City crime is a tough issue to get a hold of. Trying to deal with city crime is like trying to deal with LA gridlock. Cities have problems with crime because they are cities. One of the best predictors for crime in an area is its urbanicity. Population density increases the ability to easily and effortlessly acquire targets, increases the number of potential conflicts that can escalate, often strains and stresses people, and allows for anonymous attacks.
In general, when people ask me about policy ideas, I always try to pick policy ideas that are voluntary, community-oriented, non-coercive, low-effort and low-cost. That's because these are the easiest to implement and tend to stick around the most against political entropy. I also value ideas that don't just involve having some big institution impose something onto organic communities. And I value ideas that don't involve coercion or smashing people into submission, especially since those ideas routinely end up being not just brutal but also classist and racist as a result.
First of all, beautification and civic improvement can help. While parks can potentially be quiet places for an assault (if not properly policed), in general making city spaces look better and feel better seems to have a real impact on the general sense of well-being in the area. This is connected to the idea of "broken windows", and while the "broken windows" theory is often proposed in a very reductivist way, it is still valid.
Second, I think that making it easier to promote good health and self-defense would be reasonable. Guns in a city space are especially problematic, but martial arts would be a means of making the average person more able to defend themselves. That could easily be promoted with health grants by NGOs or government and with PSAs.
Third, promoting jobs is incredibly crucial. Unemployed people are going to increase the amount of vagrancy, the amount of people out in public serving as either victims or potential threats, etc. While human motivations are complicated and sociology is even more so, it's not a big stretch to say that people who are poor and desperate for work are more likely to turn to underground economies, theft, or other means to survive.
Fourth, improving anti-gang efforts in schools while also improving education is utterly crucial. Schools can't be prisons. We've tried all too often in the United States (and elsewhere) to deal with problem children by treating them like convicted criminals ahead of time. That destroys trust and eliminates the incentive to go to school. Urban schools need to be safe places where children can grow.
In essence, most of criminology is really an effort to try to get people to their mid-twenties without committing serious crime. Once you're an adult with a job and an education, you are incredibly unlikely to become a hardened criminal. There are just too many social, economic, and practical inducements keeping you in that world. So stopping crime, in the city and in the country, hinges on making sure that children are given the opportunities to rise to their greatest potential.
Fifth, it is vital to build city infrastructure. This is obviously a situation where national and regional governments must be heavily involved.
Sixth, racial conflicts, religious conflicts, etc. need to be dealt with. This is a job for civil society to a huge degree, but business and the law must also be involved. There needs to be a real effort to keep communities from becoming segregated. There must be proactive communication between groups and efforts to stop hatred. In my opinion, this is one of the issues we talk about in the West the most poorly. The degree of racial resentment and racialized inequality that one has in a society is in my opinion a poorly understood and utterly vital variable for crime.
Finally, the police have to be willing to do the hardest policing jobs. They have to be willing to work with the community instead of against it. They have to be willing to listen to community leaders. They have to be willing to work with children and adults, and do outreach all of the time. They have to be willing to do the hard undercover investigations and try to go after the real bad guys, not just the bad guys who are easy to catch. They have to be willing to report honestly and not "juke" the statistics.
The police are often blamed for the failures in this regard, but anyone who has watched The Wire knows that their failures are the tip of the iceberg. When politicians make no effort to keep racist cops from ascending the chain of command, make it easier for cops to go after low-level drug dealers than big bad guys, discourage difficult undercover investigations that might end up imprisoning politically connected people, and provide perverse incentives for police to "juke" the statistics, the problem cascades.
A truly indicative example of why we so often fail to get a real handle on crime is the case of Operation Greenback. Forensic accounting efforts were stopped (by then-drug czar George H.W. Bush) because it would go after rich (disproportionately white and male) bankers. Crime isn't something that's just the problem of "those people": It's everyone's problem. Delinquency is omnipresent. Most adolescents are going to have done some drugs, or done some binge drinking, or broken some speed laws, or even committed much more serious misdemeanors and felonies. A little bit of people being delinquent is actually good from a developmental perspective. Recognizing that crime is a truly pervasive problem that affects everyone and that gives all of us collective responsibility may be the single most important transformation we must engage in.
It's tempting to say that we should try to lower city population densities. The issue with that is that I think that cities are actually going to have to be a big part of our future. The more evenly spread out a population is, the harder it is to service. You have to have more roads, more transportation, more consumption of carbon, more infrastructure that has to be maintained, etc. Ecologically speaking, cities are likely to be really important to sustainably facilitate the human life of around nine billion people. Moreover, most schemes to try to reduce city sizes end up having classist and racist assumptions and implementations.
When we start realizing that we can't just run from crime into the suburbs and start realizing that crime is a collective problem that requires collective responses, we may see a real reduction in human suffering.