Noise barriers have been built in the United States since the mid-twentieth century, when vehicular traffic burgeoned. The first was installed in 1968 along a section of
I-680 in
Milpitas, California. In the late 1960s, analytic
acoustical technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specific
roadway. By the 1990s, noise barriers that included use of transparent materials were being designed in Denmark and other western European countries. measures sound in noise barrier design study,
Santa Clara County, California. The best of these early computer models considered the effects of roadway
geometry,
topography,
vehicle volumes, vehicle speeds, truck mix,
road surface type, and micro-
meteorology. Several U.S. research groups developed variations of the computer modeling techniques:
Caltrans Headquarters in
Sacramento, California; the ESL Inc. group in
Sunnyvale, California; the
Bolt, Beranek and Newman group in
Cambridge, Massachusetts, and a research team at the
University of Florida. Possibly the earliest published work that scientifically designed a specific noise barrier was the study for the
Foothill Expressway in
Los Altos, California. Numerous case studies across the U.S. soon addressed dozens of different existing and planned highways. Most were commissioned by state highway departments and conducted by one of the four research groups mentioned above. The U.S.
National Environmental Policy Act, enacted in 1970, effectively mandated the quantitative analysis of
noise pollution from every
Federal-Aid Highway Act Project in the country, propelling noise barrier model development and application. With passage of the
Noise Control Act of 1972, demand for noise barrier design soared from a host of
noise regulation spinoff. By the late 1970s, more than a dozen research groups in the U.S. were applying similar
computer modeling technology and addressing at least 200 different locations for noise barriers each year. , this technology is considered a standard in the evaluation of
noise pollution from highways. The nature and accuracy of the
computer models used is nearly identical to the original 1970s versions of the technology. Small and purposeful gaps exist in most noise barriers to allow
firefighters to access nearby
fire hydrants and pull through
fire hoses, which are usually denoted by a sign indicating the nearest cross street, and a
pictogram of a fire hydrant, though some hydrant gaps channel the hoses through small
culvert channels beneath the wall. ==Design==