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Public Roads - March/April 2016

March/April 2016
Issue No:
Vol. 79 No. 5
Publication Number:
Table of Contents

Toward More Flexible Design

by Elizabeth Hilton and Dan Goodman

The controlling criteria for highways are undergoing a makeover to provide engineers with greater flexibility to create best-fit solutions.


The city of Austin, TX, encouraged pedestrian travel in this corridor by providing a wide, clear, and accessible sidewalk on this downtown street. Roadway design is increasingly focused on flexible solutions that incorporate multimodal transportation options.


July 11, 2016, marks the 100th anniversary of the creation of the Federal-aid highway program. As this major milestone approaches, it is interesting to look back at how the program has evolved over the years, especially in the design arena. From the first standards issued more than 80 years ago to a recent push for greater flexibility in design, geometric design of highways and streets has come a long way. And it is still evolving.

Today’s transportation engineers need to strike a balance between standards and addressing the specific circumstances and context of each project. In addition to considering standard roadway characteristics, their designs have to account for economic, environmental, and social impacts, as well as support larger community and regional goals, including meeting the needs of all users.

But how do they succeed with so many challenges and expectations? Flexibility is key. In response to the call for greater flexibility in the design process, the Federal Highway Administration recently proposed to revise a key aspect of its highway design policy--the controlling criteria--making it easier for engineers to develop designs that meet community needs.

Here’s a look at how the industry realized the need for more flexible design and what the revised controlling criteria would mean for road designers.

The Early Days of Design

The initial focus of the highway program was on linking parts of the country, upgrading wagon trails for use by the automobile, and providing better access for farmers to connect with markets. In the early days, most projects involved upgrading existing roads built initially for wagon traffic. By and large, States were more concerned about road surface and drainage than geometric design.

By the 1920s, the number of more powerful motor vehicles was growing, and State and Federal highway officials recognized the need to design roads to accommodate higher speeds. State highway agencies also realized the need for more consistent designs, including improved sight distance, curvature, and superelevation. In 1928, the American Association of State Highway Officials(AASHO) (now the American Association of State Highway and Transportation Officials) published the first “standards of practice” topromote uniformity in design.

AASHO policy guidance continued to evolve and focus on the design of rural highways, and later urban arterial highways and freeways. AASHO worked cooperatively with the U.S. Bureau of Public Roads and its successor, FHWA, which adopted AASHO guidelines for use on Federal-aid highway projects. Design guidance focused on provisions only for motor vehicle travel.

A Coming of Age for Design

During the 1960s, the context in which State highway agencies operated began to change. Community awareness of the impacts of freeway construction through established neighborhoods resulted in public opposition that many engineers had not previously encountered. The public also began to express concerns about the environmental impacts of projects, such as the movement of large amounts of earth to provide flatter, wider roadways. Increasingly, public pressure mounted for State departments of transportation to adopt practices that the industry knows today as context sensitive solutions, which are collaborative, interdisciplinary approaches that involve all stakeholders in providing a transportation facility that fits its setting. Solutions also emerged that increased opportunities for nonmotorized transportation and use of transit.

Beginning in the late 1960s, as highway agencies became part of departments of transportation, the focus shifted to a broader concept of transportation to include the movement of people and goods by a variety of modes of travel--aviation, rail, highways, city streets, transit, bicycling, and walking. AASHO became AASHTO with the addition of “Transportation” to its name in 1973, reflecting the growing emphasis on intermodal transportation.

Today, engineers find themselves challenged more than ever to deliver multimodal projects that take into account environmental and social impacts. They also must consider the economic aspects of their designs in the face of constrained transportation funding. Rather than simply following standards from a book, engineers are developing unique solutions to address the specific circumstances of each project and achieve the best overall solution, while also maximizing the return on investment.

