Induced demand unaccounted for in traffic models even when public demands it

Conventional practice fails to account for induced traffic, even when the public demands it [more] By Eric Sundquist

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 tl;dr: 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 (keeps speeds and Level of Service up) than they really are (induces traffic and VMT).

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.

All-way stop signs reduce crashes by one-third

From 2009 to 2016, the Washington, DC, District Department of Transportation converted 60 intersections from two-way stops (allowing free flow on the major route) to all-way stop control. A new study in the Transportation Research Record looks at 53 of those intersections with available data and shows promising safety benefits. [more]

County wants to make informal paths safer, more formal

Montgomery County, MD, is asking residents to mark the informal paths that they have been using to reach destinations. Many paths are worn through woods, cul-de-sac ends, and beaten into the grass where good pedestrian connections don’t exist. The county is updating its Pedestrian Master Plan, and wants the walking environment to be improved for those who may not want to or be able to negotiate these dirt tracks. [more]

Recent trends in school travel mode choice

A recent study analyzed trends in travel to school in the U.S. using 2017 National Household Travel Survey Data and explored factors that impact mode choice. Among the findings, the study shows that the share of students walking and biking to school has increased since 2009 and the share of students traveling to school by car has decreased. [more]

In their new study, they wrestle with two conflicting perspectives: 1) driving rates are lower in highly accessible places, which should translate into fewer crashes, and 2) these places also concentrate traffic and other activities, creating more potential conflicts. Using data from Knoxville, Tennessee, they found crash rates are ultimately lower in more accessible places, defined as those with larger numbers of jobs within a 10-minute drive.

The researchers note that common characteristics besides accessibility also tend to make these places safer, such as more sidewalks and paid parking (which discourages car use). Our recent work also indicates that high driving accessibility often translates into much better walking, biking, and transit access. In other words, safety is about much more than a 10-minute drive.

There is a caveat, however. To test the effects of traffic exposure, the researchers introduced a new measure, called “accessibility to vehicle miles traveled (VMT),” which focuses on the amount of traffic on freeways and arterials within different travel thresholds. Crash rates are higher in places with higher access to VMT within 30-minutes, according to the study, potentially offsetting the benefits of good access to jobs and services. Traffic on local roads did not have the same effect.

The authors sum things up this way:

“Accessibility, therefore, appears to have two contrary effects on household crashes; it reduces the amount of vehicular exposure by allowing households to drive and travel less but increases the amount of ambient traffic in the surrounding built environment. High-accessibility locations might be safer if such locations allowed a shift away from vehicular modes, because then accessibility could reduce travel-related exposure without increasing exposure to ambient traffic.”

Chris McCahill is the Deputy Director at SSTI.