The expressway I-5 bridge in Seattle is notoriously loud and can be heard for miles around. Interestingly, the sound from the busy topmost lane doesn't seem to make it to the ground. You can verify this during the brief pause when the express lane changes directions. I believe the traffic noise from the express-lane is reflecting off the underside of the upper lane. Seems like a diffusing layer on the underside of the upper lane would reduce the sound getting to the city below.