Failing to Account for Induced Traffic, Even When the Public Demands It

Induced traffic occurs when new highway capacity speeds up traffic, allowing travelers to drive further, increasing sprawl. A review of major highway project plans finds that most fail to accurately account for induced travel effects.

3 minute read

July 5, 2020, 9:00 AM PDT

By Todd Litman


Bay Bridge Toll Traffic

Aaron Kohr / Shutterstock

The problem of induced traffic, aka induced demand, is well-documented in the literature. Yet it is too rarely accounted for in practice. A new paper by Jamey M.B. Volker (University of California-Davis) and co-authors examines the environmental documents from five major highway projects to see how they addressed induced traffic. The results: not well.

Induced traffic occurs when new highway capacity speeds up traffic, allowing travelers to drive further and land uses to spread out. These rebound effects eventually reduce travel speeds again, leaving travelers with longer trips in congested conditions. Failing to account for induced traffic makes highway capacity projects look much more beneficial than they really are.

Unfortunately, the environmental documents from the five projects reviewed in the paper generally ignored induced traffic except when prompted by public comment. Even then, discussion of the problem was sometimes internally inconsistent and failed to reflect findings from the literature. In three of the five cases, environmental documents did make an estimate of induced traffic, but these estimates were much less than the literature would suggest—and in two cases by orders of magnitude.

The paper points to reliance on demand models as an important source of bias in induced traffic estimates:

Current travel demand models do not fully account for induced vehicle travel, as detailed by Milam et al. (6) and Litman (15). The issue is that most models do not include all of the feedback loops necessary to represent the secondary effects of capacity expansion. These models were designed to estimate the effect of capacity expansion on travel times for a given population and employment level for the region. Many models now feed the estimated travel times back into the mode split stage of the model, thereby accounting for potential shifts away from transit and active modes resulting from improved travel times. Few models feed the estimated travel times back into the trip distribution or trip generation stages of the model, thereby ignoring the possibility that improved travel times will increase the number of trips that residents choose to make or the possibility that they will choose more distant destinations for their trips. Few models feed estimated travel times back into assumptions about the distribution and growth of population and employment that also influence the frequency and length of trips. In short, the models may do an adequate job of accounting for changes in route and shifts in mode, but they underestimate increases in VMT attributable to increases in trip frequencies and lengths that the capacity expansion will induce.

Induced traffic estimates can be improved by using lane-miles-to-VMT elasticities, such as those in the Induced Travel Calculator at UC-Davis, the paper suggests. The calculator’s estimates are based on areawide averages and may miss facility-specific conditions, but they can be used as default estimates or in combinations with other methodologies. The calculator is currently populated only with data from urbanized counties in California. But other states could readily develop their own calculators using available data.

Wednesday, July 1, 2020 in Transportation Research Record: The Journal of the Transportation Research Board

Aerial view of homes on beach in Maui, Hawaii

Hawaii Passes First Legislation Regulating Short-Term Rentals Statewide

The new law will give counties the power to limit number or short-term rentals and convert existing short-term rental units back into long-term residential housing.

May 13, 2024 - USA Today

Aerial view of Oceanwide Plaza skyscrapers covered with graffiti tags.

LA’s Abandoned Towers Loom as a “$1.2 Billion Ruin of Global Capital”

Oceanwide Plaza, shuttered mid-construction after its developer filed for bankruptcy, has stood vacant on prime Los Angeles real estate since 2019.

May 21, 2024 - The Architect's Newspaper

Entrance to a drive-through car wash at night with green 'Enter' sign.

Ohio Towns Move to Ban New Car Washes

City officials in northeast Ohio are putting limits on how many car wash facilities can open in their towns.

May 16, 2024 - News 5 Cleveland

People on bikes and walking on 606 Bloomingdale Bike Trail on sunny day in Chicago, Illinois.

Chicago Leads Nation in Biking Growth

Cycling as a mode share grew faster in the Windy City over the last five years than in any other major U.S. metropolis.

42 minutes ago - Momentum Magazine

Clpse=up of Uber and Lyft stickers inside a car windshield.

Minnesota Lawmakers Reach Ride Share Compromise

A law awaiting the governor’s signature establishes wage rates for drivers. Ride share companies say if the law passes, they plan to continue service in the state.

1 hour ago - Minnesota House of Representatives

Green and silver CTfastrak bus at a station in Elmwood, Connecticut.

Connecticut Bus System Earns Top Ranking

Hartford’s CTfastrak system wins out over other US bus networks for service quality and connectivity.

2 hours ago - CT Insider

News from HUD User

HUD's Office of Policy Development and Research

Call for Speakers

Mpact Transit + Community

New Updates on PD&R Edge

HUD's Office of Policy Development and Research

Urban Design for Planners 1: Software Tools

This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.

Planning for Universal Design

Learn the tools for implementing Universal Design in planning regulations.