Backyard Noise Barrier. A sound barrier doesn't just physically reduce traffic noise, it also tricks the mind into thinking your backyard is quieter. To reduce low frequency noise like air-conditioning units or hot water heaters, an acoustic enclosure (typically sound absorbing) will be best.
The winding two-lane road in front of This Old House's TV project house in Carlisle, Massachusetts, is hardly a superhighway, but it is a well-traveled commuter street.
The most effective barrier you can install is a fence.
Adding thick bushes helps with sound attenuation. It may also be the best solution in a side yard, where space is tight, since fences have a smaller footprint than plantings. A fence intended to serve as a noise barrier or "sound barrier" will typically use thick tongue-and-groove boards, in order to form an impervious barrier.