Page:America's Highways 1776–1976.djvu/408



While not always indicated separately, safety in highway vehicle operations has been an important objective of highway design since the beginning of the highway development programs. The identification by the 1920’s of the need for clear stopping distances, for widths to pass without major pullover, and for usable curve radii led to early efforts to determine and incorporate such appropriate features in the next highway improvements. With reasonable rapidity, the design engineers accepted the identified highway safety needs, developed newer design controls reflecting them and put them to use. In retrospect over the last 50 years, it should be said that designers never were able to catch up with nor were the construction programs able to provide the highways needed for the rapidly changing vehicles, volumes, speeds, travel habits and driver attitudes. The history of highway design control development shows a progressive series of adjustments that reflect the concerns for highway safety evolving from the latest experiences and accident data. The same process continues today.

There is space here only to indicate a few examples of design controls and practices that were pointed toward better safety. In 1938–1944 initial design policies of AASHO had many such items. The 1941 primary standards preamble urged that in using the design values, “unquestioned adequacy rather than strict economy should be the criterion.” The overall concept of design for expected volumes, for the character of traffic and for an assumed design speed was a major effort for highway safety. The enumerated values and controls for lane width, shoulders, use of a dividing median, pavement crown, curve radius and superelevation, minimum sight distance, safety passing sections, guardrails where roadway slopes were steep, appropriate intersection details and layouts, and for grade separations with interchange ramps all embody features patterned for safe vehicle operations. The 1954 rural design policy book refined these and provided additional details; advocacy of controlled access design was a major addition. The 1965 revision expanded treatment of these details, primarily in the realm of the higher volume and higher speed operations which were then being experienced. Both of these editions keyed into the separately developing standards for the several types of traffic control devices which have a high safety orientation. Both the 1957 and the 1973 urban highway policy books reflected design features needed for the higher volumes and greater land space restrictions in urban areas. They particularly stressed the design details of grade separations, interchange patterns, frontage roads, and practical geometries relative to expressways and freeways.

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