Any Retractable Leash
There are a handful of retractable leashes out there, and a handful of injuries from them.
Both dogs and humans have gotten gnarly leg burns. There are some made to reduce the chances of burns, but I personally just avoid them all together. Generally speaking, most leash laws follow the 6ft leash rule unless in a designated off leash area like a dog park. Using it on the sidewalks makes it easy to forget the 6ft rule. The leash also makes it really easy to allow the dog to walk out in front of you when crossing a road which can be dangerous. I did not know any better with my first dog, she walked only a few feet in front of me when a speeding car whipped around a turn not noticing her, nearly striking her! It is much safer to cross the road close together as a tall human is far easier for a driver to see.