Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Flexibility in Highway Design

Chapter 2: Highway Design Standards

The Colonial Parkway connects Jamestown, Williamsburg, and Yorktown, VA.

photo: The Colonial Parkway in Virginia

THE AASHTO GREEN BOOK

What the Green Book Contains

The reference most often used by designers during the design of a highway project is commonly referred to as the Green Book. Its official title is A Policy on the Geometric Design of Highways and Streets. It has been published by the AASHTO, in one form or another, since the late 1930's, with the most recent edition issued in 1994. Although often viewed as dictating a set of national standards, this document is actually a series of guidelines on geometric design within which the designer has a range of flexibility. As stated in the forward to this document:

The intent of this policy is to provide guidance to the designer by referencing a recommended range of values for critical dimensions. Sufficient flexibility is permitted to encourage independent designs tailored to particular situations.

In order for the design criteria in the Green Book to become a standard, they must be adopted by a particular State (or may be set by court decision). The FHWY has adopted applicable parts of the Green Book as the national standard for roads on the NHS. These roads comprise all the interstates and some other primary routes. The design of roads other than those on the NHS is subject to the standards of the particular State. The standards adopted by a State are usually based on the Green Book criteria.

book cover: A Policy on Geometric Design of Highways and Streets, 1994

What the Green Book Does Not Contain

The Green Book is not a design manual. It provides guidance on the geometric dimensions of the roadway. This includes widths of travel lanes, medians, shoulders, and clear zones; the width and shape of medians; turning radii; and other dimensions. There are many aspects of design that are not directly addressed in the Green Book. A number of these items are as follows:

  • Problem definition
  • Project definition
  • Definition of the termini of the project
  • Development of a project concept
  • Aesthetic treatment of surfaces
  • Design within the appropriate context
  • Selection of the appropriate guardrail/bridge rail
  • Determination of functional classification
  • Determination of the appropriate functional requirements, capacity, and level of service
  • Structure design
  • Landscape development
  • Selection of light fixtures
  • Roadside development
  • Traffic operations

Some of these items are addressed by reference in the Green Book, including:

  1. The Roadside Design Guide, AASHTO, 1996.
  2. The Manual on Uniform Traffic Control Devices for Highways and Streets (MUTCD), FHWA, 1988.
  3. A Guide for Transportation Landscape and Environmental Design, AASHTO, June 1991.
  4. Guide for the Development of Bicycle Facilities, AASHTO, August 1991.
An example of a road designed within the appropriate context as a gateway to a beach resort. This is a design aspect the Green Book is not intended to address. (Clearwater, FL)
photo of a road with palm trees, Clearwater, FL

Many fundamental questions that affect roadway design must be answered before the design phase begins and the application of the Green Book and/or State standards comes into play. Decisions are made during planning and project development, and it is then that road design starts. Decisions that are made before the design phase include:

  • Whether the proposed improvement will be two, four, six, or eight lanes
  • Whether the facility will have a median
  • Whether roadway junctions will be atgrade intersections or grade separated interchanges.

Design involves the difficult process of merging these previously determined design decisions with the appropriate design criteria used in the Green Book, working within the existing environmental and other important constraints, and using a designer's best judgment and experience to make decisions.

TYPES OE HIGHWAY IMPROVEMENT PROJECTS

There are four basic types of physical improvement projects, some of which must comply with standards and others that do not have to comply. These types of improvement projects are discussed in the following paragraphs.

New Construction

As its name implies, this action involves the construction of a new highway facility where nothing of its type currently exists. This might take the form of a bypass constructed to carry throughtraffic around an existing town or it might be a new twolane access route linking an existing arterial highway with a State park.

Four types of highway construction:

  1. New Construction
  2. Reconstruction
  3. 3R
  4. Maintenance

Reconstruction

This typically involves a major change to an existing highway within the same general rightofway corridor. In many parts of the country, roads that were originally constructed in the early 20th century as twolane "farmtomarket" roads have been reconstructed over the past few decades into multilane divided arterials to better accommodate the travel demands generated by suburban development. Reconstruction also may involve making substantial modifications to an older highway's horizontal and vertical alignment in order to eliminate safety and accident problems.

