There are a few things to consider when deciding whether to use a fixed length leash or a retractable one (“Flexi-Leash” being one brand).
More and more we see the retractable kind. While these do give your dog more freedom, and save you from having to pull in the excess fixed leash when yoiur dog is closer, there are downsides! If your dog pulls and you are trying to train him not to, you need to use a fixed leash and keep him close to your side. You want him to look at you often, and pay attention to you. You have no control if your dog is twirling around at the end of 16 feet of cord!
And then there is the safety factor, concerning both owners and the dogs; I personally know of instances where the cord has caused injuries, and drawn blood. Everyone should take a few moments to educate themselves a bit on this. A good start is to read the compelling report in Consumer Reports, but be prepared for a somewhat gruesome beginning.
Another really good article explains the risks to your dogs as well. I know from experience, having dropped the handle of the flexi-leash that I formerly used, and seen my dog bolt in fear of it “chasing her”! I can understand how some dogs have died as a result. of rushing out onto the road, in fear.
Is it likely that one of these things will happen to you? Maybe not, but is it worth the risk? I decided it wasn’t, but take a few moments and decide for yourself!