The First Geometric Design Standards

In 1928, AASHO members approved the following four standards of practice to achieve some uniformity in highway design:

  • Wherever practicable, shoulders along the edges of pavements shall have a standard width of no less than 8 feet (2.4 meters).
  • On pavements, 10 feet (3 meters) shall be considered the standard width for each traffic lane.
  • The crown of a two-lane concrete pavement shall be 1 inch (2.5 centimeters).
  • No part of a concrete pavement shall have a thickness of less than 6 inches (15 centimeters), and all unsupported edges shall be strengthened.

Source: America’s Highways 1776–1976.


These illustrations show before-and-after cross sections of an urban street where narrowed lanes provide space for bike lanes without increasing the overall width of the road. Revisions to the controlling criteria will allow engineers to make such decisions without requiring approval for the design exception from FHWA.


To this end, FHWA and many DOTs are embracing performance-based practical design. This approach focuses on including features necessary to achieve performance goals for the project, rather than designing based solely on the standards. For a more detailed look at how geometric design of highways is changing, see “The Evolution of Geometric Design” on page 28 in this issue of PUBLIC ROADS.

Flexibility, such as that afforded by performance-based practical design, is crucial for engineers to develop solutions that balance the needs of all roadway users and meet the goals of the greater community. It facilitates a connected network of both motorized and nonmotorized transportation infrastructure that enhances access to jobs, schools, and essential services in a cost-effective manner.


The city of Overland Park, KS, added bicycle lanes to Switzer Road by narrowing the travel lanes when workers restriped the road after placing an ultrathin bonded asphalt surface. The proposed updates to the controlling criteria will make it easier for transportation agencies to make these kinds of decisions without requiring Federal approval for exceptions.


13 Controlling Criteria
(Adopted In 1985)

  1. Design speed
  2. Lane width
  3. Shoulder width
  4. Bridge width
  5. Horizontal alignment
  6. Superelevation
  7. Vertical alignment
  8. Grade
  9. Stopping sight distance
  10. Cross slope
  11. Vertical clearance
  12. Horizontal clearance/lateral offset
  13. Structural capacity

Updating the Controlling Criteria

FHWA regulations incorporate the geometric design standards for projects on the National Highway System (NHS) by reference. The standards are comprehensive, covering a broad range of design characteristics, while allowing for flexibility in their application.

Since 1985, FHWA has emphasized a subset of the design criteria contained in adopted standards by designating them as the 13controlling criteria: design speed, lane width, shoulder width, bridge width, horizontal alignment, superelevation, vertical alignment, grade, stopping sight distance, cross slope, vertical clearance, horizontal clearance, and structural capacity.

The controlling criteria are the same for all projects on the NHS regardless of roadway type, surrounding land use, or other context. Design exceptions are required when any of these controlling criteria are not met. In addition, the Moving Ahead for Progress in the 21st Century Act (MAP-21) in 2012 expanded the NHS to include many urban roadways, which made them subject to the controlling criteria.

Through research conducted since 1985, however, engineers and planners have gained a better understanding of the traffic operational and safety effects of the controlling criteria. In 2014, the National Cooperative Highway Research Program Report 783 Evaluation of the 13 Controlling Criteria for Geometric Design examined the safety and operational effects of the controlling criteria. Consistent with the research findings and FHWA’s efforts regarding performance-based practical design, FHWA recently proposed changes to the 13 controlling criteria.

On October 7, 2015, the following proposed changes were published in a notice in the Federal Register:

  • Three criteria were proposed for elimination from the list of controlling criteria: bridge width, vertical alignment, and horizontal clearance (lateral offset to obstruction).
  • Three criteria were proposed to be renamed to improve clarity and understanding: horizontal alignment to horizontal curve radius; grade to maximum grade (to eliminate minimum grade as a controlling criterion); and structural capacity to design loading structural capacity.
  • Design speed and design loading structural capacity were proposed to be retained as controlling criteria for all NHS roadways.
  • The following criteria were proposed as controlling only on high-speed (design speed greater than or equal to 50 miles per hour, mi/h [80 kilometers per hour, km/h]) NHS roads: lane width, shoulder width, horizontal curve radius, superelevation, stopping sight distance, maximum grade, cross slope, and vertical clearance.

