• Dave

Marginal Improvements for the Centennial Park Drive and Portman Intersection

What many people understand is that we separate traffic in space when we don’t want them to crash into each other. People (should always) have sidewalks separate from the roadway, and vehicles travel in opposite lanes. Sometimes we can’t physically separate movements. People need to walk across streets and make turns. In those cases, we use signal timing to separate vehicles in space.

Signal timing, while most engineers think about it in terms of efficient vehicle flow, is actually for safety. If we don’t think through timing carefully we can put people’s lives at risk. When timing lights, it’s important to take human behavior into account. For example, you’ll notice that there is nearly always a few seconds when everyone has a red light. This is because engineers understand that human beings are naturally impatient and will “sneak” through the light at the last second. Those few seconds have saved countless lives.

Signal timing for bicycles is a relatively new idea in the United States, and it is a welcome development. I’ve seen a few popping up in Atlanta along the PATH trail through downtown and I love them. Unfortunately, most engineers barely learn how to time signals for pedestrians, and almost certainly not cyclists (I know of one professor that actually teaches cyclist signal timing).

This presents a serious problem.

The Portman Cycle Track runs through downtown along many one-way to one-way intersections. One-way to one-way intersections are fairly unusual, and present some very risky situations for people walking and cycling. For example, at Centennial Park Drive and John Portman Boulevard NW, there is a light dedicated to people walking or cycling from Portman Boulevard to Centennial Olympic Park or northbound on the PATH. This is critical juncture for visitors coming from hotels on Peachtree to visit the park, and for everyday commuters cycling to or through downtown. I regularly see a woman on a bakfiets transporting her children on this route.

Large one-way to one-way intersections can be very dangerous for people (small neighborhood one-ways serve an entirely different purpose). When the light is green for people walking and cycling, drivers have a red light and are not supposed to turn left on red, as they are crossing a lane of traffic (the cycle track). However, drivers regularly do this without stopping. See video below from my morning commute. You can see that the driver doesn’t stop and proceeds through the intersection. I’ve even seen police officers make lefts on red at this intersection and have seen MARTA drivers do the same at similar intersections.

From the video, we can see that the signal provides about 20 seconds of green time for people to cross. That 20 seconds probably comes from the fact that the MUTCD design walking speed is 3.5 feet/second. In other words, someone should have ample time to cross. However, this doesn’t take cycling, and nearby signals into account. This signal is coordinated with the walk/bike signal at Portman and Williams 1 block east of here. Both lights turn green at the same time.

Figure 1. Stopbar to Cycletrack distance on John Portman Avenue in Atlanta (Google Maps)

When the light turns green a cyclist on the PATH at Williams is likely to begin riding downhill. The distance between the cyclist stopbar at Williams and the PATH entrance is about 500 feet.

Figure 2. Potential conflict between cyclist and vehicle

To cover 500 feet a cyclist needs to travel at about 17 mph, on average. That’s pretty fast. That’s not something I’d want to do with children, but isn't extraordinary since this is a steep hill. What happens if a person is traveling just a bit slower, say 15-16 miles per hour on average? At that speed, one would travel about 470 feet, or right in the center of one of the southbound lanes on Centennial Park Drive, potentially resulting in a horrific crash. We need to take human behavior into account, just like we do with other signals. When a cyclist sees green, they will proceed into the intersection. With how these signals are timed, we're putting people in a very dangerous situation.


One-way to one-way intersections are unusual and unsafe. When installing the cyclist and pedestrian signal, people were trying to do the right thing. However,the signal timing at this intersection compounds the risk of the one-way to one-way. This intersection and the signal needs to be fixed before a tragedy happens. There are a few things that I think could improve it:

  1. Post and enforce a “No Turn On Red” sign for southbound vehicles.

  2. Change the signal timing. Cyclists trying to proceed through the light are in serious danger. The cycle for this intersection could start 5-10 seconds after the Williams intersection so that cyclists will hit the intersection with plenty of green time and not try to rush through at the tail end.

  3. Install a curb extension in the easternmost lane on the northbound lane to increase sight distance for drivers to detect people walking and cycling. This lane is almost always occupied (illegally) parked vehicles that block sightlines. This could be completed in the short-term using flexposts, with plans for hard infrastructure in the future.

This area is dangerous as is, with large one-way streets intersecting near a large park and some of Atlanta's most popular attractions. We need to make it safer, and we can do so with some small, quick improvements.

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