Turn signals mounted on exterior rearview mirrors are increasingly being used as original equipment on passenger cars and light trucks. The potential for mirror-mounted turn signals (MMTS) to improve the geometric visibility of turn signals is examined in this paper. A survey of U.S. and UN-ECE regulations showed that the turn signals of a vehicle that is minimally compliant with U.S. regulations are not visible to a driver of a nearby vehicle in an adjacent lane. Measurements of mirror location and window geometry were made on 74 passenger cars and light trucks, including 38 vehicles with fender-mounted turn signals (FMTS). These data were combined with data on driver eye locations from two previous studies to assess the relative visibility of MMTS and conventional signals. Simulations were conducted to examine the potential for signals to be obstructed when a driver looks laterally through the passenger-side window. With a vehicle population that is fifty percent light trucks, MMTS are visible 52 percent more often than FMTS in this scenario. Based on the lateral visibility analysis, the optimal height for geometric visibility of a signal in the adjacent-vehicle scenario is 124 cm above the ground, approximately the average height of a light truck mirror. A plan-view eccentricity analysis showed that an MMTS is much closer to the viewing driver’s forward-directed line of sight than rear-mounted turn signals when the viewing driver’s vehicle is in or near the blind zone between the signaling driver’s mirror and direct peripheral fields of view. MMTS can improve the geometric visibility of turn signals in the adjacent-vehicle scenario that is believed to precede many lane-change/merge crashes. Further research will be necessary to determine if improved geometric visibility is accompanied by improved detection of turn signals by viewing drivers, and whether improved detection of turn signals has important safety implications.