Aggressive title, right? Well, retractable leashes can be a real problem, especially for city dwelling dogs. I'm not saying you have to go and throw away your retractable leash, but I would encourage you to read on and decide whether you may not want to switch it out for a leather or nylon leash instead.
Retractable leashes have a thin cord that retracts into a plastic handle, which allows your dog to have up to 30 feet of distance between you and them. Even if it's only half that, 15 feet is a big deal when your dog suddenly comes face to face with another dog turning a corner. Or if he spots a squirrel across the street and makes a break for it. Or when she thinks she's able to run a head because she feels no resistance, and then chokes herself to an abrupt stop.
For dogs in the city, retractable leashes and distracted owners mean numerous incidents that are avoidable. I have personally treated a dog that was hit by a car while the owner was walking him on a retractable leash - the owner turned her head to look at something for a second, and when she turned back her dog was halfway across the street and she watched him get hit. It was very traumatic for everyone involved, and cemented my feelings about those types of leashes in a city environment. More commonly, though, blind corners and long leashes end up with dogs coming nose to nose with other dogs/bikes/joggers/skateboarders with no reaction time possible for the owners at the other end of that 15 feet.
The injuries associated with retractable leashes are multi-fold. As I've already mentioned, for your dog: dog fights, hit by cars, choking incidents, neck trauma and even blindness. For the human holding the leash: cuts, burns, drag incidents and, ridiculously commonly, digit amputation. That's right, amputation. That sharp little cord that so conveniently disappears into the plastic handle? Works just like a guillotine on fingers that are in the wrong place when your dog sees that tempting little squirrel.
I think the concept of the retractable leash is fine for certain situations, however. Hiking on trails or walking in big parks where there is room to run but no physical boundary? Perfect to let your dog have 30 feet of freedom without risking him taking off after a deer or another dog. You still have the risk or choking themselves, but with training they can learn the boundaries of the leash. But walking down 5th Street? No, thank you.
And, as a matter of etiquette, if you are aware that your dog is not good with other dogs or people or children PLEASE do not use a retractable leash. The amount of control you have over your dog is NOT good enough. And if your dog is not well trained (or even just squirrel motivated), the injury you may be avoiding is your own. Watch those fingers!