In the 1920s and 1930s, car tires were relatively quiet. They were narrow—just two or three inches—and generally didn’t spin down roads fast enough to make much noise. But in the 1940s tires started getting wider, and roadways stretched farther and passed through more neighborhoods. Traffic noise started to be a problem, writes Nate Berg for re:form on Medium.
Leslie Kendall, chief curator of the Peterson Automotive Museum in Los Angeles, told Berg:
Every time a tire rotates, the patch that adheres to the pavement, whatever surface that might be, sticks to it, then it has to peel off. Every time. It sticks and peels, it sticks and peels. And that peeling is the noise you hear.
Starting in the late 1960s, transportation departments starting muting that noise by installing barriers—usually the slightly textured concrete walls you see along highways and freeways. These barriers can’t completely block noise, the U.S. Federal Highway Administration explains, but the good ones can cut down the sound of peeling and sticking tires by five to ten decibels. That's enough to cut the "loudness of traffic noise by as much as one half."
But someone had to be the first to suggest such barriers. Turns out that in the city shaped by cars, movie studios and the Hollywood Bowl were the first to search for a solution. Berg writes:
Working with the California Department of Public Works to study road noise near the Hollywood Bowl, acoustical physicist and UCLA professor Dr. Vern O. Knudsen would recommend in a 1945 study that “a wall or parapet, built to a height of at least 10 feet above the promenade walk, be constructed along the southeast corner of the Bowl, thus enclosing the entire seating area with a substantial wall which will serve as a sound screen against street and Freeway traffic noises.” Such a wall, Knudsen suggested, would reduce noise inside the Bowl by up to 6 decibels, equivalent to the noise drop of turning off the TV in your living room.
Studio lots were grappling with freeway noise as well. The Association of Motion Picture Producers called in the California Department of Public Works to study the problem in the 1950s and 1960s. But the group that finally got a barrier installed was much larger—homeowners. Building on the work commissioned by the Bowl and the studios, Interstate 680 near Milpitas has "what is believed to be the first sound wall along a freeway," Berg writes.
The walls that stand between noisy roadways and quiet communities are now standard and usually made concrete or cinderblock. There have been some improvements: research about how the barrier affect drivers has led some Western European countries to add transparent panels at eye level. And trees and vegetation help with aesthetics—plants alone can’t beat the sound blocking-powers of a solid barrier, but they're a lot more pleasant to look at.