Sightline’s Highway Widening Analysis – A Lesson in System Boundaries

Although intriguing, I found Sightline’s analysis on greenhouse gas emissions associated with highway widening inherently flawed. While I was reading the article, and subtly shaking my head in disapproval, I was thinking a lot about the concept of system boundaries. Sightline seems to use very squishy and undefined system boundaries to make very broad assumptions that support their theory.

My first objection is to their analysis on new traffic. The assumption that the construction of new roadways is the major “cause” for new traffic seems like an enormous stretch. I don’t doubt that the capacity of the new highway will be absorbed eventually, but it is much more likely that the cause of this is due to economic drivers in the area rather than transit oriented development. I will not disagree that additional infrastructure would support future growth and potentially sprawl. But the quantification in this article seems widely underdeveloped and one that I must take with a grain of salt.

My second objection comes from Sightlines analysis on indirect fuel consumption. This objection really stems from the first. Simply stated, any indirect fuel consumption due to added traffic should be a fraction of the overall new traffic estimate. They make an assumption that for every 10 miles traveled on a highway the indirect distance to and from and on ramp would be approximately 1 mile. Basic arithmetic says that the carbon output should be 1/10th of that in the new traffic scenario. But Sightline’s estimate is actually greater, topping out at 100,000 tons of CO2. They seem to justify this number again by referencing development of low-density suburbs. If they are going to allocate all of the additional traffic coming from those suburbs to the new highway, why stop there. Shouldn’t the CO2 emissions from the construction of the suburb development also be included in this analysis? Where do we stop?

I think an important point was made in this article, but by failing to specifically define the system boundary by which these measurements were made, the argument falls apart quickly.

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