I used to be a Police Officer, and my first posting was to a station in a certain English city, not so many years ago, where there are large ethnic populations. My beat specifically covered one such area with an overwhelming Afro-Caribbean population. It is one of the cities where there were so called "race riots" in the 80s.
WPCs were not allowed on foot patrol there, and males had to do foot patrol in pairs. The actual square footage for the area was comparitavely small, but nevertheless there were always 4 pairs of foot patrols on duty 24/7, plus patrol cars, plain clothes Officers, and Task Force vans.
Why? Because primarily of the extremely high crime rate, including street robberies("muggings") which in my 5 years there were always perpetrated by black youth on white victims. There was also a roaring drugs trade, thefts, burglaries, assaults, and many prostitutes littering the street corners. In my time, at least two of my colleagues were knifed in the back(literally), I was once hospitalised, and many other Officers were assaulted. I have not exaggerated any of these details.
I could outline far more details, but the point I'm making is that not only is racism a two way street, but filling areas with only one ethnic group is counter productive and divisive.