Tuesday, July 28, 2009

Where the Metro Riders Are, and Aren't

How full are Metro’s trains at any point in the system? What routes do riders take when confronted with a choice between two transfers, or between a longer one-seat ride and a transfer? Last year, in discussing I created about the proposed Blue Line reroute, a reader asked about this, but Metro hasn’t collected the data.

To answer this question, classmate and frequent GGW commenter Reza and I created a non-scientific survey, and used the data to build a diagram of passenger traffic on Metro system. This survey was unscientific and the results should not be considered as absolute facts. It does show potential trends, but a larger and broader sample would be necessary to validate these results.

The survey first asked respondents to state a certain preference, like the top factor they use to decide on a route. Later, it presented a specific scenario and asked respondents which route they would choose. Overwhelmingly, survey respondents primarily prioritized getting to their destination in the shortest time. A plurality of 44 percent chose minimizing transfers as the second most important factor. For the third choice, a plurality of 37% said, given the option, they’d choose a line with more frequent trains.

Based on the responses, Reza and I created a decision tree to assign the trips from the 2007 Ridership Survey to the links of the Metro system. On the segments of the system without alternative routes, the diagram is 100% accurate from Metro’s origin and destination data. For example, ridership between Takoma and Silver Spring is definitely almost twice as high as between Silver Spring and Forest Glen, which is why Metro turns back half of Red Line trains at Silver Spring.

Inside the area with alternative routes (bounded by Fort Totten, Metro Center, L’Enfant Plaza, Rosslyn, and Pentagon), the diagram relies on the non-scientific and possibly non-representative survey.

This information is very important to helping Metro make good decisions about service. Dropping the percentage of rush hour Blue Line trains at Rosslyn 40 percent to only 20 percent, as Metro proposes, would make the most sense if about 80 percent of riders at Rosslyn were on the Orange Line. But some readers of Greater Greater Washington and elsewhere commented that their Blue Line trains between Rosslyn and Arlington Cemetery seem pretty crowded, and that they probably would become even more so if Metro halved the number of trains.

Metro’s May 2007 ridership survey lists the number of trips made from any station to any other station, on average, but not which path riders take to get there, when they have a choice. We don’t know what emphasis riders place on factors transferring versus travel time, and therefore can’t ascertain what routes people would choose if given an option. Of course, a trip from Shady Grove to Dupont Circle can only happen via the Red Line, but from Van Dorn Street to L’Enfant Plaza, the rider has to choose between the one-seat Blue Line ride and a transfer to Yellow, or a trip from Woodley Park to Prince George’s Plaza involves a transfer at either Fort Totten or Gallery Place.

Metro needs accurate models to make decisions about service levels given its budget and infrastructure constraints. They periodically take a statistical sample of riders to determine where people are boarding, where they’re exiting, and how they get to and from stations. But they have little data about how people get from point A to point B. One method that Metro uses to determine the ridership on certain line segments is to station workers on the platform to count passengers on trains, but this doesn’t capture all information about route choices. WMATA should consider adding those questions to their ridership surveys.

The ridership per link estimated by our survey and assignment model show some interesting relationships.

As mentioned above, ridership drops by half on trains going northbound through Silver Spring. The decision to short turn trains there (because of the presence of a pocket track) was a good one. The phenomenon does not repeat itself on the other side of the Red Line. Volumes never drop significantly at any one stop, although they do taper as the line approaches Shady Grove. During rush hours, half of all trains turn back at Grosvenor, but unlike at Silver Spring, there is no major drop off in volume there. In fact, there are more riders in the link south of Grosvenor than there are in the link south of Silver Spring and ridership is higher at every single link north of Grosvenor than it is on the link between Silver Spring and Forest Glen.

Downtown, the Red Line is very busy. As one would expect, there is a significant jump in ridership at Union Station when coming from Glenmont. Ridership jumps by almost half from the link north of Union Station to the link south of Union Station. It might be worthwhile to find a way to insert a pocket track into the southern tip of Brentwood Yard and run some rush period trains from Shady Grove to New York Avenue.

In Virginia, there are significant drops in ridership west of Ballston and west of West Falls Church. West of Ballston, ridership drops by approximately one-quarter, and then by another third west of West Falls Church. Currently Metro does operate some trains from/to West Falls Church during peak periods. It might be helpful, especially after the Silver Line opens, to construct a pocket track in the median of Interstate 66 between Ballston and East Falls Church. This would allow some trains from or to downtown to serve the crowds of the Wilson Boulevard corridor. This will be especially important once the SIlver Line starts to reach ridership targets because Arlingtonians will find it harder to get on already crowded trains from the suburbs.

Volumes also drop on the Blue and Yellow Lines south of King Street. The combined ridership south of King Street is 20 percent lower than on the segment north of King Street.

What about Rosslyn, the spot which started this whole endeavor? Based on our analysis, of the riders traveling on the two links immediately outbound from Rosslyn, 62 percent are on the Orange Line and 38% are on the Blue Line. This matches closely current service levels.

At Pentagon, a similar look shows us that of passengers traveling on the two links inbound of the station, some 54 percent are on the Blue Line and 46 percent take the Yellow Line Bridge. This also closely matches current service levels.

This brief analysis demonstrates some of the difficulties with understanding ridership patterns on the Metro. It shows why it is so important for Metro to find some way of surveying patrons on how they travel, not just where they travel. Unfortunately, because of the limitations on our surveying it is difficult, if not impossible, to draw substantive conclusions about ridership patterns themselves. However, it does offer an interesting glimpse at a better way to plan for service alterations.

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