DOT - Traffic FAQ

Traffic Engineering FAQ

How does the Flashing Yellow Arrow Traffic Signal work at Cooper Lake Road and East/West Connector?

What are the responsibilities of Traffic Engineers in Cobb County?

What are traffic control devices?

What is the MUTCD?

How are speed limits established on County Roads?

Will a lower posted speed limit slow traffic?

What exactly are rumble strips and could they help slow traffic?

When should a traffic signal be installed?

How do traffic signals work?

How many traffic signals are there in Cobb County?

How do I request a traffic signal or report a traffic signal malfunction?

I see video cameras at some intersections around the County; what are they for and does the County record and keep images?

Why does the County not operate traffic signals in flash late at night or in light traffic times?

Can a flashing light be added to improve roadway safety?

What is a modern roundabout?

Can stop signs be placed on streets to slow down speeding drivers?

Cars are speeding down my street, what can I do?

What can I do to get a crosswalk installed?

Can I get a "Children at Play" traffic sign on my street?

How can I find out about traffic volumes along a certain street?

Do I need a permit for conducting activities in the street such as lane closure or road closures for construction related activities?

Road Closure Alert	Find out about the latest road closures and detours. Click on the detour maps to find the exact construction locations.
Traffic Cameras	Click here to view Traffic Cameras and Live Streaming Video at over 40 locations throughout Cobb County.
Cobb Traffic Counts	Cobb County Department of Transportation collects and maintains a traffic count database for most roads within the County. The Traffic Count Map, launched through the County's Geographic Information Systems (GIS) portal, includes counts taken from the last 3-4 years and is periodically updated. The counts are the Average Daily Traffic (ADT) which is a 24-hour count of vehicles for both directions of travel.
Project Updates	Learn more about the various roadway and bridge projects happening right now throughout Cobb County. Concept maps, fact sheets and contact information are available for most projects.

FAQ Answers

How does the Flashing Yellow Arrow Traffic Signal work at Cooper Lake Road and East/West Connector?

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Cobb DOT installed a new left turn signal at the Cooper Lake Road and East West Connector intersection in West Cobb County. The flashing yellow arrow is part of a new signal display system that more clearly indicates to drivers when they must yield to make a left turn across traffic. This new traffic signal was installed in April 2013.

Click here to view project fact sheet.

Click here to view an informational video from MDOT.

What are the responsibilities of Traffic Engineers in Cobb County?

Cobb DOT Traffic Engineers are responsible for the installation, operation and maintenance of traffic control devices and an Advanced Traffic Management System (ATMS) for over 2,500 miles of road (approximately 8,800 streets), which includes approximately 530 traffic signals and over 60 closed circuit cameras, for the general safety and mobility of the traveling public. Traffic Engineers also operate a Traffic Management Center for centralized monitoring and control of traffic signals, and to facilitate effective incident and event management, provide rapid and real time communications of traffic conditions, and facilitate traffic engineering studies.

What are traffic control devices?

Traffic control devices are signs, signals, pavement markings and other devices that regulate, warn, or guide traffic.

What is the MUTCD?

The Manual on Uniform Traffic Control Devices (MUTCD) has been prepared by the US Department of Transportation, Federal Highway Administration, to set forth the basic principles which govern the design and use of traffic control devices. The Manual was developed with input from technical experts from many organizations including state, county, and municipal governments. This manual is the national standard used to provide clear and consistent uniform guidance to the traveling public including simplifying driver decisions by promoting instant recognition and understanding. The MUTCD is available at http://mutcd.fhwa.dot.gov/

How are speed limits established on County Roads?

The Cobb County Department of Transportation makes a recommendation for posted speed limits for the Radar and Speed Limit Ordinances for review and approval by the Georgia Department of Transportation (GDOT) and the Cobb County Board of Commissioners. Only GDOT approved speed limits may be enforced by Public Safety. The radar and speed limit ordinances are updated approximately every two years. The criteria used by both transportation departments to establish the posted speed limit is based in part upon the characteristics of the roadway and its design speed. The key elements include: road alignment, sight distance, lengths of merges and tapers, road surface characteristics, shoulder condition, grade (i.e., steepness of the road), roadside development, parking practices, driveway spacing, functional classification of the street, and crash experience. Additionally, speed limits are established in part by drivers themselves. In order for a speed limit to be effective, it must be reasonable and the majority of drivers must voluntarily comply. Most drivers tend to regulate the speed of their vehicle relative to road, traffic and weather conditions. National studies have found that the speed at which 85 percent of the motorists travel is reasonable and safe. The determination of the 85th percentile speed is made by conducting a speed survey of vehicles traveling along the roadway in question during normal operating conditions.

