There's an excellent book, Defensible Space: Crime Prevention Through Urban Design (1972) by the late architect and city planner Oscar Newman. . Newman set out to study why large public housing complexes had so much crime. The findings reported in his book became famous among planners and academics.

Among his conclusions was that in addition to being too large to be manageable, the housing complexes were far too accessible to strangers and there were no private outdoor spaces that residents could call their own and effectively monitor. His remedies, which proved successful in a number of make-overs across the country, involved turning crime-ridden public housing and sections of cities into mini-neighborhoods with gates and fences. In effect, these became gated communities where criminals were easily identified and felt uncomfortable. Residents developed a sense of pride and investment in their homes and neighborhoods, and they were finally able to feel safe.

The relevance for trails is that trails create the same hazards for abutting neighbors as crime-ridden public housing and neighborhoods do for residents. Too much public access and not enough private space equals big trouble. Trails are essentially unpatrolled alleyways open to anyone and everyone, with major domino effects.

Oscar Newman's work is widely respected in planning circles, even if his remedies are often ignored by many planners because they interfere with property-redistribution plans.

This is an article by Newman explaining the history of his defensible-space concepts:

Excerpts from Newman's most recent book Creating Defensible Space, (also on Amazon

Page 13:
Also in St. Louis, I came upon a series of turn-of-the-century neighborhoods where homes are replicas of the small chateaux of France. They are the former palaces of St. Louis’ commercial barons, the rail, beef, and shipping kings. These chateaux are positioned on privately held streets, closed to through traffic. St. Louis in the mid-1960s was a city coming apart. The influx of people from the rural areas of the South had overwhelmed the city. It had one of the Nation’s highest crime rates, but the private streets appeared to be oblivious to the chaos and abandonment taking place around them. They continued to function as peaceful, crime-free environments nice places to rear children, if you could afford a castle. The residents owned and controlled their own streets, and although anyone was free to drive or walk them (they had no guard booths), one knew that one was intruding into a private world and that one’s actions were under constant observation. Why, I asked, could not this model be used to stabilize the adjacent working and middle-class neighborhoods that were undergoing massive decline and abandonment? Was private ownership the key, or was the operating mechanism the closing-off of streets and the creation of controlled enclaves? Through research funded by the National Science Foundation (Newman, Dean, and Wayno, 1974) we were able to identify the essential ingredients of the private streets and provide a model that could be replicated throughout the city. This was done in both African-American and white areas, and its implementation succeeded in stabilizing communities in transition . . .

Page 41:
In my presentations, I explain what the restructuring of streets to create mini-neighborhoods accomplishes: It alters the entire look and function of the community; it completely removes vehicular through-traffic (the only traffic remaining will be seeking destinations within each mini-neighborhood); and it completely changes the character of the streets (instead of being long, directional avenues laden with traffic, they become places where children can play safely and neighbors can inter-act). By limiting vehicular access, the streets are perceived as being under the control of the residents. Fewer cars make it easier to recognize neighbors and strangers. I explain that access to the newly defined mini-neighborhoods, which will contain three to six streets, will be limited to only one entry off an arterial street. People will only be able to drive out the same way they came in. It is important to explain, again and again, that the gates will only restrict vehicular traffic: Pedestrians will be able to freely walk everywhere they did before.

Limiting access and egress to one opening for each mini-neighborhood means that criminals and their clients would have to think about coming into a mini-neighborhood to transact their business, as they would have to leave the same way they entered. There would no longer be a multitude of escape routes open to them down every city street. A call to the police by any resident would mean that criminals and their clients would be meeting the police on their way out. Such a street system will clearly be perceived by criminals, and particularly by their clients, as too risky in which to do business. . . .

It is very important to make clear to residents that most of their internal streets will be converted to cul-de-sacs and that in the first few months following the modifications residents, their outside friends, and service people will be inconvenienced. During this initiation period, many residents will want the gates removed, including some of those who voted to have them installed. But after 4 months and after residents and their friends have had a chance to learn to find their way around, people will not be able to believe the improvement in the quality of their livesp roduced by these changes and will insist that the gates remain. . . .

