Posted by Matt Williams
Caltrans Planning Horizons: Induced Vehicle Travel in the Environmental Review Process
Empirical research shows that expanded roadway capacity attracts more vehicles. However, environmental impact assessments of roadway expansion projects often ignore, underestimate, or mis-estimate this induced travel effect and overestimate potential congestion relief benefits. A very recent ITS-Davis presentation on September 29, 2021 covered an online tool developed by UC Davis to facilitate estimation of induced vehicle travel impacts of roadway capacity expansion projects. For more information email firstname.lastname@example.org.
Increasing Highway Capacity Unlikely to Relieve Traffic Congestion
ITS-Davis reports that reducing traffic congestion is often proposed as a solution for improving fuel efficiency and reducing greenhouse gas (GHG) emissions. Traffic congestion has traditionally been addressed by adding additional roadway capacity via constructing entirely new roadways, adding additional lanes to existing roadways, or upgrading existing highways to controlled-access freeways. According to ITS-Davis numerous studies have examined the effectiveness of this approach and consistently show that adding capacity to roadways fails to alleviate congestion for long because it actually increases vehicle miles traveled (VMT).
An increase in VMT attributable to increases in roadway capacity where congestion is present is called “induced travel”. The basic economic principles of supply and demand explain this phenomenon: adding capacity decreases travel time, in effect lowering the “price” of driving; and when prices go down, the quantity of driving goes up. Induced travel counteracts the effectiveness of capacity expansion as a strategy for alleviating traffic congestion and offsets in part or in whole reductions in GHG emissions that would result from reduced congestion.
Environmental Reviews Fail to Accurately Analyze Induced Vehicle Travel from Highway Expansion Projects
ITS-Davis reports that induced travel is a well-documented effect in which expanding highway capacity […] induces more driving. This increase in vehicle miles traveled (VMT) increases congestion (often back to pre-expansion levels) and air pollutant emissions, reducing or eliminating the purported benefits of the expansion. Yet highway expansion projects continue to be proposed across California, often using congestion relief—and sometimes greenhouse gas reductions—as a justification for adding lanes. These rosy projections about the benefits of highway expansion projects indicate that the induced travel effect is often not fully accounted for in travel demand models or in the projects’ environmental review process.
With this problem in mind, researchers at the University of California, Davis developed an online tool to help agencies estimate the VMT induced annually by adding lanes to major roadways in California’s urbanized counties. The researchers also applied the calculator to estimate the vehicle travel induced by five highway expansion projects in California that had gone through environmental review within the past 12 years. They then compared their estimates with the induced travel analysis completed for the projects’ actual environmental impact assessments. This policy brief summarizes findings from that research, along with policy implications.
Take it from Caltrans: If you build highways, drivers will come.
The analysis found that adding a lane in each direction border-to-border will save I-95 travelers well over 14 million hours of delays by the year 2040. Likewise, the widening of I-84 will save travelers over 4.7 million hours of delays during the same period.
Bloomberg goes on to report that congestion relief itself is a dubious claim when it comes to road expansions. Transportation experts have repeatedly found that building new roads inevitably encourages more people to drive, which in turn negates any congestion savings—a phenomenon known as “induced demand.”
So it’s refreshing — and rare Bloomberg goes on to say — that CalTrans by way of a link to a policy brief outlines important research findings from years of study into induced demand. The brief, titled “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion,” compiled by UC-Davis scholar Susan Handy found that:
- There’s high-quality evidence for induced demand.
- More roads means more traffic in both the short- and long-term.
- Much of the traffic is brand new. From leisure trips that drivers aren’t willing to make in bad traffic.
Bloomberg noted that the 2014 assessment of Caltrans, conducted by the State Smart Transportation Initiative (SSTI), specifically cited induced demand as a research finding that had yet to filter down “into the department’s thinking and decision making”:
For example, despite a rich literature on induced demand, internal interviewees frequently dismissed the phenomenon.
Ronald Milam of the transportation consultancy Fehr & Peers told Bloomberg that CalTrans has recognized the shortcomings of traditional traffic models and tried to improve its analyses to better account for induced demand. State of Connecticut officials also seem to understand that expanding roads won’t resolve the state’s traffic problems. “You can’t build your way out of congestion,” Tom Maziarz, chief of planning at the state DOT, recently told the Connecticut Post. And yet the interstate widening project moves forward.
Induced Demand — When traffic-clogged highways are expanded, new drivers quickly materialize to fill them. What gives? Here’s how “induced demand” works.
Bloomberg News reports that with 26 lanes at its widest point, the Katy Freeway in the Houston metro which made it at that time the widest freeway in North America, was the result of an expansion project that took place between 2008 and 2011 at a cost of $2.8 billion. The primary reason for this mega-project was to alleviate severe traffic congestion.
Unfortunately, after the freeway was widened, congestion got worse. Data from Houston’s official traffic monitoring agency found that travel times increased by 30 percent during the morning commute and 55 percent during the evening commute between 2011 and 2014. Bloomberg reports that as a textbook example of induced demand, which refers to the idea that increasing roadway capacity encourages more people to drive, thus failing to improve congestion.
Nearly all freeway expansions and new highways are sold to the public as a means of reducing traffic congestion. It’s a logical enough proposition, one that certainly makes plenty of sense to anyone who’s stuck in traffic: Small communities served by small roads grow bigger, and their highways need to grow with them. More lanes creates more capacity, meaning cars should be able to pass through faster. But that’s not what always happens once these projects are completed.
In situations of induced demand means the new roadway capacity creates new demand for those lanes or roads, maintaining a similar rate of congestion, if not worsening it. The principle is simple economics … when you provide more of something, or provide it for a cheaper price, people are more likely to use it.
Transportation researchers have been observing induced demand since at least the 1960s, when the economist Anthony Downs coined his Law of Peak Hour Traffic Congestion, which states that “on urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity.”
Bloomberg reports that there has also been a de-facto stance of most public officials and departments of transportation in the United States and much of the world, which have largely avoided reckoning with induced demand in their long-term planning. But the public and their elected representatives could be starting to see the writing on the sound barriers. Many departments of transportation are instead touting the benefits of toll lanes, a more au courant form of roadway capacity expansion.
How It Works
Induced demand is often used as a catch-all term for a variety of interconnected effects that cause new roads to quickly fill up to capacity. In rapidly growing areas where roads were not designed for the current population, there may be a great deal of latent demand for new road capacity, which causes a flood of new drivers to immediately take to the freeway once the new lanes are open, quickly clogging them up again. In aggregate, these choices put more cars than ever before on the newly expanded road, increasing net vehicle miles traveled (VMT) (and greenhouse gas emissions).
In the longer term, roadway expansions make an impact. Businesses that rely on trucking are more likely to locate near these new roads. With those new jobs, and access to countless more via the higher capacity road, housing developments and shopping centers spring up nearby. Urban form responds to existing infrastructure: Roadway capacity expansions spawn autocentric development patterns that utilize the new roads.
How quickly does new road capacity get filled up?
Bloomberg reports that it is important to note that measuring induced demand is a somewhat inexact science. Most studies provide ranges that estimate the amount of road capacity that is filled by induced demand over a given period of time. One literature review, conducted by Susan Handy of UC Davis for CalTrans found that a 10 percent increase in road capacity yields a 3 to 6 percent increase in vehicle miles travelled in the short term and 6 to 10 percent in the long term.