Will a lower posted speed limit slow traffic?

National studies have shown that drivers are more likely to regulate their speed based on road, traffic and weather conditions, than by the posted speed limit. Posting a lower speed typically will not slow traffic, without constant enforcement.

What exactly are rumble strips and could they help slow traffic?

Rumble strips are raised or grooved patterns in the travel lane and/or shoulder that create noise and vibration. With the proper design and construction, rumble strips may provide some traffic calming benefit. As an example, shoulder rumble strips along the interstate may help prevent run-off-the road crashes. Due to the noise created by rumble strips, we do not recommend the use of rumble strips in residential land use areas to slow traffic. They do create a noise that may alert drivers, but also agitate the surrounding neighbors every time a vehicle crosses the device (even throughout the night). Rumble strips are more appropriate in a rural setting or well outside of a neighborhood setting.

When should a traffic signal be installed?

Cobb County installs traffic signals when traffic volume and other criteria meet national standards established in the MUTCD. An engineering study, which includes analysis of vehicular volumes, pedestrian traffic, intersection safety and vehicular traffic flow, is performed to identify locations for installation of a traffic signal. Although not common knowledge there are disadvantages when installing a traffic signal and therefore we do not recommend installation when these criteria are not met. Installation of signals not satisfying these criteria may increase delays and congestion, and reduce overall intersection safety.

How do traffic signals work?

Traffic signals utilize an electronic controller to assign the right-of-way to a particular traffic movement, vehicular and pedestrian, based on a signal timing plan. All traffic signals in Cobb County include vehicle detection typically in the form of an inductance loop in the pavement or a video camera to optimize traffic flow and minimize delays. The traffic signal controller is programmed to reduce time or skip time for a phase of traffic or traffic movement based on input from the vehicle detection equipment. Some signals operate in a system or collection of signals in a coordinated manner to further optimize traffic flow through a corridor or in an area; some operate independently from other traffic signals. Generally coordinated systems favor the predominant direction of traffic flow.

How many traffic signals are there in Cobb County?

Approximately 700 in Cobb County (including signals operated and maintained by the Cities of Marietta and Smyrna), 8,200 in Georgia and 300,000 in the United States.

How do I request a traffic signal or report a traffic signal malfunction?

If you desire a traffic engineering study for the installation of a traffic signal, or observe traffic signal malfunctions or poor signal coordination, please contact Traffic Operations at http://dot.cobbcountyga.gov/work-request.htm,

or email us at This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or call 770-528-3666.

I see video cameras at some intersections around the County; what are they for and does the County record and keep images?

There are two different types of cameras used for traffic purposes in Cobb County. One type of camera is used for video detection by electronic traffic signal controllers to detect whether or not a vehicle is present at a traffic signal. They do not record and they are not routinely monitored. The second type of camera is monitored in the County’s Traffic Management Center for incident and event management, and for traffic signal management. These cameras are used for traffic monitoring and the video is not recorded or archived. These cameras can be viewed at http://dot.cobbcountyga.gov/traffic-cameras.htm

Why does the County not operate traffic signals in flash late at night or in light traffic times?

The Department does not operate traffic signals in “flash mode” for the following reasons:
1. Standard traffic engineering practice is to only flash signals for emergency conditions, or prior to a new installation. An emergency condition can occur when the electronic monitor detects a problem with the traffic signal controller and automatically places the traffic signal in flash mode.
2. In general, the traveling public recognizes the signal flash mode as a malfunction and not as normal operation.
3. The traveling public is unclear on how to travel through a signal in flash mode; some treat the condition as an all-way stop and others as a side street stop; and as a result a safety risk is created and unnecessary traffic delays can occur.
Alternatively, the Department accommodates side street traffic during light traffic conditions by operating signal systems in either a very short cycle length of approximately one minute, or by operating signals in “free mode” which permits a rapid change from the main street green to serve arriving side street traffic.

Can a flashing light be added to improve roadway safety?

Flashing beacons (also called flashers or flashing lights) are frequently requested in the belief that they will reduce vehicle speeds and make drivers more alert of the road conditions. Unfortunately, national studies indicate that this is frequently not the case. It has been found that flashers can yield an initial benefit, however they soon lose much of their effectiveness (typically after 3-months) and the flasher is ignored because it becomes part of the normal driving environment. Therefore Traffic Operations does not recommend installation of unjustified flashers. For any traffic control to be effective (including flashers) it must command the respect of the traveling public and provide a useful function. If you have a safety concern that you would reviewed, please contact Traffic Operations at http://dot.cobbcountyga.gov/work-request.htm, or email us at This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or call 770-528-1600.

What is a modern roundabout?

