Sunday, August 9, 2009

Purple Line Greener Future

Maryland Governor Martin O’Malley recently decided that the Purple Line, a planned transit corridor connecting towns in Montgomery and Prince George’s Counties in Maryland, would be light rail transit, rather than the bus rapid transit alternative. This announcement marked the culmination of years of deliberation, community activism, and analysis by residents, local politicians, and even a nationally-recognized environmental organization, the World Resources Institute. Interestingly, WRI submitted a detailed public comment that endorsed the Bus Rapid Transit (BRT) alternative rather than LRT. Let’s examine WRI’s assessment of the Purple Line’s Alternative Analysis/Draft Environmental Impact Statement (AA/DEIS) and why they favored BRT over LRT.

By doing a Monte Carlo sensitivity analysis on the AA/DEIS by creating a scenario where the AA/DEIS projections of ridership and cost are optimistic, WRI found that the LRT option carries more risk of not meeting cost-effectiveness goals, and the BRT option is more robust. In other words, for the expected value of benefits, ridership, and cost-effectiveness based on different scenarios of independent variables, BRT trumps LRT. However, WRI did not add a “pessimistic scenario” where the AA/DEIS actually under-predicts daily ridership. I found this curious, but then looked into projections more and found that Metro ridership projections in 1969 predicted 959000 daily trips in 2004, while the actual figure stood at only 653000 (“The Great Society Subway” by Zachary Schrag). Maybe there is something to Washington area transit planners being overly optimistic.

WRI goes on to analyze the greenhouse gas emissions projections of different scenarios. In the AA/DEIS scenario, the BRT alternatives are the only ones that produce decreases in CO2 emissions. However, WRI points out that the AA/DEIS used an all-Maryland emissions factor for electricity, rather than the finer granularity offered through the EPA’s e-GRID database. They find that by looking at individual zip codes, the LRT alternative is less emissions-intensive but still positive. While WRI mentions potential changes in electricity generation emissions in Maryland through 2030, I think it is important to note that this is where the LRT alternative has a big advantage over the BRT alternative: if steps were taken to connect light rail to a carbon-free electricity source, such as a potential new nuclear reactor at Calvert Cliffs, the LRT option would become virtually emissions-free, while the BRT option would be locked into whatever type of vehicle (diesel or CNG) is chosen for operation.

For the last year I’ve lived very close to one of the planned future stops of the Purple Line in East Silver Spring. Nearly every day I see the purple “No Train on Wayne” signs planted in front yards interspersed with the less common purple and green “Purple Line/Greener Future” signs. If my condo association allowed signs in windows, I’d have put a few of the “Purple Line/Greener Future” ones there, because I’ve spent the last two years wishing there were a better way to traverse the criss-crossing boulevards connecting Silver Spring and the University of Maryland. A dedicated right-of-way transit route directly linking Silver Spring with Bethesda will be a boon for commuters who today need to bear the congestion on East-West Highway either in cars or on a J2/J4 MetroBus. Personally I’m happy with the governor’s decision because I think the LRT option will, over the lifetime of operation, yield higher emissions reductions and reduce road congestion more than the BRT alternative.


  1. Did the WRI study look at the impacts on development of BRT vs light rail (I can't get the link to open)? This seemed to be one of the big advantages of light rail from a lifetime carbon perspective, that I imagine WRI would usually go for, but maybe didn't think was appropriate for a pure value-for-money assessment. If light rail stops nurture more transit-oriented development, leading to the creation of walkable town centers which reduce the need for car journeys over a long time frame. In my neck of the woods, switching the focal point of Riverdale from the grotty intersection between East-West and Kenilworth to the new station might finally spur the redevelopment they've been investigating for decades but never initiated.

    Of course, it's still not a done deal - federal funding is still needed, and apparently UMD is back to stalling over the 'vibration' concerns they've raised intermittently over the past year or two. Eventually, the damn thing will get built, I'm sure. But I wouldn't put your yard signs away yet...

  2. Simon, you beat me to the development angle!

    I sadly haven't gotten around to reading Schrag's book, yet (though I do know he debunks the "rich people killed the Georgetown Metro stop myth"). But I think one of the reasons planners in the 1960s overestimated ridership may have had to do with transit-oriented development, or rather the lack thereof outside of the District, Arlington, and MoCo. Prince Georges County and Fairfax have done very, very little to promote denser, more walkable development around Metro stations. The end result is more people living farther from the Metro, and more people relying almost exclusively on cars for transportation, leading to the region's famous traffic and congestion.

    LRT, and other rail-based transit, has a psychological advantage over BRT. Actually, it has several. Besides the stigma that unfortunately plagues buses in this country, if the rails are in the ground, that's where they're staying, and developers recognize this; investment is a lot more likely to flow to areas abutting a fixed route than it is to a bus route that might change at any time.

    However, I'm with Ryan Avent that a heavy rail Metro line tunneling under downtown Bethesda and Silver Spring (and maybe even crossing the Potomac to link up with the Silver Line at Tysons Corner) would have been a better long-term investment. Still, given the limited universe of options studied, LRT was a clear winner once development and ridership were both considered.

  3. AMT, I think this picture sums up the point you're making:

    I'd be interested in knowing if the WRI analysis considered lifecycle emissions. I wonder how construction emissions compare for LRT and BRT--I'd assume LRT would be higher.

  4. Roger, that picture is exactly what I'm talking about. Arlington County has been a model of smart growth for years. Illustrative of the difference in planning and foresight between jurisdictions is the Arlington-Fairfax county line. Arlington ran the Orange Line under the streets and put stations relatively close together. Development sprung up around the stations, and then filled the gaps, creating the Rosslyn-Ballston Corridor. Fairfax ran their portion of the Orange Line in the median of I-66 and surrounded stations with acres of parking lots, because people had to drive to get to the Metro, even if they lived relatively nearby.

    On your lifecycle emissions question, I think LRT would absolutely beat BRT on any lifecycle score. Even if the initial construction emissions are greater for LRT, rails tend to require less, and less-intensive ongoing maintenance. Asphalt and concrete roads deteriorate over time, especially when used by heavy vehicles (semi trucks, buses, etc.) I think these costs and emissions for BRT, and anything that runs on roads, push the calculus back toward LRT (all else being equal).