Reconstruction can involve a major change to the roadway appearance.

Reconstruction of 7th Street.
(Phoenix, AZ)

photo of 7th Street, Phoenix, AZ
photo of a street before reconstruction

Before reconstruction

After reconstruction.
photo of a street after reconstruction

Resurfacing, Restoration, Rehabilitation (3R)

3R projects focus primarily on the preservation and extension of the service life of existing facilities and on safety enhancements. Under the classification of 3R projects, the types of improvements to existing federalaid highways include: resurfacing, pavement structural and joint repair, minor lane and shoulder widening, minor alterations to vertical grades and horizontal curves, bridge repair, and removal or protection of roadside obstacles.

Transportation Research Board Special Report 214, Designing Safer Roads, Practices for Resurfacing, Restoration, and Rehabilitation (1987), documents the result of a study on the cost effectiveness of highway geometric design standards for 3R projects.' Each State was invited to develop and adopt minimum design criteria for nonfreeway 3R projects. The result is that States typically employ design criteria for 3R projects that are lower than those contained in the AASHTO Green Book.

photo of tree lined Rt. 136 in Wesport, CT

Ongoing maintenance is necessary to keep facilities in good condition.

(Rt. 136, Westport, CT)

Maintenance

Typically, maintenance activities consist of those actions necessary to keep an existing highway facility in good condition. Maintenance activities include repainting lane and edge lines, removing accumulated debris from drainage inlets, repairing surface drainage features, mowing, and removing snow. 1 Transportation Research Board Special Report 214, Designing Safer Roads, Practices for Resurfacing, Restoration, and Rehabilitation (1987)

Design criteria apply only to the first three of these actions: new construction, major reconstruction, and 3R projects. In addition, because 3R projects generally do not involve more than minor changes to roadway alignment and geometry, except to improve safety, FHWA and the State DOTS acknowledge that the AASHTO Green Book criteria do not always have to be adhered to for these projects. Because 3R projects have minimal impact, application of the Green Book design criteria may affect character of a roadway.

As stated in the Green Book, existing roads that do not meet the guidelines for geometric design are not necessarily unsafe and do not necessarily have to be upgraded to meet the design criteria:

The fact that new design values are presented herein does not imply that existing streets and highways are unsafe, nor does it mandate the initiation of improvement projects ...For projects of this type (resurfacing, restoration, or rehabilitation [3R]), where major revisions to horizontal and vertical curvature are not necessary or practical, existing design values may be retained. (p.xliii)

RELATIONSHIP OF THE GREEN BOOK TO STATE AND LOCAL DESIGN MANUALS

As stated earlier, in order for the Green Book to become a standard for a particular State, it must be adopted by that State (or may be set by a court decision). Although all the State DOTS may not specifically use the Green Book as their standard, State highway design manuals do derive from, and explicitly reference, the AASHTO Green Book. Despite this common origin, there are some variations in terms of the degree of adherence to all the contents of the document within the State standards. For example, Table 2.1 presents a comparison of the minimum stopping sight distance values for a range of design speeds, as defined in the 1994 edition of the AASHTO Green Book, compared to the most recent versions of the highway design manuals for California, Oregon, and Virginia.

Minimum Stopping Distance (m)
Distance Speed (km/h)
AASHTO (1)
California DOT (2) Oregon DOT (3) Virginia DOT (4)
60
74.3-84.6
85
80
80
80
112.8-139.4
130
120
120
100
157-205
190
160
160
120
202.9-285.6
255
210
210
130
N.A. (5)
290
N.A.
N.A.

Table 2.1

Example of design criteria differences among several States.

