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.

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

Chicago Commute

Planning for Congestion Relief

The third and final installment of Planetizen's examination of the role of the planning profession in both perpetuating and solving traffic congestion.

May 12, 2022 - James Brasuell

Twin Cities

Minneapolis Housing Plan a Success—Not for the Reason You Think

Housing advocates praise the city’s move to eliminate single-family zoning by legalizing triplexes on single-family lots, but that isn’t why housing construction is growing.

May 13, 2022 - Reason

San Francisco Houses

‘Mega-Landlords’ Threaten Housing Stability for Renters

As institutional investors buy up a larger share of single-family homes, the families renting them are increasingly vulnerable to rent increases and eviction.

May 15, 2022 - The Hill

Downtown Dallas

Short-Term Rentals Vex Dallas City Council

Residents complain that vacation rentals exacerbate the city’s housing shortage and bring traffic and noise to residential neighborhoods, calling on the city to impose—and enforce—stricter regulations.

May 17 - The Dallas Morning News

Traffic Safety Advocates

Traffic Fatalities Set Records as Pandemic-Era Road Carnage Shows No Signs of Stopping

An estimated 42,915 people died in automobile crashes in 2021, according to recent federal data. The increasing fatalities continue a trend that began with the outset of the pandemic.

May 17 - National Highway Traffic Safety Administration

Los Angeles County Metropolitan Transportation Authority

Driver Shortage Undercuts the Potential of L.A.’s Recent Bus System Redesign

The Los Angeles County Metropolitan Transportation Authority implemented a complete overhaul of its bus system in three waves over the course of 2021. A shortage of drivers for the system has made it impossible to implement that vision.

May 17 - TransitCenter

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.

Hand Drawing Master Plans

This course aims to provide an introduction into Urban Design Sketching focused on how to hand draw master plans using a mix of colored markers.