In the late 1970s....I didn't have an issue walking around Frankfurt, Amsterdam, or Tacoma (Washington) at 10 PM. The concept of no-go (where you admit there's potential stupidity for violence or death) didn't even occur to me.
I probably made it to the 2004-to-2008 period....where I started to notice certain little things. There would be people 'zonked-out' on drugs....streets trashed-up....people starting to talk about no-go in a strategic way (how to walk around this part of town).
By 2010...with my three-year period in DC....I got various impressions to be vigilante and plan trips/walks. I wanted to go over to Baltimore, and after asking a co-worker who knew the city....about where to venture....he gave me this map of downtown Baltimore....using a red-pen....he crossed off approximately two-thirds of the city map....saying it's best you don't go here.
My local town of Wiesbaden? Ten years ago...it existed without a no-go area. Today? We have a 'Waffenverbotszone'.....it's a zone where after 7 PM...cops are free to stop you....force an ID-check, and frisk you (got a knife....it could be several thousand Euro in a fine).
You want drugs....I can pinpoint three areas of Wiesbaden where you can get anything you desire.
The act of putting drugs, crazy-violent behavior and public safety into a mix? You go to a logical mindset....always looking over your shoulder and asking....what's the max threat you can accept?
The old stomping grounds of Tacoma and Seattle? If you ask locals in Tacoma....they will rattle off North End, South Tacoma, Hilltop, Central Tacoma, Eastside, and Northeast Tacoma...as places to generally avoid after dark.
Seattle? They now have two phrases often used....safe areas (meaning a serious police presence) or no-go (where there's a active drug scene). The Belltown area? Day or night....it's suggested to just avoid this area.
New Orleans, Houston, Atlanta, Chattanooga, Birmingham, Memphis, St Louis.....they all have no-go areas. If you ask why....the answer always revolves around drugs, gang action, addicts on the loose, and crazies looking for a 'friend'.
Just a US thing? No.
There are various parts of Hamburg, Koln, Berlin, Frankfurt, or Dresden....that I'd suggest asking for local advice on safe places/no-go areas. There's one entire city park in Berlin....that I'd tell you to avoid....day or night.
Authorities just letting this go? That's the million-dollar question....just how much are you willing to spend....to impress upon people....no-go activity is unacceptable?
The landscape ahead? Just odd....where you see 'Escape From New York' as a true story....where you are skeptical of law-and-order.