2“It’s raining, and he’s just walking around, looking about,’’ Zimmerman told the 911 dispatcher during his first exchange. Martin was in front of the clubhouse at the Retreat at Twin Lakes. He may have been looking for a sidewalk.

3Depending on which way Martin entered the subdivision, he would have found at the clubhouse either a rare length of sidewalk merging into a parking lot or leading away into a sort of jogging path encircling an artificial lake. If Martin chose simply to cross the street from the corner where he was, he would have been forced to transgress in the most literal sense. The 30-foot street (enough for two driving lanes and one parking lane on Mass. Ave.) doesn’t have a painted crosswalk. Probably because the other side only has private lawns and driveways.

4Most of the Retreat at Twin Lakes lacks a conventional sidewalk—a public pedestrian thoroughfare parallel to vehicle traffic but protected by a curb. Together with a landscaped tree belt, parking lanes, and occasionally bike lanes, sidewalks and roads make up what is called the public right of way. Without public rights of way, we would all be constantly having to trespass on private land or pay tolls to get anywhere. This was the situation Martin faced inside and outside the gated subdivision. On his mile walk to the nearest convenience store, the sidewalk ends twice and becomes a no-man’s-land of grassy highway shoulder. If Martin were trespassing, he had no choice but to do so.

5However, city planners and designers know that “public’’ is not just a legal definition but also a quality. The similarity of the word “public’’ to the word “people’’ belies the original association. A legally private place like a mall atrium on a Saturday is public in a practical sense; that’s where the people are. People sharing a space will naturally carry out a kind of surveillance that enforces standards of behavior. That tendency is part of why planners and designers promote private activities like outdoor café seating and music festivals in public places.

6Some environmental interpretations of the Martin-Zimmerman story lay blame on the gated community: An unknown black teen on private property triggered an aggressive instinct in Zimmerman. But the interaction between the two wasn’t private in an abstract way; it was private in a physical way. No one else was there. Witnesses heard the fight, but the two traversed the equivalent of several city blocks by themselves. Without sidewalks, there were no bystanders.

7After a tragedy, we try to imagine alternatives. What if we change the laws? What if we raise awareness? To those important questions I would add: What if we design places differently, places for people?

8Houses with front porches rather than driveways bring residents outside even in rainy weather and put “eyes on the street,’’ as the pre-eminent urbanist Jane Jacobs described it. When houses are closer to the property line and on narrower streets, residents feel like they are more responsible for what happens outside. Zimmerman was a self-titled neighborhood watch volunteer. Design can make residents neighborhood watch volunteers naturally.

9In a place meant for people with a denser residential street, maybe the man and the boy might have felt less like they were all alone. In a place meant for people with sidewalks and street lights, maybe they would have been less alone. Maybe a couple of neighbors could have stopped the altercation before it got out of hand.

10Maybe with a small convenience store or café in the clubhouse, Zimmerman wouldn’t have gotten into his car to go to Target. Maybe he would have walked to the clubhouse, and simply passed Martin on a sidewalk designed for him to be there.