President Obama just signed legislation - the "Pedestrian Safety Enhancement Act of 2009" - that requires all motor vehicles to emit a minimum amount of noise (to be determined by the Secretary of Transportation). The law is primarily intended to benefit blind folks, who depend on their hearing to know of the proximity of approaching vehicles. (Obviously cyclists are expected to watch out for blind folks, since we aren't required to emit noise. Although maybe that will change - maybe all bikes will be required to have playing cards in the spokes.)
That got me to thinkin' about road noise.
There's an interesting Wikipedia article about roadway noise: "In the USA it contributes more to environmental noise exposure than any other noise source, and is constituted chiefly of engine, tire, aerodynamic and braking elements." It explains how vehicle speeds, vehicle types, roadway surface and geometrics, and tire types all affect overall road noise.
For every extraordinarily quiet vehicle, there are 20 extraordinarily loud ones. Some roadway users seem to get satisfaction from making an obnoxious amount of noise - bikers, punks in their rice-burners, and rednecks in their huge pickups come immediately to mind. (I ride a Harley, so I'm sensitive to that issue from several aspects. I've got an aftermarket exhaust pipe that "rumbles" more than the factory setup, but it's not annoyingly loud. My neighbors wave at me when I ride by, rather than shaking their fists.)
This time of year, tire studs generate a painful amount of noise. Drivers of those cars probably aren't aware, since their windows are rolled up and they've got the ol' CD player and heater blasting away.
Although I usually feel safe when I'm sharing the road with motor vehicles, riding on dedicated bike/pedestrian paths always provides some noise relief. It's a sweet thing when the roar of roadway noise is a distant rumble, rather than constant and immediate.