United States of America/Highway Design

Highways in the United States (and its dependent territories) are designed according to a number of standards at both federal and state level. At the national level, standards are either published directly by the Federal Highway Administration (FHWA) or developed by the American Association of State Highway and Transportation Officials (AASHTO) and then accepted by FHWA as mandatory guidelines for federal-aid highway projects. A number of states also have their own standards which either supplement or replace the national standards. As a general rule of thumb, state standards which compete with the established national standards should reflect a more conservative approach to design (e.g. taller legend on traffic signs, or flatter vertical curves).

Although there is some limited use of design-build construction methods in the US, the most popular process for constructing a highway in the US calls for the owning agency (or one of its consultants) to produce a set of construction plans. It is accompanied by a verbal description of the standards for materials and workmanship applicable to the work, called a "bidding proposal." The two comprise a contract which is put out to tender ("let" or "put out to bid"), sometimes in monthly tranches called "lettings," or on a rolling basis with contract advertisements and bid openings taking place weekly or semiweekly. The practice in most states is to award the construction contract to the lowest responsive bidder ("responsive" in this case meaning that the contractor has successfully completed all required prequalification procedures and has submitted the bid in good order).

Design standards

Geometric design

AASHTO publishes Geometric Design for Highways, familiarly known as the "Green Book" since it used to be published in hardcover with a gold-stamped green cloth cover. The Green Book in its current form is a consolidation of two separate manuals, Geometric design for rural highways, known as the "Blue Book" because of its blue leatherette cover, and Design of Urban Arterials, known as the "Red Book" because of its red leatherette cover. FHWA requires that federal-aid projects be designed to the minimum criteria given in the latest edition of the Green Book. AASHTO has a separate publication which enumerates the specific criteria which are to be used on Interstate highways, and there is a separate Roadside Design Guide which addresses the design of crash barriers and other roadside safety appliances.

There have been AASHTO standards on geometric design since publication of the various "Policies on Highway Design" pamphlets, beginning in 1940. Prior to this, the various states had their own standards, while the Bureau of Public Roads (FHWA's predecessor agency) applied various kinds of suasion to make sure best practice was disseminated as soon as it became recognized as such.

Traffic signing

In the US the national standard for traffic signing is the Manual on Uniform Traffic Control Devices, traditionally published by AASHTO but now published directly by the FHWA. It has a companion volume, Standard Highway Signs, which gives details of signface design for standard signs and so is functionally equivalent to the working drawing series in the UK. The remote predecessor of these manuals is AASHO's 1927 signing and marking manual for the US highways, which itself synthesized design standards which had been developing at the state level for more than a decade.

Plans production

The main activity in final design of a highway project, which typically begins when the project has received the environmental approvals it requires, is production of the construction plans, sometimes called just "production" among state DOT engineers. FHWA has a set of high-level guidelines for the information that must be contained in construction plans. This is the supplement to § 630b of the Federal-Aid Policy Guide, or sometimes "FAPG 630b Supplement" for short. Although these guidelines are, strictly speaking, applicable only to federal-aid projects, they define the terms of trade in design offices at state DOTs, turnpike agencies, and engineering consultants, and are used for projects which have no federal funding.

The usual approach toward plans production is to generate a set of basemaps which reflect the finished state of the project, with each functional discipline typically having its own basemap. A functional discipline is an aspect of construction which is usually considered self-contained for purposes of plan production in the sense that designers working on this aspect of the project do not necessarily have to be in close communication with designers working on other aspects. The functional disciplines typically encountered in a major highway project include geometric design, hydrology/hydraulics (drainage), traffic (signing, marking, illumination, workzone traffic management), and structures.

Once a basemap is available for a given functional discipline, drafters (or CAD operators) then apply detail to the basemap which shows clearly, and in a graphical manner, the work which needs to be done to accomplish the finished state. Then the basemap is partitioned into pieces which will fit on standard-size plan sheets at the adopted working scale. Other sheets, called "detail sheets," are produced for each functional discipline which show information which cannot conveniently be shown on the plan sheets produced by partitioning the basemap. Examples of detail sheets include signface designs for guide signs, pipe connections and utility accesses for drainage plans, etc. Elevations and cross-sections are also prepared for certain design elements for which vertical dimensions are important—e.g. overhead sign gantries, culverts, bridges, etc.

The plans for a given functional discipline are generally held together. Most state DOTs have a standard ordering for functional disciplines which is applied to all plans sets produced by that state DOT and its consultants. When plans production is finished, plan sheets for the various functional disciplines are assembled in the stipulated order under a title sheet which gives the name or caption of the project, the name of the agency building it, certain basic geometric design parameters, and a map showing the location of the project.

The sheet counts of project plans sets vary considerably according to the complexity and cost of the project, and also whether the state DOT requires that standard plan sheets (which show recurring design details which have been standardized for economy in construction) be bound with the individual plans set. (Some state DOTs do, while others keep the standard plan sheets separate and incorporate them into the contract by reference.) As a generalization, a simple resurfacing project will have less than 100 sheets while a project to build a four-level stack interchange may have more than 4000 sheets. The culture of design, construction supervision, and contract administration has evolved at state DOTs through the twentieth century, with the result that projects from the 1920's and 1930's have far fewer sheets than modern projects of comparable scope.

Plans production for design-build projects is somewhat similar, except that it takes place concurrently with construction and thus the owner of the design-build (typically the state DOT) must review individual plan sheets as they are produced and release them for construction—often within a comparatively short period of time. These released-for-construction plans are typically altered by the design-builder after construction to reflect any revisions which may have occurred as a result of construction, and then used to prepare a set of records drawings which is deposited with the owner and serves as the as-built plans for the project.