To the editor:

The times are a “changing,” but so are the speeds on the Perimeter’s “Top End” this month.

We all know the Perimeter (I-285) speed limits on the half running below I-20 were raised to 65 mph several months ago. Georgia Department of Transportation’s assessment of the traffic on the “Lower-Half” of the Perimeter justified raising the permanent speed limits based on many criteria, which included safety, number of exits and congestion considerations.

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 up from 55 to 65 mph, peak hours suggested downshifting to a lower speed.

Thus, drivers of the Perimeter Top End may have noticed new signs being installed at strategic intervals. These signs are electronic and will be able to 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 (in 5 mph increments) 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.”

It also reduces crashes caused by multiple lane changes and quick stops.

What will not be intuitive to drivers is that 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 the congestion is experienced.

This is the same concept applied to the ramp meters stopping drivers before they enter the Perimeter; even when it isn’t 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 upon several circumstances.

First, a lane-blocking incident will trigger speed reduction miles before the incident. Second, adverse driving conditions such as a popup shower or blinding sunshine slowdowns can be accommodated.

Third, and the most impactful, is prior to the 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 current 55 mph speed limit, anticipate the new 65 mph being incrementally lowered to 55 mph at 6:30 a.m., then to 45 mph at 7 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.”

Before we protest too much, lowering the speed limit pre-peak is the most effective way to improve performance to avoid 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 which accommodates the most vehicles is just under 45 mph.

The concept is to slow down to get there faster. As counterintuitive as this is, it works.

Bob Dallas

Bob Dallas, who headed the Governor’s Office of Highway Safety under former Gov. Sonny Perdue, serves on the Dunwoody Planning Commission.