Induced Travel

“They will come even if you don’t build it.”
-Anonymous Simpleton

Let’s be realistic. New roads have an impact on an area’s transportation system. If they didn’t there would be no reason for even considering their construction. That said, one of the most hotly debated topics among planners and policy makers is the question of whether the construction of new roads or the significant expansion of existing roads offers viable congestion relief or if such construction is part of a never ending cycle that just leads to more travel and ultimately more congestion.

Induced Traffic is a term that has caused a great deal of confusion. In lay circles it has come to be a catchall phrase for any new traffic generated by new road construction. In the scientific community, the term is often not even found and is instead replaced with the more specific term, Induced Travel. However, in and of itself, Induced Travel does not describe the entirety of the potential impact of new transportation systems.

In reality, there are three separate “usage inducing” components, which need to be considered when planning transportation projects. Although each component must be considered separately, to a large degree they are interrelated and often interact. It should also be noted that although this discussion focuses on road construction, the same principles apply to all modes of transit.

The following definitions will help to clarify the differences.

Induced Travel – any increase in total travel that results from a change in transportation system capacity within a defined geographic area. This may result from new trips that previously did not occur or from additional mileage from existing trips.

Included in this category would be existing trips from outside the defined geographical area that chose to use the new facility over former routes because of time and/or cost savings. In essence, these are new trips for the area even though they will result in a decline in usage in another region.

Although usually limited in impact, Induced Travel includes Mode Shifts whereby the user(s) changes transportation modes to take advantage of reduced travel time and/or costs. For example, a rail user may switch to driving because a new road makes it more “profitable” to take the car than to take the train.

Redistributed Travel – a shift in existing travel from existing roads to the new facility. Although this may result in additional travel on the new road and can cause congestion on that facility or its supporting road system, redistributed traffic does not generate an overall increase in travel within the defined region.

There are two components of redistributed traffic:

Diverted Travel – Route switching to reduce drive times
Time of Day Shifts – Drive time switching to take advantage of lower congestion.

Redistributed traffic should not be considered to include existing trips from outside areas, as those trips would more appropriately be included under the label Induced Travel.

Induced Development – increased travel that results from residential, commercial or industrial development that occurs or is encouraged as a result of increased access to a transportation facility.

Although this could be included under the category Induced Travel, there is planning value in separating traffic that results purely from increased capacity and that which results from new development.

The Economics of Travel Decisions

Although very few drivers think about roads in this manner, the reality is that most transportation decisions are fundamentally economic in nature. We choose destinations and routes based on cost, whether that cost is one of time, money or both.

Americans tend to discount the financial cost of automobile travel because of the relatively low price of gasoline. In most cases where the financial cost is the determining factor for a particular route, tolls are a bigger determinant, especially for trucks, than is the per mile cost of fuel and operations.

Consider for a moment how Americans view travel distances. Ask almost any European how far it is from point x to point y and you will give a response in miles or kilometers. Ask an American the same question and they will almost invariably get a response that is time based. How far it is from Philadelphia to Washington D.C.? Three hours.

If we ignore the fact that the answer is totally wrong, it points out a very fundamental difference in the way travel is perceived between various cultures. Americans place a high economic value on time and are likely to disregard distance when deciding where to go if there is not a time premium associated with the trip. For example, most shopping malls measure their sphere of influence not in terms of miles but in terms of travel time. When a new roadway opens that shortens the travel time, the sphere of influence of the mall expands and more shoppers are attracted. When new shoppers are attracted, road usage obviously increases. This is a prime example of Induced Travel.

The same holds true for work trips. Let’s say that a worker was looking to locate within a 45 minute commute from his/her place of work. The distance of that commute could vary greatly depending on traffic conditions. Lower speeds and/or more congestion would result in a “need” to be closer while access to free flowing traffic at higher speeds could greatly expand the “acceptable” area. If a new expressway opens and that worker moves further out into the suburbs because he/she can still make his/her commute in 45 minutes despite the longer distance, then we have another example of Induced Travel.

