It assigns successively more and more trips to congested links even as they become more and more congested. In an important sense, static trip assignment is a kind of “lemming” model of travel behavior. That’s because four-step models, like the one used to create the Rose Quarter I-5 estimates, uses something called static trip assignment, STA that takes almost no notice of the effects of congestion and delay. That is, they won’t be deterred by congestion from changing the timing, mode, destination, or whether they take a trip at all. It predicts that as population increases, more and more people will drive more and more miles, and that they will all be oblivious in their trip making behavior to any changes in congestion. The most important one is that it allows traffic to grow without limit in the “base” case. The traffic forecasting model used by ODOT is inherently structured to over-predict traffic congestion, and presents a distorted picture of what’s likely to happen with freeway widening.The model, a classic four-step traffic demand model (albeit with a few tacked on bells and whistles, like a microsimulation package) is decades old technique with a couple of critical and well documented weaknesses. As we detailed earlier, one problem is that the model (without saying so) is based on the assumption that a 12-lane, 5 mile long freeway was built (reality check: it wasn’t) between Portland and Vancouver in 2015, and is funnelling thousands and thousands more additional cars into the Rose Quarter than is actually the case.īut there’s a more fundamental problem. How do they know that? Because they have a model that says so.īut how realistic is that model? As it turns out, not very. The traffic projections made as part of the Environmental Assessment for the $500 million Rose Quarter I-5 widening project make an audacious claim that the project will reduce traffic congestion and greenhouse gases, and completely unlike any other freeway expansion project, won’t induce any additional demand.

Highway planners use a deeply flawed “lemming” model of traffic that rationalizes highway widenings

Lemmings jumping off cliffs youtube