While looking for some information on the typical spectrum of traffic noise I found this great online noise generator at mynoise.net. This well designed site has a host of noise types (including the traffic noise samples I was looking for). Apart from the simple interface and wide selection of noise types, one of the things I like about this site is the calibration feature. This allows you to compensate for weaknesses in either your audio hardware or hearing by cutting or boosting the various frequency bands as necessary. The calibration is stored as a cookie so is browser/hardware specific. If you are looking for a good noise generator to help you focus while working or looking for a specific type of noise, or just like to play with noise generators (I have no shame in admitting I fall under all three categories), then this is well worth checking out.