A Signal Timing Primer

Guest post by Jonah Finkelstein, EIT Spack Consulting

One of the services offered at Spack Consulting is Traffic Signal Timing and Tune-up. We often receive questions from clients and other engineers regarding what our timing and tune-up entails. While there is a lot written on this subject, the team of engineers in the office thought this would be a good opportunity to share our process and best practices with Mike On Traffic readers. This article is geared to those who are just getting started with signal timing projects, but more seasoned professionals who haven’t worked with signal timing in a while may also find this primer interesting. The process of a signal timing project can be divided into four overarching sections. We’ll unpack those and explain each piece of the process.

1. Collecting and Reviewing Reports.

The first step in a signal timing project is collecting and reviewing the signal related documents the city or client may provide. There are two primary documents that a city, or client generally provides:

 Signal Layout

This document is one that shows the physical characteristics of the signal being retimed. The signal layout sheets include:

1. Signal head locations and type.
2. Lane configuration.
3. Detector location and operation.
4. Controller phasing and basic operation.

This information is necessary for proper modeling of the intersection and can save a lot of time spent in the field collecting         this information manually.

 Timing Report

The client may also provide you with the current signal timing. This document includes:

1. Minimum and maximum green times.
2. Yellow clearance time.
3. All red time.
4. Pedestrian timings.
5. Detector delays and operation.

The phase split times and offset values are also included, when the signal is running coordinated.

These documents, when provided, allow accurate modeling of the existing signal and give a good base from which to start the     new . However, in some situations these documents are not available. When this is the case the information needs to be pulled   directly from the signal controller through the use of programs such as Aries or manual through the signal controller monitor.

2. Data Collection and Analysis.

After gathering as much signal information as possible our engineering team analyzes the intersections. A 24- or 48-hour turning movement count at each of the intersections is collected and analyzed to create an accurate and up to date volume representation from which to build new timing.

From the turning movement data collected, we then created a volume profile. This profile allows us to split the day into segments, based on the changing volume that passes through the intersection throughout the day. By being able to see different volume peaks throughout the course of a day, we are better able to create timing plans that service the volumes needed as they change throughout the day.

Spack Consulting Tip: We recommend collecting 48-hour turning movement counts over 24-hour counts. By collecting two days of data, a better representation of the traffic going through the intersections on a “typical day” can be determined through averaging the two days and smoothing out any irregularities that may have occurred on a single day.

3.  Create a New Timing Plan.

Once the collected data has been organized and reviewed, we then move into creating new timing plans. The goal when creating a plan is to move the largest number of vehicles through an intersection using the shortest cycle lengths possible.

Our first step is to create an accurate model of the existing signal using the information collected in the signal layouts and timing sheets. Then, with the volume periods determined from the profile sheets, we create a master volume sheet. This sheet lays out each volume period of the intersection and assigns an analysis volume to use within our model. The volume periods taken from the profile sheets often include the AM and PM peaks, the Bal low (which is the midday low value), and the Bal peak (which is the midday peak value), however each intersection will have unique volume cases.

This volume information is then input into separate models to create a complete and accurate existing conditions model to begin retiming with.

The phase timings are determined individually for each movement with the highest efficiency and short cycle length in mind. We start with updating the timing for the yellow, all red, and pedestrian walk and don’t walk timings, based on MnDOT (Minnesota Department of Transportation) standards. With the updated yellow, all-red, and pedestrian timings (YARP) completed the green times can finally be adjusted. With the goal of supplying the most green time possible to each phase, we work towards finding the optimal cycle length.

Spack Consulting Tip: Most signal modeling software has built in optimization tools, which should only be used as a starting off point for signal optimization. Through manual adjustments a more efficient signal timing can be determined

4. Optimizing the Retiming.

The final step in the retiming process is optimization. Rather than simply relying on the program to optimize the timing plan, we look at the existing vehicle movements through intersections and adjust signal timing minutely to find the optimal timing for all phases. By personally looking at the timing and making adjustments, we are able to change timing and vehicle movements by seconds or less. While it may not seem like adjusting a phase timing by a few seconds would make a difference, when adding the combined delay of all vehicles sitting at red lights throughout the day, those few seconds can add up very quickly. By adjusting the timing, we are saving hours of unnecessary time spent waiting at an intersection as well as lower emissions and most importantly decreasing driver frustrations (that’s right, traffic engineers make the world a happier place, or at least that’s what I tell myself).