The Case for Reforming Our Nation’s Deadly Traffic Manual

by Sara Cecilia Bronin

As a new faculty member, I’ve spent a little time getting to know the town you, fine reader, know so well. Driving and taking the bus lets me experience the city one way, but by far the best way to understand the city is by walking. A walker finds that Ithaca’s dedicated trails are very comfortable. Many of the sidewalks are serviceable, too. But they often flank very wide streets, with cars zipping by at a fast pace. And I don’t just mean auto-oriented Meadow Street. Even West Green and Seneca Streets downtown, which in theory should be pleasant walking streets, are urban highways. Overall, there seem to be only a handful of traffic calming measures throughout the city. Why aren’t Ithaca’s roads better for the many people who walk and bike?

In recent research, I contend that road infrastructure like Ithaca’s is shockingly common.1 The whole country has been locked in 1950s-era inertia when it comes to the rules that govern road design. These rules are largely developed at the federal level and written by several nongovernmental membership associations, with little public input. They haven’t fundamentally changed in decades, and they don’t incorporate the latest techniques for making our streets safer for nondrivers. Most importantly for a legal audience, perhaps: they don’t really allow states, much less cities like Ithaca, legal cover to innovate. If cities design roads to meet the federal standards, they are virtually absolved of legal liability for design. So cities stick with the federal standards, even if they result in roads designed more for cars than for people.

So What is the Manual on Uniform Traffic Control Devices?

Although I cover several different federal road-design standards in my work, I would like to focus on just one standard here: the Manual on Uniform Traffic Control Devices for Streets and Highways (“the Manual”).2 First published in 1935, the Manual has been administered by the Federal Highway Administration (FHWA) since 1971 and is in its tenth edition. Its influence is felt not only on federal highways but on nearly every neighborhood block in America because it is adopted (and adapted, in often very minor ways) into state and local laws governing road design.3

On its face, the Manual is a straightforward technical document. It contains over eight hundred pages of engineering guidance on everything from traffic-light placement to the font of highway signs. It also prescribes acceptable methods for modifying speed limits. Millions of Americans entrust their lives to a road system that is governed by the Manual. And why shouldn’t they? It is purportedly in place to make life better for those who use the American transportation systems. But while such provisions may sound benign, some of them have life-or-death consequences.

After spending decades in relative obscurity, the Manual has received a flood of scrutiny over the past year. In 2020, the FHWA announced a proposed revision to the Manual (“Proposed Manual”).4 Over 26,500 public comments were submitted, including one from myself and Iowa Law professor Gregory H. Shill. We argued that the Proposed Manual would perpetuate some longstanding arbitrary, capricious, or discredited rules, even as it introduced new ones bearing the same defects. On the whole, the Proposed Manual would carry over to the twenty-first century some of the direst transportation policy failures of the twentieth. Moreover, it would make it nearly impossible to accomplish our nation’s official goal of “zero deaths” on the roads.

A Critique of the Manual

In our view, the biggest issue is that the FHWA prioritizes maximizing “throughput,” namely, the number of vehicles able to pass through a given marker, such as an intersection, in a given period. The Manual (and the Proposed Manual) achieve this priority. They establish a hierarchy of vehicular speed over public safety, vehicular mobility over other uses of public space, and driving over other modes of mobility.

One glaring example of bias in the current and proposed Manuals alike is the “85th Percentile Rule,” a method that empowers traffic officials to adjust the speed limit to the speed at or below which 85 percent of vehicles are traveling “in free-flowing traffic.” If as few as 15 percent of drivers are driving faster than the current posted limit, for example in light traffic, then this method would counsel raising the speed limit to legalize the conduct of that minority, even if doing so would be reckless. The 85th Percentile Rule does not merely favor driving over other forms of mobility; it delegates the interpretation of the law to the subset of drivers who are violating it most aggressively. Worse, as it erases the interests of anyone who is not driving.

Beyond the speed limit, other road-design standards also establish a maximum-throughput vision in the Manual. For example, the Proposed Manual leaves in place current guidance that “[c]rosswalk markings should not be used indiscriminately” and that an engineering study must be done before a crosswalk is installed. Such study requirements entrench the status quo by raising the cost of installing new crosswalks. Similarly, the Manual sets forth an array of factors to be taken into account by an engineer to determine when a new traffic signal or crosswalk is warranted. One factor is pedestrian activity. Justifying a midblock crosswalk signal on a major street, for example, requires the observation of at least 107 pedestrians crossing the street per hour.

An alternative method of justifying a crosswalk is to require a certain amount of human and material sacrifice. Five or more serious crashes—crashes that exceed the reporting threshold for injury, death, and/or property damage—within a twelve-month period are necessary, though not sufficient, to add a crosswalk under this “Crash Experience” warrant. Since they lack the protection of an automobile, pedestrians bear the brunt of the burden of this minimum-crash requirement. It is aggravated by other rules about striping and signalization that put pedestrians and bikers in dangerous situations, where they are less visible to cars, less protected from cars, or not accounted for at all.

The Proposed Manual charts new terrain for throughput by adding an entire new chapter on automated vehicles. But this technology is too experimental, too unsafe, and too likely to produce racially disparate outcomes to mandate that our roads be rebuilt around it. The fact that the Proposed Manual drafters have included an entire chapter on automated vehicles underscores the drafters’ car-centric bias.

