How the ITE overestimates traffic

The Institute of Transportation Engineers (ITE) manual for trip generation radically overestimates traffic spurred by new development, measuring "phantom trips" that never materialize.

Planners have long known that mixed-use, transit-oriented developments tend to generate far fewer trips than transportation manuals would suggest--44 to 48 percent fewer, we reported in articles in 2006 and 2008. 

But the problems with the ITE manual—the standard tool to estimate traffic generation—go deeper than that, according to in Access Magazine. Comparisons between actual trip generation, as revealed in the National Household Travel Survey (NHTS), and ITE data points to a 55 percent overestimation across all household and commercial development. NHTS is the most comprehensive travel survey in the US.

Source: Adam Millard-Ball, Access Magazine

Moreover, as automobile travel have leveled off in the last decade and a half while development has continued apace, the gap between ITE estimates and actual traffic has increased, according to the research by Adam Millard-Ball, Assistant Professor in the Department of Environmental Studies at the University of California, Santa Cruz.

The contrast is even starker in more recent years: an increase of 2 million trips between 2001 and 2009 according to NHTS, but 90 million by the ITE-based method.

For those who are keeping track, that's a discrepancy of 4,500 percent. As US travel habits change, the ITE data keeps pointing to ever-increasing traffic, as developers pay impact fees and transportation planners anticipate more congestion.

Use of the ITE manual has a profound affect on new development--opposition often centers around traffic generation. But the bigger impact is on overbuilt roads and the construction of too much parking. Not only is this wasteful, but also it diminishes sense of place and walkability, which in turn affect quality of life and economic development. The design of the road itself can result in more cars on the road. Safety is affected--streets that are too large encourage speeding, which boosts the severity of collisions and ultimately .

Millard-Ball explains why ITE numbers differ from reality:

First, the rates appear to greatly overestimate the number of vehicle trips that can be attributed to any development project, most likely because ITE’s data are based on a biased sample. Engineers and planners who use ITE rates are likely designing streets to cater for phantom trips that will never materialize.

Millard-Ball explains that ITE estimates are likely weighted toward suburban, automobile-oriented developments and big, controversial projects on key sites. ITE numbers also assume that projects will be successful, whereas in the real world many are not. This data, in other words, is based on measuring sites that have more traffic than typical sites.

Moreover, many developments don't generate new traffic--they just shuffle it around. A new courthouse will not increase the number of people in the county judicial system, Millard-Ball explains. Schools generate surprisingly little new traffic compared to what ITE suggests. A new supermarket may not increase actual shopping for food, but shift customers from one location to another instead.

In other words, ITE’s data reflect the average trip generation rate, not the marginal trip generation rate. The average rate is simply the number of trips that could be expected at a development of a particular type. The marginal rate is the increase in total trips as a result of that same development.

All of these factors: An overestimation due to biased survey samples, a focus on average rather than marginal trip generation, and a failure to take into account long-term transportation trends lead to what can only be called a gross disparity from actual travel behavior.

A new approach is needed for traffic analysis, air quality, and greenhouse gas emissions at a scale beyond the immediate vicinity of a development project, he says.

Instead of conceptualizing new housing, offices, or retail centers as generating traffic or emissions, it seems more useful to judge them against the baseline of existing development.

In a sprawling metropolitan region, new high-density, transit-oriented housing will probably reduce total vehicle travel. Adjusting down the number of trips generated for developments of this nature is certainly a step in the right direction, but fundamentally it is misleading to think that such transit-oriented housing generates any additional vehicle trips at a regional scale. A more reasonable starting point is to consider that new development is just as likely to reduce traffic, air pollution, and greenhouse gas emissions as it is to increase them.

This is especially true as overall vehicle miles traveled as plateaued. By ITE’s logic, each new house, apartment, office, and factory should be generating additional trips, thus leading to significant travel growth throughout the country. But in reality, total trip-making in the US has been relatively flat since 1995.

Rethinking the assumptions behind trip generation studies may not only avoid wasting resources on over-sized roadways, but can also support efforts to promote transit-oriented, livable communities, he concludes.

Robert Steuteville is editor and founder of Better Cities & Towns.