two field studies were conducted on public roads to measure driver head movements while using the left outside passenger car mirror. The first study measured the effects of mirror width in the presence or absence of overtaking traffic. Driver head movements during left lane-changing maneuvers were recorded from a lead vehicle equipped with a motion picture camera and a telephoto-zoom lens. Results showed that, in addition to the head turning motions, the drivers made on the average about 2.0 inches of lateral head movements while using one of the four left outside mirror sizes which ranged in width from 2.3 to 10.6 inches. The drivers were also found to make larger lateral head movements when no other vehicles were present in the mirror as compared to when an overtaking vehicle was present. The second study was conducted in no overtaking traffic with one mirror width and used an improved photographic technique. Head movements during left lane changing maneuvers were recorded by a motion picture camera which was mounted in the back of the subject's car and which photographed the image of the driver's head and eyes in the left outside mirror. The data of 25 drivers using a 5.1 inch wide left outside mirror showed that drivers moved their head laterally from 0.4 to 5.0 inches, with a median of 1.5 inches, after turning their heads to search in the mirror. In both field studies, large individual head movement behavioral differences were observed.