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Date: Nov 5, 2013

aerial image of a Towson intersectionKris Nebre
Engineer I, Public Works, Bureau of Traffic Engineering

Around 350 traffic signals are maintained by the Baltimore County Bureau of Traffic Engineering. Part of maintaining the signals is determining and programming signal timings to ensure that there are appropriate green times for the assigned movements so that traffic may flow efficiently. Traffic engineers determine the signal timings with assistance of a computer program called Synchro.

Synchro is a computer program that calculates the Level of Service (LOS) of an intersection based on traffic volumes, lane geometry and signal timings. Traffic volumes at an intersection are collected on one-year or three-year cycles depending on the severity of congestion. This traffic intersection data can be viewed on the County’s web site. The most heavily congested intersections are counted every year.  At the same time, traffic engineers also verify the lane geometries and signal timings. If there is a proposal to change lane geometries, such as adding turn lanes or installing a roundabout or traffic signal, Synchro can determine if the proposed changes will improve the traffic flows at the intersection.

Because growth is natural and expected in metropolitan areas, traffic signal timings need to be revised from time to time to accommodate the change in traffic demands based on increases that are apparent from the traffic counts. Synchro is used to determine if signal timing changes are needed.  However, achieving the lowest delay at a particular intersection does not necessarily represent the optimal signal timing, especially at coordinated signals, because the main objective is to move traffic through the corridor.  In coordinated signal systems there is more green time given to the mainline traffic.  Synchro can analyze a series of coordinated intersections and plot out a graph that shows how traffic flows from one intersection to the other. In addition, Synchro has an auxiliary program called Sim Traffic that can help visualize traffic flows.

Sim Traffic is a simulation software that takes data parameters from Synchro and puts them in an animated presentation. Roads and intersections are laid out in relative scale in top view, with simulated cars running through the intersections. Because flows from signals in a coordinated system are dependent on one another, it is good to see in action how the queues dissipate and move through the corridor, and where congestion tends to build up. Like the applications used in Synchro, we can also see how different lane geometries and signal timings affect the flow of traffic.

We can also make a comparison on the effectiveness of a proposed signal or roundabout at an existing intersection. This is very helpful because the animations visually show whether the changes create more congestion or improve traffic flow which can be very helpful when explaining traffic engineering decisions to the public. 

Although Synchro and Sim Traffic are very helpful in determining timings, they are not a substitute for field observations.  Ultimately, a field observation is necessary to verify improvements from timing changes, and further adjustments to timings will be done based on field observations. Synchro and Sim Traffic are not substitutes for engineering judgment; however they are great tools in the traffic engineer's tool box.


Revised April 6, 2016