Friday, May 3, 2013

Longfellow Bike Traffic

I usually cross the Longfellow Bridge by bicycle around 7:30 a.m. It's before the peak of the rush hour, and while I'm not alone on two wheels, it's not too crowded. Twice this week, however, I've been crossing the bridge eastbound around 8:30. And both times, when I've reached the light at the end of the bridge, there was a veritable traffic jam of bicyclists, with a lineup of 10 two-wheelers waiting to turn right on to Charles or go straight up Cambridge. We've had a long string of great cycling weather (sunny, dry and cool), Hubway is in full swing, and we're recovering from the marathon fiasco. So there are a lot of bicyclists.

From this small sample, I'm going to make some big extrapolations. I sleuthed out the traffic counts from the intersection from a Red Line / Blue Line connector document (pdf) and sussed out that it is a 100 second light cycle—that it repeats 36 times per hour. Assuming a constant ten cyclists per light cycle for an hour, this would equate to 360 bicycles across the Longfellow in an hour. Is this a big number? I think so. Here's why:

  • This is more than half as many bicyclists as vehicles. Peak morning eastbound car traffic is 707 vehicles per hour. Now, the Longfellow is mainly a transit bridge, and at peak hour the Red Line carries more than 10,000 vehicles. Plus, vehicle traffic decreases (as it has in the Kendall area) and bicyclists' numbers continue to climb.
  • Bicyclists have a sub-optimal facility on the Longfellow. In other words, the bike lane kind of sucks. It's bumpy, narrow and squeezes down at the Boston end of the bridge (although it is better-paved there). The future lane will be a bit wider, although to preserve two lanes of inbound traffic it won't have a buffer built in. Still, it won't be as squeezed as it is now. (MassDOT pdf)
  • This illustrates the importance of keeping the bridge open to cyclists during construction, as is the plan. Even as traffic is limited to one direction, bicyclists and pedestrians will be allowed to cross the bridge in both directions. With hundreds of bikes per hour, it's a vital link in the regional bicycle infrastructure.
  • Finally, the roadway is currently more efficient at carrying bicycles than motor vehicles. 707 vehicles use two lanes per hour, at a rate of 354 per hour. Bicycles use one lane, and there are (by my assuredly crude calculations) 360 bicyclists. But wait! Aren't traffic lanes a lot wider than bike lanes? Yes. 360 bicyclists traverse the Longfellow in only 5 feet of bridge width, at a rate of 72 vehicles per foot. The 707 cars have 24 feet of bridge width, a rate of only 29 vehicles per foot. Even if we assume 1.25 people per car, bicycles are still twice as efficient at transporting people. (And, yes, the Red Line inbound, in 14 feet of bridge width, transports more than 10,000 people, making it ten times as efficient as the bike lane.)
Of course, the bike lane is certainly not at capacity (neither are the vehicle lanes; although the Red Line is quite crowded). Leaving two lanes for vehicular traffic in the new bridge design is contentious, and a single-lane design with a wider, buffered bicycle facility—akin to the outbound side of the bridge—would do more to encourage cycling. Even narrower lanes—and a wide bike lane—would help cyclists (and slow speeding motorists, as well). But even without that encouragement, bicyclists don't seem to be shunning the Longfellow.


  1. The Red Line isn't quite at capacity there either, depending on how you count it. If you look at what the current line signalling allows, then yes, the current 4.5 minute headways are probably the best that could be done (giving 13 trains per hour). But if the signal system were optimized a bit better, it should be possible to run at least 24 and possibly as many as 30+ trains per hour, with the limiting factors being the terminal at Alewife, the junction south of Andrew, and the number of trains available to provide the service. I have to wonder if more people would ride if the trains were less crowded and more frequent.

  2. True, but even with significant capacity increases, the Red Line still could only carry 25,000 passengers, tops. Once you get to headways much less than what they are currently, any delay at a station (mainly due to overcrowding) has significant ripple effects upstream, so you have to keep the trains relatively empty. So while you can conceivably run 15 trains per hour with 1200 passengers per train (18,000), if you double the trains to 30 per hour, you can probably only carry 800 or so each (24,000). There is a point of diminishing returns. (See also, Line, Green)