In a leash-optional park, it will be more difficult for a victim to establish that the dog owner was negligent by not having the dog on a leash. Negligence is the lack of ordinary care; that is, the absence of the kind of care a reasonably prudent and careful person would exercise in similar circumstances. If a person's conduct in a given circumstance doesn't measure up to the conduct of an ordinarily prudent and careful person, then that person is negligent. In a leash-optional dog park, the dog owner still must do what is reasonable, such as keep his or her dog on a short leash if the dog is unruly, easily scared, not socialized, unneutered, has provoked fights with other dogs, or has any known dangerous propensity, such as the habit of jumping on people.
If the owner of such a dog fails to keep it on a leash in a leash-optional park, or allows it to cause injury to person or dog in a manner that could have been prevented, the victim should still prevail on a claim that the conduct of the dog owner was unreasonable, and therefore negligent. To read more about negligence, see the discussion of it in Legal Rights of a Dog Bite Victim.