The Outdated Models That Continue to Guide Transportation Planning — News

“Despite them being a legal­ly required por­tion of any trans­porta­tion infra­struc­ture project that gets fed­er­al dol­lars, it is one of plan­ning’s kept secrets that [ ] mod­els are error-prone at best and fun­da­men­tal­ly flawed at worst,” writes Aaron Gor­don.

Gor­don says the pri­ma­ry issue is not so much whether these mod­els pro­duce bet­ter results but rather why they are con­sid­ered the best guides to make plan­ning and pol­i­cy deci­sions. “TDMs, its crit­ics say, are emblem­at­ic of an anti­quat­ed plan­ning 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,” says Gor­don.

Gor­don describes major issues with TDMs: assump­tions about pop­u­la­tion, land-use pat­terns, and trav­el deci­sion-mak­ing as fixed and pre­dictable. The flip side, he says, is induced demand, where expand­ed capac­i­ty does not relieve con­ges­tion but rather ampli­fies it. “To ful­ly appre­ci­ate the absur­di­ty of quest, look no fur­ther than the $2.8 bil­lion project in Katy, Texas that was sup­posed to reduce com­mute times along the expand­ed 23-lane free­way, the widest in the world. All too pre­dictably, con­ges­tion only increased, and com­mute times are longer still.”

TDM crit­ics say the focus should be on dif­fer­ent mea­sures that reflect qual­i­ty-of-life issues such as access to pub­lic tran­sit, parks, and walk­a­ble neigh­bor­hoods. “The ques­tion is not whether the pre­dic­tions of how they will behave are accu­rate, but what of behav­ior we want to have more of,” adds Gor­don.

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