ArcNews Online

Spring 2006
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Gwinnett County, Georgia, Uses GIS for Traffic Analysis

A growing suburb of a city known for traffic snarls doesn't want to mess around with its traffic analysis systems. It wants a system that takes its existing data and applies analysis that will help it improve the safety and function of its roads.

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Example of a midblock diagram created that shows aerial photos and road centerlines.

Until recently, Gwinnett County, Georgia, did not have the capability to truly analyze traffic accidents. It was using a traffic diagramming system that did not incorporate the spatial orientation of the roads where the accidents occurred. The problem with the old generic system is that not every intersection consists of two roads that come together at a 90-degree angle. If this were the case, any generic diagramming solution would work. But accidents occur in the real world where roads intersect at irregular angles and have curves. Gwinnett County is no exception. As part of the expanding Atlanta metropolitan area, the county has a complicated transportation infrastructure.

When analyzing traffic accidents, transportation engineers must have a clear understanding of the contributing factors. How would analysts know if a road angle contributed to the accident if every intersection is plotted as a generic four-lane, 90-degree intersection? How would analysts know if the sun could have been in the driver's eyes or if a tree was blocking a driver's view? They wouldn't. They would have to know the spatial orientation of the intersection or drive to the site.

In addition, the county had no way to plot midblock accidents. Midblock accidents are classified as all accidents that occur between intersections. The most common midblock accidents occur when someone pulls from a parking lot onto a busy street. The old method of diagramming midblock accidents meant running database queries and drawing the diagrams by hand. This process could take hours, depending on how many accidents occurred at a specific location.

Despite having an established GIS program, the managers at Gwinnett County understood that they needed some expert guidance in developing a GIS-based traffic-analysis program. Following extensive investigation, the county picked Esri Business Partner Burns & McDonnell Engineering of Kansas City, Missouri, to help develop a real-world solution for collision diagramming.

The county's new consultant then applied its ArcGIS Desktop (ArcView) extension TrueTraffic, which produces spatially oriented diagrams of accidents where they happened, whether at intersections or midblock.

The extension works by querying the county's Oracle database for all the accidents that have occurred at a selected intersection or midblock location. The data stored in Oracle contains all the characteristics of an accident, including accident type, vehicle direction of travel, street names, vehicle maneuvers, and more. The application then analyzes each accident and places it in the appropriate location on the map. The map layout view in ArcView can include any additional features that the analysts want to evaluate. These features might include edge of pavement, road centerlines, signs, or aerial photos.

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Example of an intersection diagram that was created for 2001-2004. Accident symbols are colored by year. In addition to the road centerlines, edge of pavement is shown.

Says Martin Bretherton, traffic studies engineer for Gwinnett County, "The county is no longer limited by a simplified schematic that doesn't accurately represent the real world of curves and angles in streets."

The ArcView extension takes into account the data obtained by the county by incorporating it with the diagrams, including aerial photos. The county is already seeing benefits from the extension's ability to transform its existing data into a more useful and reliable resource. The extension also helps improve the data in the county's system by flagging invalid accidents that are missing crucial details or have incorrect spatially oriented values.

"On a generic diagram, you can't see contributing factors—pavement edges, signage, curves, or blind spots," says Derrick Beasley, project manager in the Burns & McDonnell Business & Technology Services Group.

TrueTraffic performs complex spatial algorithms against actual Gwinnett County road directions and angles to ensure accidents are correctly entered into the transportation engineers' database, such as a report describing an accident in which a driver was going north on a street that actually runs east and west at the accident location. The system flags such anomalies for county correction and does not allow any accident to be incorrectly represented. Once identified, the accidents can be quickly corrected and diagrammed again. Correct interpretation of the data is key to Gwinnett County's analysis of traffic accidents. Furthermore, the program runs on numerous workstations throughout the county and allows Gwinnett County to address the safety concerns of its citizens by giving officials a clearer picture of where accidents happen and what might be the contributing factors.

For more information, contact Barry Puckett, GIS manager, Gwinnett County (e-mail:, tel.: 770-822-8031); Derrick Beasley, project manager, Burns & McDonnell Engineering (e-mail:, tel.: 816-822-4363); or Jim Trimble, client coordinator, Burns & McDonnell Engineering (e-mail:

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