The Self-Fulfilling Prophecy of Car-Dominated Communities

Colin Fiske, Coalition for Responsible Transportation Priorities Executive Director

This is the sixth and final article in a series about the history of how American communities were designed for cars. Previous articles in the series can be found at

The widest roads in our region are also often the most congested. Adding more lanes only makes things worse.

Over the last six editions of EcoNews, we have explored key historical developments that led to most American communities being designed almost exclusively for cars. We’ve seen that decisions made by politicians, engineers, industry leaders and wealthy “early adopters” of the automobile led to the creation of laws that prioritized the unobstructed movement of cars in the street, while other traditional street users, notably pedestrians and bicyclists, were relegated to small designated areas or ignored entirely. Local officials and land developers adopted zoning rules that led to sprawling, low-density development as part of a conscious effort to keep lower income residents and people of color – who were less likely to own cars – out of wealthy White neighborhoods. These development patterns were reinforced by rules requiring every new building to provide excessive amounts of free parking, and by unprecedented levels of federal investment in building highways which sped White commuters in and out of cities while destroying thriving black neighborhoods.

As a result of these massive changes in land use patterns, laws, and transportation infrastructure, vehicle ownership quickly became a requirement in many places for people to access jobs, schools, shopping, hospitals, and other key destinations. By 1976, the vast majority of American households had at least one car. So it is perhaps not surprising that in that year, transportation engineers published a Trip Generation Manual which assumed that almost everyone would drive to any new home or business that might be built. 

The Manual was produced by the Institute of Transportation Engineers, which has been one of the most consistent promoters of pseudo-scientific justifications for car dominance since the early twentieth century. Just as with the Institute’s Parking Generation Manual (discussed in a previous article in this series), the Trip Generation Manual used a small number of mostly suburban observations to generate predictions of how many car trips would start or end at any particular type of building or development. And just as with the parking manual, the trip manual’s predictions were adopted wholesale by local governments and other public agencies across the country.

Ironically, the most common use for trip generation predictions was to measure the negative impacts of cars. As car travel had become the norm, American communities were forced to confront some of the negative impacts, including rising deaths on the street as well as air, water, and noise pollution. But the impact Americans seemed to care most about by the 1970s was congestion—the fact that more cars on the street would keep them from driving their own cars as fast as they liked. So when new development was proposed, many communities began using trip generation predictions to estimate how many more cars would be put on the road, then requiring more road capacity to be built so that the new cars wouldn’t increase congestion. Traffic engineers even came up with a subjective grading scheme called “level of service” (LOS), which assigns an “A” to streets with completely free-flowing traffic and an “F” to those with frequent gridlock, to help in the all-out campaign against traffic delays.

Remarkably, in California, the landmark California Environmental Quality Act (CEQA) of 1970 was interpreted in such a way that traffic delay was considered an “environmental impact.” If a new development was predicted to generate a significant number of car trips—usually using figures from the Trip General Manual—CEQA therefore required developers to increase roadway capacity as an environmental mitigation. Incredibly, this remained the prevailing method of transportation analysis under CEQA until 2020.

The Trip Generation Manual is based on the idea that there is a fixed and measurable “demand” for car travel associated with any given land use. This belief has been widespread among engineers and planners for generations. But since the early days of the automobile, some observers suspected that it was completely backwards. They saw that when new roads were built, or new lanes added, congestion wasn’t relieved for long. Instead, more cars ended up on the road and filled up the new space. By the end of the twentieth century, this phenomenon of “induced demand” was well-studied, documented, and quantified. We now know that you can’t actually reduce congestion by building more road capacity, mainly because the temporarily faster travel times encourage more people to drive until delays return to prior levels.

In spite of this knowledge, level of service standards and congestion management goals remain entrenched in the official plans and policies of local governments across the country, including those in Humboldt County. Our limited transportation dollars are still being spent on capacity-increasing projects—from new lanes to intersection re-designs—meant to reduce congestion, although the research clearly shows they will fail. And the Trip Generation Manual remains the standard for predicting traffic “impacts” of new development, despite its well-documented lack of scientific rigor and the overwhelming evidence that actual trip generation is highly variable and dependent on factors like location and design.

By predicting more driving, which leads to the construction of new road capacity that virtually guarantees the predicted driving, the Trip Generation Manual has become something of a self-fulfilling prophecy. It’s a pattern we’ve seen repeatedly in transportation planning history. And while many early decisions which promoted car dominance—from traffic laws to the interstate highway system—were more or less explicit about their aims, recent policies—like parking and trip generation predictions—have often relied on a veneer of empiricism to obscure their ideological slant. Studying the history of cars and communities puts the lie to these modern assumptions by revealing that car dominance is not a permanent, inevitable or beneficial condition, but instead a relatively recent phenomenon resulting from specific individual and collective decisions. In other words: we got ourselves into this mess, and we can get ourselves out of it too.

Recommended Reading:

  • Shoup, Donald. 2003. Truth in Transportation Planning.  Journal of Transportation and Statistics.
  • Blumgart, Jake. 2022. Why the Concept of Induced Demand Is a Hard Sell. Governing Magazine.