• Dave

Atlanta needs to bring it's traffic calming policy into the 21st Century

Updated: May 10, 2019

Yesterday, I wrote about establishing principles for Atlanta’s streets. Principles are important, but policies can dictate how we operate. City policy is frequently overlooked, but can be a powerful tool for creating a safer, healthier city.

City transportation policy is typically related to local speed limits, permits for using right of way, and maintenance of the right of way. However, traffic calming is part of a city’s policy toolbox to increase safety and make our neighborhoods a safer, more enjoyable place to live.

Atlanta’s traffic calming policy merits a fresh look. The policy is horribly outdated. How outdated? The “Traffic Calming Device Implementation Guidebook” was written in 1999, and is based on ITE guidance written in 1993 (Note: the ordinance states that any subsequent revisions apply, but I cannot find an updated document for the city of Atlanta). Atlanta, transportation, and Atlanta’s transportation priorities have changed drastically in the past 20 years. We live dynamic world, and policy is frequently not-so-dynamic. But, local governments can respond to changes in a way that other levels of government cannot.

So how does traffic calming typically happen right now? Let’s walk through it.

First, a resident needs to request a traffic study. Then, the Department of Public Works will conduct a “traffic study,” which consists of collecting data on vehicle speeds, the number of vehicles on the street and those surrounding it, quantifying the number of crashes, and collecting turning movement count data. The location of the study is determined by “field reconnaissance” by an engineer who determines where the issues are located. The recon also requires that the location of traffic control devices in the area be noted. This is no small task.

Then, the Department of Public Works uses data from this study to determine whether or not traffic calming is warranted. This guidance document begins quite well, actually, describing traffic calming as “an engineering methodology to physically change the character of streets, improving pedestrian safety and encouraging drivers to obey speed limits.” Exactly! Unfortunately, the document unwinds from there.

First, a street must experience most of it’s traffic during a peak hour, and have a relatively high number of vehicles on the street during that hour. Figure 2 below from the City’s Traffic Calming Device Implementation Guidebook (page 9) describes the zone labeled “Moderate-Problem” as the sweet spot where traffic calming can be considered. Based on this chart, only small neighborhood streets that experience hundreds of vehicles driving through in an hour, or high volume streets that experience traffic for most of the day would qualify.

A road on the “low traffic” portion of the chart has 250 vehicles per day, with 70% of traffic at peak, suggesting a total of 357 vehicles per day. This likely happens on one-way streets that provide a “cut-through” alternative to a congested area. Tye Street in Cabbagetown has 410 vehicles per day, is one-way, and is frequently used as an alternative to Estoria Street. That sort of street is a great candidate for traffic calming.

On the other end of the spectrum, a street on the “high traffic” end of the chart experiences about 12% of traffic during the peak, and 1000 vehicles during the peak. That means the road has about 8,800 vehicles per day. For reference, that’s fairly similar to Ralph McGill Boulevard at Central Park Place downtown. Unfortunately, Ralph McGill is considered an arterial and would thus be disqualified from receiving traffic calming.


If a street clears the peak traffic/volume hurdle, it must exhibit another type of problem, and that problem is typically speeding. So, what determines a speeding problem? According to the guidebook:

Figure 3 once again breaks it down into tolerable, moderate, and serious (page 10, Traffic Calming Device Implementation Guidebook). Basically, if 85% of vehicles are exceeding the posted speed limit, it is considered a “serious problem” and merits traffic calming.

There is some good and some bad here. The person who authored this guide realized that a crude 10 mph increase in speed presents higher risk at higher speeds than at lower speeds. This is actually a smart, data-driven guideline and should be applauded. Frankly, not many cities in 1999 had that sort of vision. However, we can do even better.

We want to target those areas where there are excessive speeders. Finding those areas where 15% or more vehicles exceeding the speed limit are targeted seems like a simple means of doing so. But, this does not account for how speeds are distributed on a roadway. If you think about how people drive, especially on neighborhood and city streets, they try to stay close to the speed limit, and do their best to travel no more than 9 mph ("9 your fine, 10 your mine). To get a better idea of what this might look like, I simulated a few distributions with 500 observations (think "cars") on a 25 mph "street" where the cars mostly stayed within 9 mph of the mean speed. These are random samples with those parameters.

In the figure to the right, I assumed that the mean speed was 25 mph, and that people tended to err on the side of caution, and travel within 9 mph of the speed limit. The red line indicates the 85th percentile speed, which is around 28 mph, well under the 10 mph threshold. In this very simple example, nearly half (239, or 47.8%) of the vehicles on this street are exceeding the 25 mph speed limit.

I created a few simulations and almost none actually met the threshold of the 85th percentile speed being more than 10 mph over the posted speed limit. However, the one graphed below stood out. The drivers were slightly less conservative (let's pretend that this is a cut through route, so there are few local drivers traversing slowly). Again, I assumed the mean was 25 mph, with most values falling within 9 mph of the speed limit. The red line is still the 85th percentile speed (about 34 mph). Take a look.

The parameters on these two roads are very similar, but by making different assumptions, the risk on the road is far different. Over half (52%) above the speed limit, but the 85th percentile speed is only 9 mph higher than the posted speed limit, and 13% exceed 35, with some getting up to 50 or 60 mph. This road would not be considered a "serious problem."

According to the City of Atlanta's own documentation, both of these roads present serious risk. Page 5 of the Safer Streets technical memorandum notes that the likelihood of a pedestrian death increases substantially when a crash happens with a vehicle traveling at 30 mph relative to a crash occurring with a vehicle traveling at 25 mph.


Once this analysis has been completed, there is a further ranking system to prioritize traffic calming projects. I could keep going into the details, but I will stop there. The thing is, this policy is broken and doesn't work. If you follow City Council's Transportation Committee meetings, traffic calming requests are typically just approved by council because a few neighbors spoke up, regardless of whether they meet the "technical requirements." Unfortunately, many of these request get held up for close to a year, or more. Traffic calming should not be burdened by excessive studies that increase costs for the city, spend precious time during City Council meetings, and most importantly, expose Atlantans to fatal risks. Atlanta deserves a proactive policy that makes neighborhood streets safer, where neighborhood greenways, and safe walking aren't a commodity for those can afford it.

Fortunately, Atlanta’s Transportation Plan addresses poor transportation policy directly, suggesting that the city “Modify policies, regulations, and laws to prioritize safety measures” under Specific Strategy Recommendations (page 28 in the “Safer Streets” Technical Memorandum of the Plan).

If we have a newly formed Department of Transportation, speeds must be addressed, and a world-class traffic calming policy is a great place to start. It's time to #SpeakUp and lead.


I plan to share some ideas on proactive vs. reactive policies soon and how we can create a traffic calming policy to be proud of. Remember to #SpeakUp for road safety this week. Everyone can lead.

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