These proposed changes would result in only two controlling criteria remaining for NHS roadways with a design speed of less than 50 mi/h (80 km/h): design speed and design loading structural capacity. The significant reduction in the number of controlling criteria applicable to roadways with lower speeds will give engineers the flexibility they need to design solutions that address the project goals in a way that is more compatible with the community. For example, engineers will have more flexibility to narrow vehicular lanes without needing Federal approval to do so.

FHWA received more than 2,300 comments on the proposal. After carefully considering these comments, FHWA will issue a final rule on the proposed policy change in the Federal Register. Once finalized, the updated policy will be available at

By ensuring that design exceptions are required only for criteria with the greatest safety or operational impacts, FHWA will effectively remove barriers to applying greater flexibility in design. Where the remaining controlling criteria cannot be met, design exceptions are still available as tools that transportation engineers can use to help them provide the best overall design.

Seleta Reynolds, general manager for the Los Angeles Department of Transportation (LADOT) in California, says, “LADOT supports this streamlining effort and appreciates FHWA’s objective of providing local agencies with more flexibility in project design. Beyond flexibility, these changes encourage local agencies to be innovative and design projects that enhance community safety and mobility for everyone.”

Achieving Better Design

FHWA proposed only two controlling criteria for NHS roadways with a design speed of less than 50 miles (80 kilometers) per hour:

  1. Design speed
  2. Design loading structural capacity


The addition of this bike lane improved conditions for bicyclists on South Congress Avenue in Austin, TX, a key urban corridor.


The changes to the controlling criteria are a significant step in supporting FHWA’s partners and stakeholders as they work to implement projects that result in better and more sustainable outcomes, such as improved connectivity and mobility for people of all ages and abilities, enhanced safety, and increased equity. The changes to the controlling criteria also demonstrate how much the focus of the Federal-aid highway program has evolved since its creation. Today, FHWA focuses on the safety of all users of the transportation system and on connecting people to work, schools, and other important destinations in ways that meet the needs of all modes and are sensitive to community character, livability, and quality of life.

To further assist State transportation departments and partners in achieving these goals, FHWA has several additional efforts underway to improve guidance on a multitude of issues. Several products will be available soon, including a workbook called Incorporating On-Road Bicycle Networks Into Resurfacing Projects and a resource titled Achieving Multimodal Networks: Applying Design Flexibility and Reducing Conflicts. The latter will include explanatory graphics and highlight key concepts for a broad range of design topics, such as crossing treatments, school access, transitions from State highways to main streets, and intersection geometry. The intent of these design topics is to help communities build connected pedestrian and bicycle networks, apply design flexibility, and enhance safety for all roadway users.

Elizabeth Hilton is a geometric design engineer in FHWA’s Office of Infrastructure where she focuses on geometric, bicycle, pedestrian, and accessible design. She is a former chair of the Transportation Research Board’s Design Section and Geometric Design Committee and serves as FHWA’s representative to the AASHTO Technical Committee on Geometric Design. She is a licensed professional engineer in Texas. She holds a B.S. in civil engineering from the University of Texas at Austin and an M.B.A. in public administration from St. Edward’s University.

Dan Goodman is a transportation specialist on the Livability Team in the Office of Human Environment at FHWA. He is a member of the TRB Pedestrian Committee and former chair of its Pedestrian Research Subcommittee. He serves as FHWA’s representative to the AASHTO Joint Technical Committee on Non-Motorized Transportation. He has a B.S. in political science from Kalamazoo College and a master of community planning degree from the University of Rhode Island.

For more information, contact Elizabeth Hilton at 512–536–5970 or, or Dan Goodman at 202–366–9064 or

Editor’s Note: As of press time, the final rule listing specific criteria has not been published. Visit to read the final provisions of FHWA’s policy regarding controlling criteria for design.