Sometimes bad road design (e.g. lanes too wide) are to blame, but in miserable neighborhoods with no traffic enforcement at rush hour you can also end up in a situation where the majority of people on the road are simply aggressive drivers who are familiar with the road. At some point you do need to enforce the law if it isn't being respected. There is a growing subset of people in the US who not only disregard traffic law but pride themselves in a distain for it.