Intersection Angle Geometry and the Driver’s Field of View

Abstract

At a skewed-angle intersection with the acute angle to the right of a driver on the minor roadway (the approach required to stop or yield), the vehicle body may obstruct the minor-road driver’s line of sight. Body parts that may obstruct the line of sight include the door frame, a panel aft of the door, or the cargo box of a single-unit truck. If the combination of vehicle design and intersection skew angle restrict a driver’s line of sight to the right, safety may be compromised. This research project evaluated obstructions caused by the bodies of a number of vehicles. The researchers measured the angles at which drivers’ lines of sight were obstructed by the body of their vehicles. Two driving positions were used—“sit back” and “lean forward.” A 13.5-degree vision angle (with respect to a line perpendicular to the vehicle path) was selected to represent an intermediate posture between the sit-back and the lean-forward positions. With a 13.5-degree vision angle in some restrictive vehicles, the 60-degree minimum intersection angle allowed by A Policy on Geometric Design of Highways and Streets (“Green Book”) will cause the driver’s line of sight to be obstructed by the vehicle itself and offer only limited sight distance. When the acute angle is to the minor-road driver’s right, minimum angles of 70 degrees or more may be more appropriate, depending on the through-road speed.