A traffic lead is a short dog leash - often very short - at under a foot in length. Traffic leads are designed to be worn by medium to large sized dogs that have already been trained to walk on a regular leash. Traffic leashes aren't a very popular tool (it's unlikely your local Petco stocks them in stores) but they can be very useful when training, especially when teaching your dog how to heel.
As the name implies, traffic leads are particularly helpful when visiting heavily-trafficked places, like busy shopping malls, schools, and stores with your dog. But they can be useful in any situation that demands a short leash. Using a traffic lead in busy areas is convenient, because you wont have a bunch of extra leash to hold onto when you know you wont be letting your dog go that far.
Traffic leads are usually under 1' long.
Due to their short length, these are not everyday walk-in-the-park leashes. Traffic leads are training tools, typically used for short periods of time when working or training a dog. Common users of traffic leads include service dog handlers, police officers, and the military.
You can also use traffic leads for off-leash training, as your dog can run and move without getting tangled up in the leash. It's important to note, however, that traffic leads are usually not considered leashes, by law. Most leash-laws require leashes are at least 4' in length.
You can find traffic leads in multiple colors, widths, lengths, and materials. Most common materials are leather, biothane, and nylon. If you're training your dog, a traffic lead can be an excellent addition to your gear, and can be worn at the same time as another leash for the most versatility.