The times are a-’changing; and so are the speed limits on the Perimeter (I-285).
The Perimeter speed limits on the half running below I-20 were raised to 65 mph several months ago. The Perimeter “Top End” however — not so fast. There, the congestion, number of exits and safety concerns were significantly more impactful. While conditions on weekends and late nights support shifting the speed limit from 55 to 65 mph, peak hours suggested downshifting to a much lower speed.
Before we protest too much, lowering the speed limit pre-peak is the most effective way to improve performance by averting the worst congestion during peak drive times.
Why does this work? It avoids the accordion effect that creates stop-and-go traffic and significantly reduces the number of severe crashes that degrade the flow. Studies consistently show the optimal speed that accommodates the most vehicles is just under 45 mph.
Thus, drivers of the Perimeter Top End are now experiencing the new electronic signs that now display variable speed limits. Quoting the Texas A&M Transportation Institute: “Variable speed limits are enacted by signs that can be changed to alert drivers when traffic congestion is imminent. Sensors along the roadway detect when congestion or weather conditions exceed specified thresholds and automatically reduce the speed limit … to slow traffic and postpone the onset of congestion. The system’s goal is to slow traffic uniformly in a way that allows smooth traffic flow and avoids stop-and-go conditions.”
Variable speed limits also reduce crashes caused by multiple lane changes and quick stops.
What will not be intuitive to drivers is that the slowing down will occur prior to encountering congestion. Thus, a driver may perceive little congestion and question why the speed limit has been reduced.
As the goal is to prevent or delay the onset of congestion, slowing drivers down has to occur before congestion is experienced. This is the same concept applied to ramp meters stopping drivers before they enter the Perimeter, even when it doesn’t appear congested. By restricting the flow of vehicles entering the interstate, it flows more smoothly, and the impact on congestion is ameliorated.
In practice, speed limits will be lowered in several circumstances. First, a lane-blocking incident will trigger speed reduction miles before the incident. Second, adverse driving conditions, such as a pop-up shower or blinding sunshine slowdown, can be accommodated.
Third, and most impactful, is prior to predictable peak congestion, the speed limits will be lowered. For example, if the peak is at 7:30 a.m. when traffic cannot move at anywhere near the former 55 mph speed limit, anticipate the new 65 mph being incrementally lowered to 55 mph at 6 a.m., then to 45 mph or less at 6:30 a.m.
For drivers who believe speed limits are simply suggestions, or worse, anticipate more than one complaint over a citation issued for exceeding a lower speed limit when the road was “wide open.”
The concept is to slow down to get there faster. As counterintuitive as this is, it works.
Bob Dallas is an attorney who served as director of the Georgia Governor’s Office of Highway Safety under Gov. Sonny Perdue.
