U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The figure was taken from the Manual on Uniform Traffic Control Devices. It illustrates the maximum horizontal distances that signal faces can be from the stop line. At least one and preferably both of the signal faces shall be located between the two lines intersecting with the center of the approach at a point 3 m (10 ft) behind the stop line, one making an angle of approximately 20 degrees to the right of the center of the approach extended, and the other making an angle of approximately 20 degrees to the left of the center of the approach extended. Using the cone of vision specified by the 20 degrees to the left and right of the approach extended, the figure provides the following specifications: within 35 m (120 ft.) use 200 mm (8 in.) or 300 mm (12 in.) signal lenses; within 45 m (150 ft) use 300 mm (12 in.) signal lenses, unless a near-side signal face is used; within 55 m (180 ft.) use 300 mm (12 in.) signal lenses.