Engineering in the dark

Image via the Old Urbanist

The similarities of urban design across American neighborhoods is no coincidence, but neither is it the result of city planners’ uniform adherence to best practices. Infrastructure is often built based on shockingly little information about the demands of its users. And while poorly reasoned infrastructure policy in one city is bad enough, the United States’ broad adherence to poorly reasoned policies has resulted in a nation in which swaths of neighborhoods are built on poor design foundations.

Parking Requirements

In The High Cost of Free Parking, Donald Shoup explains the origin of municipal parking requirements. Municipal planning offices do not have the resources to study the amount of parking that businesses should provide. Even with more staff, it’s not clear that planners would be able to determine optimal parking requirements unless they allowed business owners themselves to experiment and choose the amount of parking on their own in a learning process of how to best serve their customers.

The Institute of Transportation Engineers is one of the only organizations that provides estimates of the number car trips that businesses generate. Given the lack of information planners have to determine parking requirements, they often rely on ITE’s information to set their parking requirements. However, ITE studies are often conducted at businesses that already provide ample free parking, ignoring the potential for businesses to manage demand for parking on their property through prices. Furthermore, ITE estimates of trip generation are typically based on a very small sample of locations, which are unlikely to be representative of businesses and cities in general.

In the example below, the ITE provides a recommendation for fast food parking requirements based on their floor area. Even though the chart includes a line of best fit for the plot of peak parking spot occupation and floor area, the ITE hasn’t demonstrated a correlation between these two variables. Shoup points out:

We cannot say much about how floor area affects either vehicle trips or parking demand at a fast food restaurant, because the 95-percent confidence interval around the floor-area coefficient includes zero . . . This is not to say that vehicle trips and parking demand are unrelated to a restaurant’s size, because common sense suggests some correlation. Nevertheless, factors other than the floor area explain most of the variation in vehicle trips and peak parking occupancy at these restaurants.

Even though this study fails to show much of anything about parking at fast food restaurants, the ITE parking requirement recommendations carry heavy weight with city planners. Shoup finds that “the median parking requirement for fast food restaurants in the US is 10 spaces per 1,000 square feet–almost identical to the ITE’s reported parking generation rate of 9.95 spaces per 1,000 square feet. After all, planners expect minimum parking requirements to meet the peak demand for free parking, and parking generation rates predict this demand precisely. When the ITE speaks, urban planners listen.”

An inefficient quantity of parking at one business is a trivial problem that can be easily rectified when the business owner observes that his customers would be better served by more or fewer spots. But when an inefficient practice is mandated across most cities in the country, it’s a regulatory burden in the hundreds of billions of dollars enforced in spite of shockingly poor knowledge of the issue.

Street Width Standards

Like their counterparts in city planning offices, transportation engineers often have to set street width requirements without reliable information about the width that best serves street users. As Streetsblog reports, the Federal Highway Administration recently published a document explaining that, contrary to many state and local transportation agencies’ beliefs, federal highway funds can be used for streets that do not follow the FHWA guidelines published in A Policy on Geometric Design of Highways and Streets, known as the “green book.”

While this may be a new found freedom for state DOT’s and local transportation agencies, it doesn’t result in a system in which transportation engineers get feedback about how their infrastructure is serving its users. A study from the Mineta Transportation Institute states:

Street standards are normally developed by engineers in departments of public works or transportation, based on various guidelines provided by county, state, or professional organizations, such as the Institute of Transportation Engineers and the American Association of State Highway and Transportation Officials. The standards are either published as a separate document or as part of a jurisdiction’s development ordinance.

Like parking requirements, street width standards adhere closely to this guideline even though it’s not binding. In their analysis of 97 cities, the study authors found that “the average minimum street width was 30.6 feet, and the median minimum width was 30 feet,” just under the ITE’s recommendation of 32- to 34-foot wide streets dating back to 1965. Some states, such as California, even have state wide minimum street widths for public streets of 40 feet.

The ITE recommendation is based on the assumption that wider streets are safer for drivers. While this is true in highway design, narrower street are in fact safer in neighborhoods, discrediting the false assumption that has led to dangerously wide streets in suburbs across America.

Jeff Speck explains the central role of the green book in influencing American traffic engineers:

For traffic engineers, AASHTO is the keeper of the flame. Its “Green Book,” . . . is the primary source for determining whether a road design is an accepted practice. As such, it is useful in protecting engineers against lawsuits; if something is in the Green Book, it’s “safe.”

Given the protection it affords, nobody questions the Green Book. Never mind that very little of it is evidence-based, and that there are no footnotes justifying its pronouncements. I mean, does the Bible have footnotes?

While state and local traffic engineers are free to deviate from the green book’s recommendations, even while spending federal funds, they rarely do because they have few other sources to base street widths on. The collective consequence of wide-laned neighborhood streets that increase the distances between destinations is a country in which its difficult for many people to live without a car. The Mineta study finds that wide neighborhood streets cost taxpayers an estimated $20 to $177 billion in maintenance costs alone.

Inescapable Knowledge Problems

Decades of planning for making car travel as easy as possible are giving way to new experimentation at the local level. For example, some cities have reduced or eliminated parking requirements in their city centers. Others, like San Francisco and Fresno have adopted “road diets” in which one traffic lane is eliminated in order to create space for bike lanes and a left turn lane. This break from lock-step planning at the local level may create opportunities for cities to learn from one another. However, as the problems with the current practices in parking requirements and street widths show, poor results can come from cities copying one another’s regulations.

While city planners and traffic engineers with new urbanist bents may feel compelled to update their city’s regulations, they won’t have good information about whether their changes are making people better or worse off. Design guidelines inspired by Smart Growth or new urbanism will face many of the same challenges that have plagued the implementation of regulations influenced by the ITE or the FHWA. When municipalities copy a set guidelines, mistakes go on unnoticed for decades. When problems come to light, it takes additional decades to begin correcting them.

The history of damage done by planning to accommodate cars without consideration for the costs that car infrastructure imposes on others should not lead planners to do an about face in favor of plans that they think will better accommodate pedestrians. Rather, a key lesson from the twentieth century planning should be humility in city planning with the recognition that it’s difficult to foresee the long-term consequences of regulations and that monotonic regulations impose a systemic risk of unforeseen consequences.