THERE is a troubling, and worsening, mismatch between Americans' love of personal transportation and the way the nation's highways are shaped. Even in 1916 -- the year Emily Post published By Motor to the Golden Gate, an account of her arduous car trip from New York to San Francisco, and Theodore Dreiser published A Hoosier Holiday, the story of his mud-splattered auto journey from New York to his boyhood home in Indiana -- it was plain that the future of transportation in the United States was the road, not rail. Motorists were starting to expect paved highways, and plenty of them.
Periodic bursts of improvement in highway design led to the creation of a number of beautiful naturalistic parkways before the Second World War and to the development in the 1950s and 1960s of interstate highways with gradual, easy curves, generous separation of opposing lanes, and much-needed restrictions on billboards. Since then the imagination of American highway builders has fallen into a long slumber, and it has been only minimally roused by a federal program of "transportation enhancements" enacted six years ago. Auto makers shoot car commercials in deserts and on mountaintops, while the highways traveled by millions become ever more prosaic.
In 1968 California built what are believed to be the first noise barriers along modern federal highways. Walls were erected on Highway 101 in San Francisco and Interstate 680 north of San Jose to shield abutting residential neighborhoods from the sound of heavy traffic. Within four years the federal government followed California's lead, adopting regulations requiring that whenever a state builds, expands, or realigns a federally funded highway, an attempt be made to curtail excessive noise that would otherwise be inflicted on sensitive neighbors, such as schools, hospitals, and residential areas. Since then a total of forty-one states have built highway noise barriers, generally with 75 to 90 percent of the cost paid by the U.S. Department of Transportation. Nineteen of those states, including Colorado, Michigan, and Connecticut, have also voluntarily built noise barriers along existing highways where no widening or relocation of the roadway was taking place. In all, noise barriers have risen along some 1,300 miles of federal highway, and they are being added at the rate of ninety to a hundred miles a year. California still leads the nation, with 438 miles of barriers.
Bruce Donohue, a landscape architect in Westport, Connecticut, is aware of the effects that prolonged exposure to highway noise can bring. "One summer when I was a student," he recalls, "I was painting a house along Interstate 95 in Stamford, and I noticed that I was tense. I noticed that the women in the neighborhood were irritated. They were screeching at the kids, and the kids were raucous. The whole neighborhood was irritable and irritated." Road noise does not damage the hearing of people who live close by, but it seems to cause physiological and psychological stress, with results that include nervousness, difficulty in sleeping, and elevated heart rates.
The effects of stress vary so widely from person to person, however, that transportation departments have avoided basing noise-control programs on them. They have focused on something they can assess more objectively -- interference with hearing and speech. Federal regulations say that if a new or expanded road will produce noise approaching sixty-seven decibels (generally measured outdoors, near buildings), the state must try to bring about a "substantial" reduction in the sound level. Sixty-seven decibels is sound so loud that people have to be within three to five feet of each other to hold a conversation without raising their voices.
The human ear usually cannot detect a change of three decibels or less, so transportation departments aim for improvements great enough to be easily noticed by the average person -- often eight to ten decibels. A ten-decibel reduction amounts to a 50 percent cut in loudness.
Researchers for the federal Environmental Protection Agency have found that 20 percent of the population is "highly annoyed" if the sound level measures fifty-five decibels. This level, as it happens, is common outdoors in urban neighborhoods well away from major highways, and is one that transportation departments would often be glad to find in neighborhoods bordering expressways.
Attempts to provide relief from noise are complicated by the fact that walls work best at blocking short sound waves -- high-pitched sounds. Low-frequency sounds, such as the deep rumble of heavy trucks, may actually become more noticeable once the higher-frequency noise has been sharply reduced. "After a noise barrier has been installed," says Robert E. Armstrong, a senior highway-noise specialist for the Federal Highway Administration, "some people complain that they now hear every truck that goes by."
To a limited extent designers of noise barriers take psychology into account. California determines heights for its noise barriers partly by calculating how high they must be to hide the sight of tractor-trailer exhaust stacks. "If there were no trucks on the road, the walls could be considerably lower," says Walter Whitnack, an engineer formerly with the California Department of Transportation. "When people can't see the source of the sound, they tend not to be as bothered by it."
Highway engineers can seldom rely on trees and shrubs to buffer noise. A band of vegetation thick enough so that people could not see through it would provide much the same psychological relief as a solid wall, and most travelers would rather look at trees and bushes than at concrete. But an expanse of vegetation such as evergreen trees with dense undergrowth would have to be a hundred feet deep to reduce the sound level by five decibels, according to William Bowlby, a traffic-noise expert at Vanderbilt University. In most instances the expanse would have to be even deeper. "You almost never can find existing vegetation along a highway that could give you a significant sound reduction in a hundred feet," Armstrong says. "If we planted the right type of trees and ground cover-at considerable expense-it would take five to ten years for it to produce a five-decibel reduction."