Human trafficking can occur anywhere, but it is more likely to occur in places where people are in distress, not especially in any certain cities and towns.
Car pollution is a major problem in cities around the world. Large cities in China and India are suffering serious issues because of car pollution.
The "travelling salesman problem" is the problem where you have to find the shortest route to visit each of several cities. Even if the distances between the cities are known, the solution is actuall quite complicated; a lot of different algorithms (methods) have been developed to optimize the problem under certain circumstances.
In cities which have not taken steps to limit the problem - at the moment some of the Chinese cities have this problem.
New York City is currently ranked as the safest major city in the U.S., but during the 1970s until the mid-1990s, it had a serious crime problem.
London & cities all over the USA
nothing
You no longer have to go to certain cities to train certain weapon skills. You automatically know the skills that your class can use.
serious
Street gangs are a major problem in the inner cities. Drugs and violence are a trademark of most gangs as they fight for territory.
mandates
mandates
It depends on where you are. There are certain areas of certain cities that I would be loathe to walk about at night.