Our freeway system has allowed many people to live substantial distances from their place of employment. For some time, the system even seemed to work well enough to encourage many others to take the plunge and move further and further out into the suburbs. However, roads do have a finite capacity and once that capacity is exceeded the system breaks down quickly and decisively. One only need look at the commuter towns serving Washington D.C. along I 270 in Montgomery County, Maryland to see how bad things can get.

In the case of I 270, the four lane road was expanded to six lanes in 1972 to handle the traffic generated from the rapid residential growth. The road expansion was met with a wave of new development (in all forms) and traffic once again filled the roadway to capacity. It was expanded to twelve lanes in 1989 but, once again, the capacity of the new road was countered with intense new development along the corridor. Now, with 258,000 vehicles a day clogging the artery, it stands (literally, since much of the time no cars are moving) as a monument to sprawl and ineffective planning.

We bring up I 270 for a purpose. First, it is one of the great examples of “you can’t build your way out of congestion.” Secondly, it is a prime example of why it is important to study road projects from an economic perspective. Had they done so they would have found that there would be significant impacts that would mitigate the purported benefits.
Yes, traffic was bad when the road was four lanes but it is no better now with twelve lanes and the communities along the corridor have been saddled with a host of new problems and costs that have accompanied the rapid development of their region.

 

The Battleground for Induced Traffic

For years there has been a face off whenever there is a challenge to a major new road project.

On the pro-road construction side, people will argue that the traffic already exists and that it is foolish to expect that 1950’s infrastructure can accommodate 21st century transportation needs.

On the other side are those who believe that new road construction leads to increased travel and development patterns that result in the new roads soon becoming as clogged as the roads that they were designed to relieve.

Interestingly, both sides are right.

The simple reality is that our current infrastructure cannot support the travel demands in many parts of the country. Pro-road advocates are correct in their analysis that 1950’s infrastructure cannot accommodate 21st century needs. No big surprise there. All you have to do is look around you to figure that one out.

Where their thinking goes more than a bit askew is the assumption that merely increasing the amount of 1950’s technology (more roads) will provide a satisfactory solution to today’s congestion. “More is Better” is a distinctly American way of looking at life and the simplicity of the argument appeals to a large group that doesn’t want to take the time or doesn’t have the capacity to look at the situation realistically.

Those who argue that new roads induce more travel and development now have the backing of a substantial body of scientific research to support their position. For example, Mark Hansen and Yuanlin Huang found in their study, "Road Supply and Traffic in California Urban Areas," that:

Every 1% increase in new lane-miles generated a 0.9% increase in traffic in less than five years, from which they drew the conclusion that "With so much induced demand, adding road capacity does little to reduce congestion."

This study was highly controversial when it was released in 1997 but has been supported by many other studies in the interim. Below, you will find links to many of those studies and we urge you to consult the references and bibliographies of each if more detail is desired.

Even the FHWA and the EPA, which denied and/or ignored the existence of induced travel and development in NEPA reviews of roadway projects for so many years have now admitted that the phenomenon exists. That’s great news, but now what?

If there is anything that we need to take from the “startling” revelation that induced travel exists, it is that we cannot plan for our infrastructure needs in a vacuum. Infrastructure and land use go hand in hand and to plan one without integrating the other can only lead us down a path where nobody really wants to go; that is, continued sprawl and continued congestion.

Equally, it is critical that the purpose of new road projects be clearly defined. If the goal of the road is to encourage growth then fine, be open and up front with that goal. In many cases, increasing transportation options will achieve the desired end. However, politicians and policy makers have an absolute moral obligation not to mask the true purpose of a new road with rhetoric about congestion relief.

As a case in point, residents of Bucks County Pennsylvania were sold a bill of goods when they were told that a proposed expressway would cure their congestion woes. On that basis and that basis alone, public support for the project was mustered and the road was approved for construction. Once the approvals were in hand, the stated purpose of the road was suddenly shifted to being one that “fuels growth” and promoters of the expressway now claim that they never promised long term congestion relief for the area, despite the fact that it is listed as the primary goal on the first page of the Environmental Impact Statement.