Round About Safety 1 Round About Safety 2
A modern roundabout is a safe and efficient form of traffic control which is relatively new to the United States, although very common throughout other portions of the world for several decades as an alternative to stop-controlled and signalized intersections. The main characteristic of a modern roundabout is the “yield-at-entry” rule, meaning that traffic entering a roundabout must yield to the traffic already within the roundabout. Roundabouts are designed for lower speeds and can have one or multiple travel lanes depending on traffic volumes. Studies have shown that roundabouts can greatly improve the capacity and safety of intersections when compared to stop signs and signals. This is primarily because of lower entry speeds, simplified decision making and fewer conflict points. Four way intersections have 32 possible vehicle to vehicle conflict points, but roundabouts only have 8, as illustrated in the below drawing.

Modern roundabouts should not be confused with the older style traffic circles. The design of traffic circles was based on larger diameters and high-speed entry and exit speeds.

Click here for more information on roundabouts.

Can stop signs be placed on streets to slow down speeding drivers?

Specific criteria have been developed by the US Department of Transportation to ensure consistency for all traffic control devices including stop signs. Safety, traffic volumes, and the classification of the road (whether it is a local road, collector, or arterial) are the three main determinants of whether all-way stop sign control is appropriate; and a traffic engineering study should be performed prior to installation of all-way stop sign control. Installation of all-way stop signs for the purpose of slowing traffic has, based on experience and national studies, been shown to yield the following negative results: higher speeds between intersections because drivers attempt to make up for “lost” time; traffic rolling through and disregarding the apparent inappropriate traffic control device; the potential for increased disrespect for appropriate traffic control devices; and unnecessary stopping which wastes fuel and creates more traffic noise. Essentially, stop signs placed inappropriately can create more safety problems than they solve; and therefore Traffic Operations does not recommend installation of all-way stop control for slowing traffic speeds.

Cars are speeding down my street, what can I do?

The County offers two solutions: speed limit enforcement and traffic calming programs.

To request speed limit enforcement please contact the Cobb County Police Department at www.cobbcountyegov.org/secure/police/traffic-complaint.htm.

To request a traffic study for one of our traffic calming programs, please contact Traffic Operations at http://dot.cobbcountyga.gov/work-request.htm, or email us at This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or call us at 770-528-1600.

Our traffic calming programs include installation of speed humps, on local roads with a speed limit of 25 mph, and deployment of speed trailers which display a driver’s speed adjacent to the posted speed limit for instant comparison.

What can I do to get a crosswalk installed?

Marked crosswalks at uncontrolled locations (locations without a stop sign or traffic signal) are sometimes requested to help delineate where pedestrians should cross. To request a crosswalk please contact Traffic Operations at http://dot.cobbcountyga.gov/work-request.htm, or email us at This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or call us at 770-528-1600. Requests will be reviewed and evaluated based on pedestrian volumes, traffic volumes, vehicular speeds, crossing width, and sight distance. In the wrong location, a painted crosswalk can cause additional risks to pedestrians as safety studies have shown that simply adding crosswalks does not make crossings safer nor do they necessarily result in more vehicles stopping for pedestrians.

Can I get a "Children at Play" traffic sign on my street?

It may seem that “Children at Play” signs protect children, and some communities have used them. However, the facts show otherwise and no evidence exists that these type signs reduce pedestrian crashes or lower speeds. Children should not be encouraged to play in the street and these signs could provide a false sense of security, suggesting that playing in the street is acceptable. For these reasons along with Federal Guidelines and the concern that the use of unnecessary signs can promote disrespect for all signs, Traffic Operations does not recommend installation of these type signs.

How can I find out about traffic volumes along a certain street?

Cobb County Department of Transportation collects and maintains a traffic count database for most roads within the County. The Traffic Count Map is periodically updated and includes counts taken within the last 3-4 years. The counts are the Average Daily Traffic (ADT) which is a 24-hour count of vehicles for both directions of travel. These can be viewed at http://dot.cobbcountyga.gov/traffic-counts.htm. The Georgia Department of Transportation also maintains a traffic count database for roads within Cobb County. GDOT traffic count data can be viewed at http://www.dot.state.ga.us/statistics/TrafficData/Pages/stars.aspx.

Do I need a permit for conducting activities in the street such as lane closure or road closures for construction-related activities?

Yes. These requests are reviewed by Traffic Operations and must be approved for any activity that would impact traffic flow. If approved, all related traffic control must comply with Cobb County standards and the Manual on Uniform Traffic Control Devices (MUTCD).

Please use the linked application for a Temporary Lane Closure Permit, and email or call to request a temporary road closure permit at This e-mail address is being protected from spambots. You need JavaScript enabled to view it or 770.528.3666.