Notes:

1 A Policy on Geometric Design of Highways and Streets, AASHTO, 1994 p. 120, Table III1.
2 Interim Selected Metric Values for Geometric Design, California DOT, December 15, 1993, p. 2, Table 201.1.
3 Metric Highway Design Manual, Oregon DOT, 1994, p. 92, Figure 410.
4 Road Design Manual, Volume 2 (Metric), Virginia DOT, August 1994, p. 27, Table III1.
5 N.A.=Not Applicable

The AASHTO Green Book provides a range of values for the minimum stopping sight distance, while each of the three States provides only one minimum value. In each instance, the California requirement is highest. Moreover, the California stopping sight distance presents an upper limit design speed of 130 km/h (81 mph), although AASHTO, Oregon, and Virginia standards do not acknowledge using a design speed above 120 km/h (75 mph).

The minimum criteria established for Oregon and Virginia typically fall into the lower to middle portion of the AASHTO range of acceptable values. This situation is encountered in most other States, because in the past most States sought to meet or exceed the minimum values established by AASHTO in their designs, and this is reflected in their standards.

New State and Local Standards

Since the passage of ISTEA in 1991, States have been allowed to develop highway design standards outside of the Green Book criteria. A number of States have either developed or are in the process of developing new standards for roads not on the NHS. Idaho and Maine have implemented such separate design standards, Colorado and Vermont are at various stages of developing separate standards, and South Carolina plans to do so in the near future. These standards will give designers a greater range of flexibility when working on improvement projects to nonNHS roads.

The Rhode Island DOT has been particularly innovative in developing new State standards for all State roads. Rhode Island has decided that, because it has historic, scenic, and cultural resources along many of its roads, it would be inappropriate to make major changes to the geometry and alignments that would negatively affect these resources. When possible, the State DOT would prefer to maintain existing facilities by keeping existing alignments and geometries. For new construction, standards based on the AASHTO Green Book would be followed. This new approach will help to preserve the resources along many of Rhode Island's older roads and help engineers maintain the roads in a way that the public feels is appropriate for the communities.

Scenic roads can benefit from new State standards.

(Snickersville Tnpk., VA)

photo of Snickersville Turnpike, Virginia

A number of local governments (primarily counties) have also developed their own geometric guidelines in recent years to allow for expanded design flexibility on local roads. In these cases, the local design guidelines are explicitly related to those of their respective State DOT; and/or the AASHTO Green Book.

For example, the Design Guidelines and Standards for Scenic and Historic Roads, published in June 1994 by the Prince George's County (Maryland) Department of Public Works and Transportation, was prepared specifically to ensure the adequacy and safety of any new or upgraded roadway within the county in accordance with the guidelines and in order to preserve the scenic and historic values of adjacent areas.

Specific criteria for scenic roads can help minimize impact on valuable resources, such as these rock fences.

(ParisLexington Road, KY)

photo of ParisLexington Road, Kentucky

THE DESIGN EXCEPTION PROCESS

Despite the range of flexibility that exists with respect to virtually all the major road design features, there are situations in which the application of even the minimum criteria would result in unacceptably high costs or major impact on the adjacent environment. For such instances when it is appropriate, the design exception process allows for the use of criteria lower than those specified as minimum acceptable values in the Green Book.

If the highway project is not on the NHS, the State does not need FHWA approval for a design exception. Under the ISTEA, the State can request an exemption from FHWA oversight on nonNHS projects.

For projects on NHS routes, FHWA requires that all exceptions from accepted guidelines and policies be justified and documented in some manner and requires formal approval for 13 specific controlling criteria. The process of justification and documentation, although not required, can be followed by States with exemption from FHWA oversight on nonNHS projects, as well. These criteria are as follows:

  1. Design speed
  2. Lane width
  3. Shoulder width
  4. Bridge width
  5. Structural capacity
  6. Horizontal alignment
  7. Vertical alignment
  8. Grade
  9. Stopping sight distance
  10. Cross slope
  11. Superelevation
  12. Vertical clearance
  13. Horizontal clearance (not including clear zone)

For the most part, design exceptions to these 13 criteria can be easily identified and defined. Two items, horizontal clearance and design speed, warrant further explanation.