 

New Roads and Sprawl

There is an obvious relationship between new roads and sprawl but it would be incorrect to assume that all new roads cause sprawl. Consider that we could build a new expressway through the northern reaches of Montana that is unlikely to induce sprawl in the traditional sense. (That doesn’t mean that the road would be without consequences or that it would be well received by those impacted!) On the other hand, a road of the same type that was built in the suburban regions of any of the nation’s cities could have a major sprawl inducing effect.

One item of major consequence that is too often ignored in traffic studies is the behavioral patterns of the people that will use the road. Over the last 15 years there have been substantial shifts in the way many Americans live their lives. Big Box retail has come storming to the forefront of American retail and has had a dramatic impact on our travel patterns and transportation needs. Virtually all Big Box retailers are auto-dependent and prefer locations that are easily accessible from multi-lane roads. New mega-centers can attract between 10,000 and 15,000 cars a day, a large number by almost any standard.

Of equal importance is the continuing migration of businesses from the urban core. Many large corporations have chosen good road access over good transit access and there is no reason to expect that the trend will not continue. Corporate facilities that house over 5,000 employees are commonplace along the US 78 and US 287 corridors in Central New Jersey and it should not surprise anyone that they are among the most congested roads in the state. Nor should it surprise anyone that the formerly rural communities adjacent to the roads have undergone massive residential development that has fundamentally changed their nature.

Changes in the nature of the American job force need to be considered as we shift from a manufacturing to service oriented economy. Obviously, there are many more issues that could be discussed but the major point is that the highway agencies need to be cognizant of social trends and integrate those trends into their projections during the initial stages of project design. Good planning requires taking all factors into account.

 

Smart Growth and Congestion Relief

“Smart Growth” advocates often promote higher density housing options as one solution to the nation’s growing congestion problems. The theory is that by doing so, a critical mass of potential users will be formed making public transportation services feasible. It is an interesting concept but whether it is one that works or not is open for debate.

The first problem is that it makes an assumption that people will want to get out of their cars. Given the track record of our society over the last twenty years, that may not occur. Secondly, it assumes that the public transportation providers will offer service. At least in Southeastern Pennsylvania, that is a questionable assumption. SEPTA (South- eastern Pennsylvania Transit Authority) has already killed train service into densely populated areas such as Newtown and Quakertown and is now threatening to discontinue service into Warminister which is both heavily populated itself and the primary station for a number of other municipalities.

Given today’s constraints on our public transportation systems, it seems unreasonable to assume that merely grouping large numbers of housing units in close proximity to one another will ensure access to and use of public transportation. For “Smart Growth” to be effective relative to congestion relief it needs to be implemented along existing transit routes. In many municipalities, this is simply not possible because public transportation options are so limited.

Surprisingly, “Smart Growth” alternatives could actually increase levels of congestion in some cases. Consider the following example on a 100 acre tract of land:

Typical Sprawl Development –
2 acre lots / 15% loss for infrastructure and right-of-way
42 houses x 10 trips per day per house (ITE) = 420 trips per day

“Smart Growth” cluster –
3 units per acre – 50% open space – 30% transit usage
150 units x 7 trips per day per house (ITE – 30%) = 1050 trips per day

It is simple math but it yields a distressing result. Something to think about isn’t?

The point of the above was not to discount “Smart Growth” as a viable alternative to sprawl but merely to point out that it is unlikely to be “the” simple answer to the issue of congestion management. Obviously, it is a very difficult problem that defies easy solutions. Proper road planning is in large part situational and dependent on numerous factors that may vary from location to location. We will need leadership and intelligence to innovate new “technologies” that will meet our transportation needs and finding that is going to be a lot more difficult than proving that induced travel exists.

 

Links

Most of the studies below provide extensive bibliographies and references that can be quite useful for further research. In addition, the February 1996 issue of the journal Transportation was devoted to induced travel and had a number of interesting articles on the subject that remain relevant.

