Saturday, March 9, 2013

Larger vehicles should have fewer stations-per-mile

On the topic of station consolidation along the Green Line, one thing I've always noticed is that the 57 bus tends to outpace the "B" branch on the shared corridor except in the case where the bus has to make every stop. The train makes every stop on almost every trip except late in the evening when demand is lower. The difference can be quite stark: I've made it from Park Street to Harvard Avenue in under 20 minutes after midnight, but I've been stuck for over 50 minutes on the same leg during the afternoon rush, on what should nominally be a 25-30 minute trip.

The reason why I (and others) advocate for station consolidation is to reduce the fixed overhead associated with making a stop. But I think there's a more general case to be made. Let's look at the capacities of the vehicles. The planning capacity of a typical MBTA bus is 54 people (75 at crush load) and the capacity of a typical 2-car Green Line train is 202 people (468 at crush load). The train could easily be carrying between 4 and 6 times the number of people as the bus.

Now consider two stations that are too close together (for example, Saint Paul Street and Pleasant Street). There are people who will prefer one or the other station for small reasons and will be inclined to ring the stop request bell at that station. On the train, the probability is much higher that a person will ring the bell for the first station and also another person will ring the bell for the second station. On the bus, with fewer people onboard, it is likelier that one or the other can be passed up.

My experience bears this out: the Green Line rarely skips stops under normal service; the 57 bus will almost always be able to skip some stops even at the height of rush hour.

That accounts for alighting, but what about boarding? The likelihood that someone will be waiting at the station and flagging down the vehicle is related to the "strength" of the route: how well it attracts riders. The top 3 characteristics that make it attractive to riders are: frequency, where it goes, and perceived reliability. The Green Line is scheduled to run at higher frequency than the bus during its entire span. In addition, the train seems to draw more riders than the bus for a number of reasons, including: it goes downtown, it has a larger presence in the minds of riders (unfair but true), and higher perceived reliability (at least for those folks without smartphones). Again, if you have two closely spaced stations, then for small reasons people will prefer one or the other and the train will likely have to stop at both. But in this case it has less to do with capacity (except, perhaps, if riders fear they may be passed up by a full bus).

So to wrap it up, I think there is a natural bias towards longer station spacing (up to a reasonable point) for larger capacity vehicles, where the stop request bell is going to be pressed virtually all of the time anyway. In addition, people tend to be willing to walk further to faster, higher quality service, and spacing out stations further will speed up the Green Line a bit. Building higher quality platforms with good access at both ends (something the Green Line sadly lacks, but ADA requires) will also help shorten those walking distances while not sacrificing service quality.

2 comments:

  1. Going to Kenmore, the 57 can often be quite crowded by the time it gets to Green Line, although I wish it was still the A Branch.

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    1. The 57 can get quite crowded, it's a top 5 bus. I typically find that if I see the coming bus falling behind schedule on the tracking display, it probably means that it is overcrowded, and I can wait for the follow-up bus -- which is catching up due to the "forces" of bus bunching. The more heavily crowded bus ends up making more stops (as detailed above) and the follow-up bus winds up beating it to Kenmore Station.

      Had a similar experience last night on the 1 bus after I typed out this post. I'm sure the same story goes for all the frequent, heavily used routes.

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