Horizontal Clearance

For clear zones, the criteria in the AASHTO Roadside Design Guide' should be treated as guidance and not as a national standard requiring a design exception if not numerically met.

Design Speed

Design speed is used to determine individual design elements, such as stopping sight distance and horizontal curvature. Therefore, a design speed exception is an exception to all the various design elements affected by it and should be justified on that basis.

A few points to remember when evaluating design exceptions are as follows:

  • Consideration should be given to the effect of the variance on the safety and operation of the facility and its compatibility with adjacent sections of the roadway.
  • Consideration should be given to the functional classification of the road, the amount and character of the traffic, the type of project, and the accident history of the road.
  • The cost of attaining full standards and any resultant impact on scenic, historic, or other environmental features should also be examined.
  • Finally, the following three issues should be considered. What is the degree to which a guideline is being reduced? Will the exception affect other guidelines? Are there any additional features being introduced that would mitigate the deviation?

For preventive maintenance projects, no exceptions are needed for the retention of existing features. 2 Roadside Design Guide, AASHTO, Washington, DC, 1996.

TORT LIABILITY AS IT RELATES TO THE GREEN BOOK

Tort claims against highway agencies have steadily risen since the early 1970's, when the AASHTO first began surveying States for information about tort liability claims. This is partly due to the trend of no longer allowing design immunity (sovereign immunity) for highway agencies in almost all States. There is evidence to believe that the majority of these cases involve allegations of faulty traffic control devices or maintenance. Even though the number of cases alleging design defects is relatively small in comparison, tort liability is still a real concern for highway engineers.

Tort is a legal term that refers to a civil wrong that has been committed, in this case by highway agencies. Negligence is a term used to refer to a classification of tort in which the injury is not intentional, but where there was failure to use due care in the treatment of others compared to what a "reasonable man" would have done. Liability is the responsibility to make restitution to the damaged party through an action or payment determined by the court. Finally, States protected with sovereign immunity for design cannot be sued for decisions regarding design. (Sovereign immunity is now only in existence in a handful of States.)

The AASHTO Green Book, other Stateadopted highway standards, Federal and State regulations and guidelines, and research publications issued by the Transportation Research Board are often used in tort cases to educate the jury about the standard level of practice for design. In addition, expert witnesses are used, who in turn rely on written text to explain the accepted standard practices for design to the jury.

Designing a highway that is safe and has a minimal impact on its surrounding environment can be a difficult task for engineers. Sometimes a design exception is necessary, as was the case for this project. (Bethel, VT)
photo of a tree lined road in Bethel, Vermont

This does not mean, however, that adherence to accepted standard practices, such as the AASHTO Green Book guidelines, automatically establishes that reasonable care was exercised. Conversely, deviation from the guidelines, through the use of a design exception, does not automatically establish negligence. The best defense for a design engineer is to present persuasive evidence that the guidelines were not applicable to the circumstances of the project or that the guidelines could not be reasonably met. (It should be noted that an economic defense is not the most effective.) It is highly recommended that designers document their rationales for decisions. If the justification documented by a designer completely describes the physical and environmental factors that make the exception or any design necessary, it is likely that this will be legally persuasive that the correct procedures were followed and ultimately the appropriate decision was made. In addition, it is helpful to have statements by other design experts who concur with the decision in the documentation.

As a result of concerns about litigation, designers may be tempted to be very conservative in their approaches to highway design and avoid innovative and creative approaches to design problems. While it is important for design engineers to do their jobs as thoroughly and carefully as possible, avoiding unique solutions is not the answer. This may undermine design practice and limit growth in the engineering profession. Designers need to remember that their skills, experience, and judgment are still valuable tools that should be applied to solving design problems and that, with reliance on complete and sound documentation, tort liability concerns need not be an impediment to achieving good road design.

Updated: 09/19/2012
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000