The mystery of Mass. Ave. traffic jams

There’s a great variation in traffic inbound on Mass. Ave. during the morning rush hour.

Most of the time traffic moves quickly, clearing traffic signals completely on the first go-round. Sometimes, however, the very eastern part of Mass. Ave eastbound is, to use a cliche, a parking lot. I think most regular commuters must have horror stories of how it took them ten, or twenty, minutes to move two blocks from Thorndike Street to Route 16.

Why are there so many more cars clogging Mass. Ave. some morning than others? Do car commuters all get up on the same morning and say, I think I’ll try Mass. Ave. today? You would think that with so many commuters, random variations would mostly even themselves out.

Also, given the fact of these traffic jams, how can traffic engineers possibly talk about there being excess capacity on Mass. Ave.?

To make a long story short, the traffic jams, which happen about once a month, are caused by problems in Cambridge–on Route 16 or Mass. Ave. Often there is a specific cause: an accident on Route 16, or a fire in Cambridge along Mass. Ave., for instance. What looks like a mysterious surge in the number of cars is really the same cars as always, only delayed and bunched together.

In terms of those nutty traffic engineers: they are saying, in effect, that there is no design change that Arlington can make to Mass. Ave. that will in any way address the problems caused in Cambridge. This is a pretty important insight for those redesigning that road. If you could somehow double the number of lanes on Mass. Ave., all you would achieve is to get to the bottleneck faster, where twice as many cars would wait twice as long (roughly) to get by.

Chad Gibson, posting at the Livable Streets blog, spells this out for us with a series of graphs comparing current and projected future traffic levels to the capacity of Mass. Ave. under the proposed design. (Note that the street names listed refer to traffic on Mass. Ave., not on the side street named.)

At just about every point along Mass. Ave., capacity would be at least double that of peak demand. See for yourself.

But, sadly, the plan does not address those Cambridge-caused traffic jams. Because nothing in Arlington can.

Update: No one has called me on this, but the figure of speech I used above about “twice as many cars” waiting “twice as long” etc. is almost certainly not precisely correct.

The basic point that there would be no appreciable gain for drivers from doubling the number of lanes is right, but I am aware of no analysis of an 8-lane (!) road design that tries to estimate actual times spent at bottlenecks and actual times getting to them.

Perhaps that’s clear from context, but consider this my footnote for the record.

1 comment so far

  1. Mark K. on

    What causes the Mass Ave congestion between Arlington High and Park Street during commuting hours? Answer: 1 travel lane/direction.

    What happens if there is an accident/breakdown, fire, or bridge closure on Rt. 16 between Mass. Ave. and Broadway or Rt. 2? Ans: worse if we reduce Mass Ave. traffic capacity.

    Bicyclists have an out, just use the multimillion dollar bike path.

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