A BEGINNER’S GUIDE TO TRAFFIC SIGNAL TIMING
It is Monday morning, and you are trying to beat traffic on your way to work when all of a sudden it seems like the whole traffic engineering world is conspiring against you by ensuring you hit red light after red light. After a few explicit words, you begin to wonder what makes these signals act like this, who times these signals, why do we need signals?, etc. Glad you asked! Below is a very basic introduction to signal timing so that, just maybe, you have a better understanding of the logistics behind your morning commute.
In the most basic terms, traffic signal timing involves determining the sequence of operation and assigning green time to each approach at an intersection while considering time for pedestrians and other users as well. In order to understand signal timing, we have to look at some fundamentals such as cycle lengths, phases, splits, peak hour trends, pre-timed and actuated signals, optimization, coordination, and communications.
Cycle Length A cycle length is the amount of time required to display all phases for each direction of an intersection before returning to the starting point, or the first phase of the cycle. Cycle lengths are based on traffic volumes and work best within a certain range depending on the conditions of the intersection. The goal of signal timing is to find an optimum cycle length for the most efficiency. Typical cycle lengths may range from one minute to three minutes. A split determines how much time each movement gets in a cycle. The split includes the green time and the clearance interval, or the time to clear the intersection, which includes the yellow and red lights. Clearance interval times are calculated based on speed limit, intersection widths, intersection grades, perception or start-up time, and acceleration rates. Clearance intervals are often referred as the change interval, when changing from one signal phase to the next. The clearance time in that sequence is also referred to as “loss time” due to vehicles coming to a stop or starting-up and the time that no vehicles are moving through the intersection.
Pre-Timed vs. Actuated Pre-timed signal timings are timings in which the sequence of operation and splits are pre-determined based on observed traffic volumes and trends and do not change based on changes in volumes. Pre-timed signals are common in downtown grid locations with closely located intersections and one-way streets or in many downtown areas where it may not be feasible to maintain the inductance detection loops (see below!) for each signal location. Actuated signal timings can be semi-actuated or fully-actuated. In case of semi-actuated timings, only the minor street has detection whereas fully-actuated signals have detection on all approaches. This means if you pull up to a red light on a minor street of a major intersection, your car will be detected and the signal will soon change to allow you to proceed. The pre-timed signals have preset timing plans that vary during different times of the day, where as in a fully-actuated signal the green times have a minimum and maximum range that gets used based on the actual traffic on the road. Based on traffic trends, various signal timing plans can be set up in the signal controller. Fine tuning these signal timing plans is critical to their success.
Coordination Signal timing is performed on two most common types of intersections – isolated and system intersections. Isolated intersections, as the name suggests, are isolated from other signalized intersections and the signal timings at this intersection have no effect on other intersections in the vicinity. System intersections are usually closely spaced intersections and any timing changes at one intersection has an effect on the upstream and downstream intersections. Signal system corridors are typically coordinated on a time of day basis for each associated peak period. The most common peak periods are the AM, PM and midday. Typically, these peak periods are driven by traffic patterns or daily commutes by direction. AM and PM peaks may be associated with “inbound” or “outbound” traffic patterns. Midday traffic patterns are most often balanced by direction.
Detection Detection systems are critical to actuated signals, using various methods to detect a vehicle’s approach. Examples include inductive loop detectors, radar, sub pavement electromagnetic pucks, and video detections. Inductance loops are wiring that is placed in saw cuts in the pavement and run back to the traffic signal cabinet. A detection card produces a magnetic field through this wiring and detects when a vehicle is present over the loop saw cut area, which is typically at the stop for side street and mainline left approaches. There are less intrusive forms of detection, such as radar detection and video detection that typically also require less maintenance. However, over the years, standard saw cut inductance loops have proven to be the most reliable form of detection, if maintained properly.
Software Within the traffic signal cabinet is a traffic signal controller that acts as the “brains” of the traffic signal. The controller tells the signal what to run, how long to run, when to run, etc. The controller collects information from the intersection through the detection system, decides how to respond, and then tells the traffic signal lights how to operate. Currently, in the state of Georgia, the Georgia Department of Transportation and local transportation agencies are going through a statewide overhaul of the signal timing operation software. Prior to this most current software, the last time Georgia did a complete overhaul of their system was in the early 2000s. At that time, the traffic signal software was upgraded to meet the nationally accepted CALTRANS 2070 traffic signal controller standards.
TMC and Incident Management Often signal system corridors can be connected via fiber optics, copper wiring, or wireless networks to local traffic management centers (or traffic control centers) where they are monitored and controlled remotely. The same software locally operating the traffic signal controller can be set up on a computer desktop located at a traffic management center. Through remote connections, the computer can communicate directly to intersections and make remote changes to the traffic signal operation. Remote communications and signal operation allow agencies to make changes to traffic plans or patterns during special events or incidents.
Traffic signal timing and coordination is rooted in science due to the sophisticated algorithms and optimization models involved. But engineers have different preferences and options when they time a signal and there is no “one glove fits all” solution. There are many factors such as local trends and driving behaviors that cannot be articulated using science and engineering, and that is why signal timing is commonly described as an art. So next time you’re sailing through green lights, take a second and consider all the complexities that have gone into creating that blissful traffic moment!
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