Tuesday, July 10, 2012

Why are we shackling our future to crude traffic models?

The Casey Overpass (project)
Potentially good news received: the Casey Arborway design may be reduced by two lanes. The at-grade intersections replace the failing Casey Overpass. Originally, MassDOT planners had pushed for an extra two lanes for the corridor that is being renovated, even though the Arborway on either side is only four lanes.
The “opening year” design, so called because it is aimed at accommodating 2016 levels of traffic, the projected opening date for the new Arborway, has fewer lanes than the full 2035 design, though it leaves space for the future lanes to be added when necessary. Combined with other changes to the total design, it is a reduction of up to 22 feet of pavement for pedestrians to cross.
The extra lanes had been something of a poison pill, leading to a questionable choice between a blighting overpass or a crummy intersection. It's not clear how they would have helped drivers either, since they would have to merge again once they passed through.

But I also want to draw attention to a few statements:
As the Gazette reported earlier this monthtwo different versions of analyses showed occasionally widely different levels of service (LOS). [...] One version of the analysis was presented to the advisory group last October. A newer version was released last month following a six-week delay after a Freedom of Information Act (FOIA) request. In the newer analysis, some intersections had much worse LOS scores while some fared better. According to the design team, as the design evolves—with traffic lanes being added, subtracted or moved—LOS will improve or worsen at impacted intersections. 
“As design progresses and is tweaked, the [LOS] results will change,” MassDOT Highway Administrator Frank DePaola told the Gazette in a prepared statement. “The good news is, by making even small changes in a sophisticated traffic modeling system, we are allowed to see the effects of those changes in relatively short order.”

It's completely backwards to attempt to project "level of service" grades for the intersection in 2035, and then design based upon that. That is a sure-fire way to overbuild the road. Since "level of service" contains no provision for walking or bicycling, basing a project on it will completely ignore any issue but pushing automobiles down the road as fast as possible. This might be appropriate for a highway in the middle of nowhere, but it is a completely inappropriate attitude towards a densely populated region where people live and work, and where they like to be able to walk and bicycle as well as drive.

It's clear that the models they are using are extremely crude and subject to wild variation. The 17% growth in traffic they are projecting "is a number not seen anywhere else in the city." That means it is a massive overestimation. Trying to build a road that accommodates so much traffic will only lead to more cars trying to use it, degrading quality of life for residents. A self-fulfilling prophecy of doom. Instead of this destructive path of attempting to pursue guesstimated "level of service" goals, MassDOT should first be working to provide a corridor which is appropriate to the neighborhood, and then working within those constraints to provide safe traffic flow. Not the other way around.


  1. I'm very pleased that MassDOT has decided to build only what we need today. Traffic volumes have been going down or at least staying level across the city. Why would we just assume growth when the trend is the complete opposite? And let's say for a moment there are more people trying to drive in 2035 and that they design this section of roadway to handle it. Will the adjacent roadways and intersections even be able to handle it? Are they going to widen those too? As we've seen time and time again, building wider roads just makes traffic worse. Let's stop the insanity now.

  2. But since building more roads create more traffic, then that creates new bottlenecks on the surrounding roads, which the DOT can offer to fix with even more expansion. After all, one of the keys to the survival and expansion of any bureaucracy is continually justifying its own existence. Induced demand is perfect for that, because do you really expect any DOT to just one day say "okay, we're done, we don't need any more roads", barring some Detroit-level urban catastrophe?