Criminal justice research does not always affect public policy. It can often be the case that criminal justice researchers think that something (alternative sentencing, for example) is a good idea but public policy makers completely reject what they are saying. Criminal justice research can affect public policy, then, only when the prescriptions of the research fall in line with what policy makers already want to do.
Two examples of this can be seen in policing. These are James Wilson's "Broken Windows" and community policing. Both of these have had (somewhat related) effects on public policy.
We can see the impact of Wilson's ideas on New York City's policies under Police Commissioner William Bratton and Mayor Rudy Giuliani. Using Wilson's theory, these leaders started to emphasize cracking down on small-scale crimes to reduce the overall feeling of lawlessness that, they said, led to more important crimes.
Similarly, community policing has become very popular across the country. Numerous studies have advocated community policing and at least some of their proposals have been adopted.
These are two examples of ways in which some criminal justice research can affect public policy.