Travel Behavior Issues in the 90’s – This U.S. Department of Transportation document is often cited in research and articles. It covers a range of topics and provides a wealth of statistics on U.S. travel patterns.

The Effects Of Added Transportation Capacity – This somewhat older document from the Transportation Library of the U.S Department of Transportation provides an overview of a conference on the issue of induced traffic held in 1991. In reviewing the comments it is clear that the problem was recognized and that there was a significant concern about the potential impact even at that early date. What is unclear is why the EPA, an agency that recognized the potential adverse effects very early on, did not require the impact of induced traffic to be included in the modeling for new road projects.

Desk Reference for Estimating the Indirect Effects of Proposed Transportation Projects – This is the reference document for National Cooperative Highway Research Program Project 25-10, which was completed in 1996 and reflects the following; “The objective of this research is to develop an analysis framework and supporting methods to facilitate identifying, understanding, describing, and evaluating indirect effects of transportation projects.”

The Impact of Bypasses on Small and Medium Sized Communities: An Economic Evaluation – A technical review of the economic impact of bypasses starts on page 57 of this issue of the Journal of Transportation and Statistics.

Relationships Between Highway Capacity and Induced Vehicle Travel – Given the reluctance of many public officials to acknowledge the phenomenon of induced traffic, it is hard to believe that the EPA actually published this report.

A Statistical Analysis of Induced Travel Effects in the U.S Mid_Atlantic – This paper (00-1289) was presented at the Annual Meeting of the Transportation Research Board, Washington, DC, in January of 2000. It details the impact of induced traffic and has a very concise conclusion that should be considered by policy makers and the public BEFORE approving road projects.

The following data is from Texas Transportation Institute and provides a very interesting insight into the extent and the costs associated with congestion. The 2003 Annual Urban Mobility Report contains a plethora of information as well as access to national congestion data including data on congestion for the Philadelphia region.

Generated Traffic and Induced Travel – This is an excellent, (relatively) easy to read paper from the Victoria Transport Policy Institute. Lots of good information.

Surface Transportation Policy Project – STPP is highly regarded and their site contains a wealth of information on congestion and induced traffic. Use their search engine, its good.

Victoria Transport Policy Institute – is an independent research organization whose website provides a wealth of information on a wide variety of transportation issues. The quality of their product is excellent and their thoughtful approach to transportation issues is to be commended.

Transportation Research Board (TRB) – This organization covers just about every imaginable aspect of transportation down to and including road surface materials. Because of the number of topics covered, finding what you want can be a bit challenging but in all likelihood it’s there.

National Transportation Library – This site is a product of US Department of Transportation and provides an enormous supply of resource material. Their search engine generates too many unrelated documents but have patience, the material you need is probably there.

Texas Transportation Institute – TTI is associated with the Engineering Department at Texas A&M and provides access to some interesting research and resources.

Road Expansion, Urban Growth and Induced Travel, A Path Analysis – This study from UC Berkley is interesting in that the researcher appears to have started out with the assumption that studies validating induced growth were wrong. In the end, he concurs that the issue is real but suggests that it is but one component in a complex system that includes land use decisions and other transportation options. He has a point that is well worth considering and integrating into all transportation planning.

You are not going to believe this – Anyone who has fought the FHWA and was rebuffed when the issue of induced travel was brought up will take some satisfaction from this little ditty that is buried deep in the FHWA website. It appears to have been written by Lamar Smith, who ignored and dismissed exactly the same arguments that he now makes when they were presented to him by opponents to the US 202 expressway in Bucks County, Pennsylvania. For those who expended so much effort in a losing battle, this is a bitter pill to swallow. (Please advise us if they remove the document – we have downloaded the document and can repost from this site if necessary.)

Tri-State Transportation Campaign - provides an excellent bibliography of induced traffic resources.

How “Smart Growth” Intensifies Traffic Congestion and Air Pollution – There are about 8,000 things wrong with the author’s logic in this article but it is often helpful to understand what the other side is thinking. (The thinking part is questionable in this case.)