Placement Design of Ramp Control Signals

Some current design guidelines specify that standard-mounted ramp control signals be placed almost aligned with the limit line. As a result, signal indications are typically placed outside of a driver's comfortable cone of vision, as required by the Manual on Uniform Traffic Control Devices (MUTCD). In this paper, the placement design of ramp control signals was analytically examined in relation to the satisfaction of a driver's comfortable cone of vision for stopped vehicles at the limit line and the satisfaction of stopping sight distance for approaching vehicles in accordance with MUTCD. Relationships between location of limit line, location of signal standard, ramp geometry, and approaching speeds were derived. Sample lookup design charts were also developed to facilitate the development and evaluation of signal placement design. A brief analysis using these relationships concluded that signal standards placed in alignment with the limit line would violate not only the comfortable cone of vision of stopped drivers but also the stopping sight distance of approaching vehicles. For a loop on-ramp with a 300-ft radius, standard-mounted signals on the left side of an on-ramp should be placed at least 22 ft downstream of the limit line to satisfy the requirements of both the stopped and approaching vehicles. In contrast, signals on the right side could satisfy only the stopped vehicles but not the approaching vehicles if placed at least 44 ft downstream of the limit line. (Left and right here are relative to the direction of travel.) Therefore, signals placed on the left side of the on-ramp curve (even with a radius of 500 ft) are more critical than those on the right side, especially when the approaching stopping sight distance is important.