What's a no-go area? I can guarantee you there are streets in big European cities where cops tend to show up only in numbers because it's pretty likely they're going to be attacked. A lot of that threat comes from Far Left anarchists who consider some streets "theirs". But there's also some housing estates where certain gangs have enough power that if the cops go to arrest someone who might be part of such a criminal entity they have to show up in bigger numbers because they would probably be ambushed.