Page 44:
Smallness is essential to identity, so a mini-neighborhood should consist of a grouping of no more than three to six streets. The optimal configuration for a mini-neighborhood is a Greek cross, a vertical with two horizontals. Only one point of the cross will remain open, the
the other five will have gates across them.

Cul-de-sac configurations should not be too large, for they take residents too far out of their way and produce too much of their own internal traffic. If a mini-neighborhood is made up of a vertical with six horizontals, for instance, residents will have to travel too long a distance to get to the end of their mini-neighborhood, and then they will have to travel all the way back to get out of it. In the process, they will encounter others doing the same thing. This will produce a great amount of internal traffic, and traffic is exactly what we are trying to avoid.

Page 45:
Mini-neighborhoods and their access arterials should be designed to facilitate access but discourage through-traffic in the overall Five Oaks community.

Page 48:
Only one entrance, or portal, is provided to each mini-neighborhood, and it is the only way out as well.

The fact that many of the houses in Five Oaks are also served by alleys, and that these alleys are used for both parking and garbage collection complicated our plan appreciably. For maximum effectiveness in facilitating community control and in reducing crime, access to the alleys had to be limited to the residents of each mini-neighborhood and to the garbage collection vehicles.

Page 52:
Once the gates were installed, police, in a concerted effort, came in and flushed out the drug dealers, pimps, and prostitutes. They had done this before in Five Oaks, but the criminals had come back a week or 2 later. However, when the criminals were removed after the gates were installed, they did not return.

Pages 56-57:
The University of Dayton’s survey found that 67 percent of residents thought their neighborhood was a better place to live, while 13 percent said it had remained about the same; 39 percent said they knew their neighbors better, while 53 percent said they knew as many as before; 24 percent said it was easier to recognize strangers; and 36 percent were more involved in the community (that is, through block clubs, civic activities, neighborhood watches). Most importantly, there was no difference in these perceptions between African Americans and whites, renters and homeowners. Drugs, theft from houses and cars, and harassment were all found to be less of a problem than a year earlier (University of Dayton, 1994).

The usual complaint about such programs, that they displace crime into the surrounding neighborhoods, also proved untrue. Crime in all the communities immediately surrounding Five Oaks decreased by an average of 1.2 percent. The police’s explanation is that criminals and their clients knew that the residents of Five Oaks have taken control of their streets, but because they did not know the neighborhood’s exact boundaries, they moved out of the surrounding communities as well. The positive effects in traffic reduction also spilled over into bordering communities as all of Five Oaks has itself become an obstacle to cut-through traffic. Other communities in Dayton are now exploring a similar restructuring.

Page 74:
Try to subdivide all the grounds and assign every scrap of it to individual families.

The reassignment of public grounds was undertaken with the intention of expanding the domain that residents felt they controlled and in which they felt they had the right to expect accountability from strangers.

Page 78:
The average monthly assault rate dropped from 0.53 per 1,000 to 0.31, a 42 percent change. The number of felonies during evening and night-time hours decreased by more than one-half. For the serious crime categoriesburglary, robbery, and assaultthe average crime rate was reduced by 61.5 percent.

The percentage of people who felt they had a right to question strangers on the project grounds increased from 27 to 50 percent. Residents’ fear of crime was reduced even more dramatically than the actual crime rates and, for the first time in years, most residents said they had little fear of walking through the project grounds at night.

The project, which was 30 percent vacant before the modifications, not only achieved full occupancy, it acquired a waiting list of hundreds of applicants.

Pages 104-105
There isn’t even minor theft among residents on the sites, and you know what it can be like in public housing: people stealing each others’ curtains. The residents now store their outdoor things openly in their individual back yards: bicycles, barbecues, lawn chairs, tents. These yards are only separated from each other by low 3-foot fences. Yet nothing disappears. That’s because everyone knows it would have to be an inside job. You can’t get into the collective rear yard area from the outside because of the high 6-foot fencing that encloses the collective of individual rear yards.