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The Colonial Parkway connects Jamestown, Williamsburg, and Yorktown, VA. |
Chapter 2HIGHWAY DESIGN STANDARDS |
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| THE AASHTO GREEN BOOK
What the Green Book Contains
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:
Some of these items are addressed by reference in the Green Book, including:
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:
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
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 alinement in order to eliminate safety and accident problems.
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.
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) 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:
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.
Notes: 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. 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 alinements that would negatively affect these resources. When possible, the State DOT would prefer to maintain existing facilities by keeping existing alinements 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.
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.
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:
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:
For preventive maintenance projects, no exceptions are needed for the
retention of existing features. 2 Roadside Design Guide,
AASHTO, Washington, DC, 1996. 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.
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. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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