You can walk around during the day and surprisingly for the most part well into the evening...considering the number of tourists that walk around almost anywhere at night in Negril, the number of incidents are surprisingly low.
Would I ever recommend walking around freely at night...NO...but I can't think of anywhere in Jamaica, Canada or anywhere in the world that I've visited would I recommend this activity. If I'm freely walking around at night, I expect the risks to be higher.
As far as walking down the lanes in Negril...I feel this to be a strange thing to do anyways when uninvited.
I know as humans we obviously have a curious nature...but to wander into purely residential areas as a tourist seems strange to me and then to question one's safety into an area that's not meant for tourism seems hypocritical.
Let's simply look at this from the local's perspective...
you're at home...your dogs start barking...you check to see why they're barking as everyone on the lane knows each other as do the dogs...you see a stranger or group of strangers walking down your lane...now what??...your evening is interrupted...you have to figure out whether it's tourists that got lost or people that are up to no good...either way you now have to keep an eye on them. You may call a neighbour or two to let them know what is happening. Why? Because simply...there are strangers in your neighbourhood...
This plays out on a daily basis in Negril and the locals have learned to accept it and are quite wonderful about it. But, please remember to be respectful of residential areas, especially at night.