These examples all illustrate how the Manual and Proposed Manual promote fast vehicle travel, trading off important social priorities, such as community vitality, safe access for pedestrians, and clean air, in the service of throughput. These other goals do not appear to be a priority of the principal group that advises the FHWA on the Manual, the National Committee on Uniform Traffic Control Devices.

Does the Manual Really Matter?

The maximum-throughput approach embodied by the Manual has had many consequences. It makes driving faster and other modes of travel slower and more dangerous, raising the level and risks of driving. One group that benefits little from this approach is nondrivers—almost 100 million Americans—who are by and large excluded from the benefits of the Manual’s emphasis on throughput.

The past year and a half is evidence of the direct danger of car-centric policy. Although driving dropped off sharply during the worst of the pandemic, 2020 saw the highest year-over-year increase in roadway death rates on a per-mile-traveled basis in ninety-six years. Lighter traffic allowed uninhibited, faster driving, a development for which the Manual helped lay the groundwork prior to the pandemic.

As has been documented extensively, the lives taken by car crashes are far likelier to be African American, Latino, Indigenous, low income, or belong to people with disabilities than the general population. Indeed, significant parts of the road system have been built in ways that have destroyed low-income communities and communities of people of color. The choice to accommodate the rise of the automobile by displacing Black, immigrant, and poor Americans for roadbuilding supercharged racial segregation and health disparities that continue to this day. In the words of U.S. Transportation Secretary Pete Buttigieg, “[t]here is racism physically built into some of our highways.”5

What Can Be Done?

In our paper, we propose several changes to the Manual so that it promotes, or at least no longer impedes, the paramount goals of safety, climate action, racial justice, and prosperity.

First, we propose the elimination of the 85th Percentile Rule, which undermines safety and the rule of law. Rather than enhancing predictability, stability, or fairness, it empowers those who violate the law to trigger an opaque administrative process that results in a change in legal regime. It is perhaps unique in American law in empowering law-breakers to activate a rewrite of the law to legalize their own unlawful conduct.

Second, we propose withdrawal of a proposal to render millions of miles of American streets more hostile to vulnerable road users in the name of an unproven technology, autonomous vehicles (AVs). The Proposed Manual seeks to maximize uniformity over and above important alternative goals. For example, the chapter warns against streets with decorative crosswalks. Many legal crosswalks are not marked at all; the Proposed Manual fails to address how AVs should interact with them, nor does it advise road engineers to mark crosswalks in preparation for AVs. Other provisions of the chapter prescribe affirmative changes that are harmful. One section encourages wider roads, which have been shown to promote faster and more dangerous driving. The Proposed Manual’s premature emphasis on AVs is even more startling given that, in our 101st year of the quest to fully automate driving, the most successful completely automated vehicle is arguably a vacuum cleaner, the Roomba.

Third, we propose that the FHWA review the remainder of the Manual through the lens of three guiding principles: fostering fairness for all types of road users; incorporating diverse expert and community opinions; and facilitating local flexibility and innovation, especially in cities and communities that are disproportionately harmed by fast vehicular traffic. Where specific provisions hinder achievement of these provisions, the Manual should be rewritten. Moreover, the composition of the National Committee on Uniform Traffic Control Devices, which guides the drafting of the Manual, should reflect more types of road users, not just drivers. We also propose that their meetings be opened to the public.


Improving safety for all road users is essential for progress on economic prosperity, climate, and racial justice in the United States, yet the current Manual undermines those bedrock goals. The Proposed Manual is no better, and, in several ways, it is worse. It doubles down on the mistakes of prior generations of the document, which embody an early-twentieth-century goal of increasing car ownership and vehicle miles traveled. In addition, it adds an entirely new chapter to accommodate a technology, automated vehicles, that remains experimental and problematic. The biases enshrined in the Manual undermine safety, equity, and economic development, and its continued narrowness reflects a decisionmaking process that has remained closed to diverse input for nearly a century.

Fundamental changes to the Manual and Proposed Manual are imperative. A freshly rewritten Manual can advance rules of design that minimize rather than amplify the unique dangers to which speeding motorists expose vulnerable road users like pedestrians, wheelchair users, and bicyclists. Over time, changes to this obscure federal road standard can help make Ithaca’s streets more welcoming to curious newcomers.

This essay excerpts portions of an article published in October 2021 in the Harvard Law Review Forum and coauthored with Iowa Law Professor Gregory H. Shill.



1. Sara C. Bronin, Rules of the Road: The Struggle for Safety and the Unmet Promise of Federalism, 106 Iowa L. Rev. 2153 (2021). 

2. The Manual, known to traffic engineers as the MUTCD, is not the only document governing road design. Most notably, the American Association of State Highway and Transportation Officials’ “Green Book” dictates road and lane width and intersection design, among other things. 

3. See 23 C.F.R. § 655.603(a) (2020). 

4. Fed. Highway Admin., U.S. Dep’t of Transp., Corrected MUTCD Proposed Text (Dec. 18, 2020) [hereinafter Proposed Manual]. 

5. Corinne Grinapol, Biden Administration Seeks to Address the Interstate Highway System’s Racist Past, Eng’g News-Record (Apr. 15, 2021)