Planning Department Knew About Traffic Measurement Flaws, Part Two

Ten years ago, the Planning Board considered whether to raise the county’s Critical Lane Volume (CLV) standards by up to 100 points. This would have allowed developers to escape traffic mitigation requirements near some intersections exceeding the CLV standards in their policy areas. But citizen activists questioned whether allowing CLV standards to rise would result in more congestion since higher CLVs are thought to indicate more congestion. The surprising answer from the Planning Department’s staff was no. Why? The staff found that CLVs were unrelated to real-world traffic delays.

In a 1998 study entitled, “Measuring Congestion and Delay: The Critical Lane Volume Method,” Planning Department staffers Richard C. Hawthorne and Ronald C. Welke looked at how CLVs compared to other measures of congestion. The authors stated:

In researching how to measure congestion, the study group selected actual delay as the best measure as perceived by the roadway user. The average stopped delay per vehicle in the peak hour is the measure used in the operational analysis of intersections in the 1994 HCM [Highway Capacity Manual], and therefore provided a quantifiable standard.

The authors then gathered data on CLVs and actual delays from 27 observations taken at 15 highly-congested intersections to see if the two measures correlated. They plotted each of these observations against each other on the chart below:


If the two measures were directly related, the observations would cluster tightly around a line rising from zero on both axes. Instead, the observations form an amorphous blob. The authors found that the coefficient of determination between delay and CLV, also known as R-squared, was only 14%. That means that only 14% of the variation in one measure is explained by changes in the other. The authors concluded, “There is little relationship between delay and CLV.”

So if CLV is such a poor predictor of “the best [congestion] measure as perceived by the roadway user,” why not stop using it? The authors said, “The problem is that delay data is difficult and expensive to gather and thus not readily available.” So because CLVs were shown to be unrelated to delay, the Planning Board raised the allowable CLV standards – a move that was opposed two years later by then-Council Member Ike Leggett. And rather than search for a better data source that truly measured actual traffic congestion, the Planning Department has continued to rely on CLVs. The situation is compounded by the fact that Richard C. Hawthorne, one of the study’s authors, was then and still is now the department’s Chief of Transportation Planning.

This has potentially severe consequences for traffic management in Montgomery County. CLVs are used by Planning staff to form recommendations on traffic mitigation for new developments. Under Local Area Transportation Review (LATR), if an intersection near a new development exceeds the CLV standard for its policy area, a developer is required to pay for traffic mitigation measures. But what if, as the above study holds, CLV is not a reliable predictor of congestion? That means there is a possibility that mitigation measures have been installed at intersections that do not need them, and have not been installed at intersections that desperately require them. And this has been going on for at least ten years even though the Planning Department KNEW that CLVs by themselves were a flawed measure of actual congestion.

Planning’s knowing reliance on a defective congestion measure is difficult to understand and impossible to excuse. But it can be fixed. Perhaps delay was expensive to collect ten years ago, but that was prior to GPS units being available for rent at $5 per day.

Our Planning Department was once the best in the country. It is important to every one of us that it produce the highest-quality information on traffic and development that is humanly possible. We do not deserve the cheapest traffic measurement system, or the quickest and dirtiest, or the one we have been using for a long time merely because the bureaucracy wants to avoid change. We deserve the best. We put our alternative on the table and now it’s the Planning Department’s turn.

The long-forgotten CLV study is not available online, but we reproduce it in its entirety below.