The Art and Science of Traffic Signal System Design

The Art

For the traffic signal system designer, one of the first things to consider is the location of overhead as well as underground utilities. The location of the underground utilities determines where the signal pole foundation can be placed, while location of overhead utilities determines where the signal mast arm can be placed. One element of the art of design is finding the pole locations to avoid utility conflicts while satisfying the elements described below.

Ideally, signal poles are laid out in a position to be readily accessible for pedestrians and handicapped users, since the pedestrian push buttons are typically mounted on signal poles. In conditions where this isn’t possible, signal poles are located where they can best service vehicular traffic, and a pedestrian push button post is installed.  Minimizing mast arm lengths is a prime consideration in this process. Optimizing signal pole locations to minimize mast arm lengths can have a substantial impact on the cost of the signal system.  It’s worth noting that mast arms come in 5-foot increments, so specifying a non-standard length mast arm will cause the manufacturer to fabricate a customized mast arm at an increased cost.

Once pole locations are established, signal head locations can be laid out on the mast arm.  The Manual of Uniform Traffic Control Devices (MUTCD) establishes criteria for head location and spacing.  Signal head configuration is dictated by the number of lanes approaching the signal system as well as the selected traffic movements of the signal system.

The Science

Once signal head locations are determined on the mast arm, the science of signal design enters the picture for determining the size of the pole foundation. Selecting the foundation diameter and depth is a function of soil conditions, design wind speed, pole height, mast arm length, the number and offset of the signal heads, type of lighting unit (if any) attached to the signal pole, and the number, location and area of the signs mounted on the mast arm. Fortunately, the Washington State Department of Transportation (WSDOT) has made a science of this task and has engineered foundation dimensions for standard pole loading and soil conditions.

The science in design also includes determining wire size to energize the signal, conduit size and number to contain the wiring, and type and number of junction boxes for the connecting to conduit. Through the years, we have developed spreadsheets to assist us with this process. However, it is still an exercise in scrutiny to ensure that everything is properly accounted for in the design.

Traffic signal design is usually an interactive process, and there is often more than one layout that will fit the site. Coordination with the client at an early stage of development is an important key to ensuring that the art and science of the design meet the client’s expectations.

A Signal System Design Solution

At the intersection of 44th Avenue and 214th Street in Mountlake Terrace, Wash., safety and delay issues were present due to peak hour commuter volumes associated with the nearby Mountlake Terrace High School. The City hired Reid Middleton to design a traffic signal system to help alleviate the congestion.

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Some of the challenges that needed to be addressed included existing utilities that required relocation of a utility pole and fire hydrant to allow for placement of a new traffic signal pole at the southeast corner of the intersection. However, even the relocated pole meant that the overhead lines would still be in conflict with a pole that had a luminaire mast arm. Coordination with the pole owner resulted in mounting luminaires on the relocated pole to address the overhead conflict. The project installed new ADA compliant access ramps, resulting in the installation of separate pedestrian poles for each of these ramps.