Parallel Bridges

David Bragdon, the Chair of Portland’s Metro Council, has a fine piece in the Oregonian on a subject of direct relevance to this region – a massive bridge across a major river.

The proposed I-5 Bridge across the Columbia (to replace and add capacity to one already there) has often been used as a justification for the Port Mann twinning.  Bragdon is a supporter of this multi-billion-dollar exenditure, but he has some qualms – particularly with respect to induced traffic and tolling. 

Writes Bragdon:

The way in which these projects have a history of defeating their purpose is when they attract more traffic than expected and create more congestion, rather than solving it. I am well aware the modelers claim that won’t happen in this case, but you need to look at the model and the modelers before accepting that assertion.

The key concept to consider is what economists call “induced demand.” Transportation infrastructure is one of the few categories of capital investment in which the supply curve (capacity) can actually influence the demand curve (traffic).

For most other forms of infrastructure, the supply curve does not have that interactive effect on demand; if we build more schools, people do not have more children; if we build more sewers, people do not flush the toilet more. …

While it seems common sense to say “if demand is ‘x’ then build to ‘x,’” experience shows that, without other controls, building to “x” causes demand to rise to “x plus.” And so it goes. That, in one sentence, is the history of 20th century transportation planning in America.

There are many reasons to believe the impact of induced demand will be somewhat limited in this case: There are good land use systems in place on both sides of the river; the new bridge would replace an established one in an existing corridor rather than opening up a long new corridor; good transit will be provided where today there is very little; and some form of tolling will dampen demand. … 

When the two state DOTs (really the highway divisions) are asked about induced demand, they cite the mitigating factors above, which are valid, but they ultimately rest their case on two key statements that are unproven — and that the two DOTs will not allow to be scrutinized independently. …

The two underpinnings they rely on are, first, the assertion that settlement patterns will not change; and, second, that the toll will dampen demand. Neither of these two assumptions, while somewhat plausible, has been validated.

The first, that settlement patterns would not change, is an output of the model only because the two DOTs insisted it be an input. They created a tautology rather than any illumination. The second, that tolls will dampen demand, is valid as a general assertion but has not been subjected to rigorous analysis on the obvious question of “how much”: How much does it dampen demand, and how does pricing relate to the question of physical capacity?

Nor has any agreement on tolling been reached, or assurance that political pressure will not reduce the tolls. It’s very curious to me, and leads me to question to motivation of the DOTs, that they have deliberately segregated two questions that are inextricably linked, capacity and pricing, and insisted that we make decisions about those two things in isolation from each other. …

The fact that the two highway divisions are so defensive about their two key assumptions, and will not tolerate anyindependent scrutiny of them, should make inquiring minds want to investigate more.

Exactly the same two criticisms can be made of Port Mann: the Ministry refused to acknowledge the legitimacy of induced demand, and they have argued that tolls will prevent congestion re-emerging, at least within the next decade.  But like Portland, have separated the amount of the toll from controlling of demand. 

The difference between Portland Metro and Metro Vancouver: at least they have some inquiring minds at the political level.

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