>In response to a question about ridership on the 3A/3B/3E line, which in some part runs along the same route as a streetcar line that I proposed earlier, WMATA’s office of customer relations replies:
For May 2008, the average weekday ridership on Lee Highway Routes 3A, B, and E was 2,752. According to the spring 2006 ride check, the most recent available detailed data by trip and stop, the average weekday ridership between Rosslyn and East Falls Church was approximately 600.
A common metric for determining what transit lines deserve or require an upgrade to higher quality service, such as streetcar, bus rapid transit, or light rail, is called “traffic density”, and is measure in passenger-miles (km) per mile (km) of route per day or per year.* Here’s an extensive listing of traffic density for various transit lines. For example, Houston’s new light rail line reported 3.5 million passenger-miles per mile per year, and Boston’s green line reports 8.2 million per year for the “B” Boston College line. Typically, a bus line with a traffic around 5000 per day or 1.5 million per year is a good candidate for upgrade to BRT, or streetcar. Higher traffic densities such as 20,000 per day or 6 million per year are candidates for grade separated “Heavy Rail” such as a Subway or Metro.
How many passengers are we talking about when we say “5000 per day?” I think the best way to envision these numbers is to imagine standing next to the route at some average point, and count the number of people in the buses that go by in a single direction in a day. Assuming a bus line with 6 buses per hour in the peak direction and operation from 6am to 12midnight. For 5000 per day, that means 40-50 passengers per bus at the average point along the line. Since transit passengers rarely space themselves out evenly along a transit line, and definitely don’t ride evenly throughout the day, I’m going to apply a time peaking factor of 3.0 (based on the WMATA data presented yesterday) and a location peaking factor of 2.0 (made up number – the worst location on the line carries twice as many people as the average) to arrive at a peak loading of 6 times the average, or about 240-300 per vehicle assuming the same 6 buses per hour (10 minute headways). A 60-foot articulated bus can accomodate 105 passengers, or 70 passengers per 40-foot standard bus. Therefore, during the peak hour, peak direction we’re going to need to increase the number of bus vehicles to 15-20 per hour, which means a bus vehicle every 3-4 minutes. So for a transit density of 5000 per day, the required service is 10 minute headways off-peak, 3-4 minute headways during peak, for 18 hours a day, all using 60-foot articulated buses. That’s seriously high capacity for a bus, so I can see how that’s the threshold for shifting the line to streetcar or light rail service.
Going back to the data reported for the Lee Highway lines (3A, 3B, 3E), WMATA stated that average daily ridership is 2,752 for the whole route or 600 for the route between Rosslyn and East Falls Church. For that portion of the line, the distance is 6.5 miles. Even assuming that all 600 passengers per day ride the entire length, that’s only 600 per day, which is far below the stated threshold. I think a future study could be to ask WMATA what the most popular bus lines are and the ridership data (average weekday boardings, average trip length, etc.) for those lines.
*Note that the length units would cancel out and therefore don’t matter. However, in order to prevent listing the units as the more cryptic “passengers per day”, the length units are sometimes not cancelled. For brevity I will abbreviate it to millions per year or thousands per day, implying millions of passenger miles per route mile per year or thousands of the same, respectively.