Driving along the freeway singing along to Whitney Houston or engrossed in the latest book on tape, you can easily forget the impacts on the city around you. Ensconced in 21st-century car comforts, it's impossible to realize how streaming traffic affects the thousands of people in nearby homes. But one of the most overlooked problems is freeway noise.

One reason we might not notice noise is that the sound of cars can be uncannily pleasing. (One friend of mine calls the sound soothing, "almost like a river.") For others, freeway noise is an unceasing torment, particularly the high whine of motorcycles or the low rumble of decelerating trucks. Over the past few decades, to prevent freeway noise, sound walls have become a ubiquitous part of our urban landscape. But the art and science of noise wall design is more complicated than you might think.

A brief history of noise walls

When highway planners first started drawing lines on maps, building wide freeways through the hearts of cites, they were seen as a largely unmitigated good. Freeways were a modern idea that would increase the economic circulation of the urban system and solve traffic congestion once and for all. The noise of cars was an afterthought.

It wasn't until the 1972 » National Environmental Policy Act that departments of transportation around the country began considering noise impacts as part of freeway projects. Since then, engineers have gone out and measured sound levels and the distance between the homes and speeding cars, and run them through sound projection models. Anytime a wall is projected to reduce at least 5 decibels of noise, it is likely to be constructed; exact noise reduction goals are typically between 7 and 9 decibels.