Wednesday, June 4, 2008

Planning Department Knew About Traffic Measurement Flaws, Part One

After last week’s marathon five-part series on traffic measurement, I was prepared to move on to other things. But an anonymous friend of the blog sent me proof that the issues I have been discussing have been known to the Planning Department for at least a decade. An internal study prepared by Planning staff back in 1998 found that Critical Lane Volume (CLV), the measure they use to estimate traffic congestion, has “little relationship” to actual delays. This revelation throws into question the entire system of traffic mitigation used by Montgomery County.

To understand the magnitude of this statistical debacle, we must first review the role played by CLVs in the county’s planning procedures. As I detailed last week, CLVs are hourly sums of conflicting movements (both through and turns) of cars through an intersection. They reflect volume: the more cars pass through an intersection, the more a CLV count will increase. But some of the county’s most congested road corridors have low CLVs because they are too gridlocked for large numbers of cars to get through. Nevertheless, the Planning Department uses CLVs to construct their regular lists of the county’s most-congested intersections and make recommendations for capital improvements.

CLVs play an even more important role when the Planning staff assesses traffic impacts of new developments. One of the review procedures that a new development must pass is Local Area Transportation Review (LATR), an analysis that explores the development’s traffic impact on a handful of nearby intersections. Under LATR, the developer is required to submit a traffic study to the Planning staff estimating the number of new trips that will be created at peak travel hours. The staff then obtains the CLV estimate for the affected intersection(s) and compares it to the standard set for the development’s policy area. If the CLV exceeds the relevant policy area standard, the developer will be required to pay for traffic mitigation measures to offset that impact. These measures might include purchasing Ride-On buses, building bus shelters, installing turn lanes, widening an intersection or other remedies.

The county’s policy area standards provide for acceptable CLVs to be relatively low in rural areas and higher in dense areas. The rationale is that dense areas are likely to have more transit options, especially when they are near Metro stations. As established in last year’s growth policy, the CLV standards by policy area are:

1350: Rural East, Rural West
1400: Damascus
1425: Clarksburg, Gaithersburg, Germantown East, Germantown West, Montgomery Village/Airpark
1450: Cloverly, North Potomac, Olney, Potomac, R&D Village
1475: Aspen Hill, Derwood, Fairland/White Oak
1500: Rockville City
1550: North Bethesda
1600: Bethesda/Chevy Chase, Kensington/Wheaton, Germantown Town Center, Silver Spring/Takoma Park
1800: Bethesda CBD, Friendship Heights CBD, Glenmont, Grosvenor, Rockville Town Center, Shady Grove, Silver Spring CBD, Twinbrook, Wheaton CBD, White Flint

Here’s an example of how this review procedure might work. Suppose a developer wanted to build a commercial project near the corner of Connecticut Avenue and Jones Bridge Road in North Chevy Chase. The Planning Department estimates CLVs for three nearby intersections: Jones Bridge at Manor Road (679 AM, 676 PM), Jones Bridge at Platt Ridge Drive (773 AM, 963 PM) and Jones Bridge at Connecticut (1731 AM, 2017 PM). The policy area standard for Bethesda/Chevy Chase is 1600. While two intersections fall below that standard, Jones Bridge at Connecticut exceeds it in both the morning and evening. This developer would have to agree to a package of mitigation measures to offset the traffic impact on Jones Bridge at Connecticut.

Now suppose the developer wanted to build the same project near the corner of Connecticut Avenue and Knowles Avenue in Kensington. The Planning Department estimates CLVs for three nearby intersections: Plyers Mill Road at Metropolitan Avenue (687 AM, 866 PM), Connecticut at University Boulevard (1335 AM, 974 PM) and Connecticut at Knowles (1433 AM, 1274 PM). The policy area standard for Kensington/Wheaton is 1600. Since none of the intersections exceed the standard, this developer would not be required to pay for mitigation (at least not under LATR).

The county’s traffic mitigation system under LATR is thus based on the premise that CLVs are reliable measures of congestion. When intersections “fail” – that is, they exceed their policy area standard CLV – traffic mitigation is required for new developments. When intersections “pass” – in other words, fall below their standard – traffic mitigation is deemed unnecessary.

But we called into question whether CLVs truly measure congestion all last week. We even revealed a list of four heavily-congested corridors with low CLVs using the Planning Department's own data.

And now we learn that the Planning Department’s own staff confirmed our suspicions ten years ago. More on their long-forgotten findings tomorrow.

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