tag:blogger.com,1999:blog-5666195730630249633.post3248007217503019964..comments2024-03-25T17:49:41.408-07:00Comments on Salem Breakfast on Bikes: Study on Traffic Forecasting Urges Embrace of UncertaintySalem Breakfast on Bikeshttp://www.blogger.com/profile/15618055627843335993noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-5666195730630249633.post-63808394266486166892020-08-31T11:56:11.192-07:002020-08-31T11:56:11.192-07:00Here is a critique of "four step" algori...Here is a critique of "four step" algorithms - <br /><br />https://www.vice.com/en_us/article/v7gxy9/the-broken-algorithm-that-poisoned-american-transportation-v27n3<br /><br />"...nearly everyone agreed the biggest question is not whether the models can yield better results, but why we rely on them so much in the first place. At the heart of the matter is not a debate about TDMs or modeling in general, but the process for how we decide what our cities should look like.<br /><br />TDMs, its critics say, are emblematic of an antiquated planning process that optimizes for traffic flow and promotes highway construction. It’s well past time, they argue, to think differently about what we’re building for."Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5666195730630249633.post-23638094674809137182020-01-25T18:38:59.805-08:002020-01-25T18:38:59.805-08:00By email, a reader forwards a comment about a kind...By email, a reader forwards a comment about a kind of control that the study omits:<br /><br />"<i>While it is frank about the errors in making projections for projects that are actually built, I think there's a subtle 'dog that didn't bark' bias in the research design here.<br /><br />As you know well from the SRC, the policy question isn't so much the accuracy of the build forecast, as it is being able to accurately compare states of the world (congestion, emissions, etc) between the build and no-build (and possible other options). This study didn't look at the accuracy of no-build forecasts for projects that weren't built. <br /> <br />My experience (and other analyses) have concluded that traffic models tend to way over-predict traffic growth under the no-build scenario. Wes Marshall attributes this to the use of static assignment, rather than dynamic assignment, which allows the model to overload successive links in a system without regard to whether capacity is available. So in the 'no build' models, they forecast an amount of traffic that one link can't possiblly handle, and then allow that impossible traffic to show up on subsequent links, when, in fact, there would be a feedback loop that would lead to some combination of route changes, mode changes, time shifts, and reduced trip making.<br /><br />The clue with this particular study is that the word 'induced' appears, as far as I can tell, exactly once in the text, and not in the context of induced demand.<br /><br />So yes, the 'build' forecasts are inaccurate, and biased. But what may be more important from a policy standpoint is that the 'no-build' forecasts--which this report doesn't consider--are even more biased, and in the opposite direction, which leads policy makers to too dour a view of conditions under the no-build, relative to the build condition.</i>"<br /><br />That's a good point!Salem Breakfast on Bikeshttps://www.blogger.com/profile/15618055627843335993noreply@blogger.com