Based on my ten years in the TL, and my roaming all over the district at all hours of the day and night, "checkpoints" is almost certainly the wrong word for what lettergram is describing.
Let me translate what lettergram mentions:
> ...whole city blocks that were barricaded...
The sidewalks on some blocks in the TL were occupied with tents, chairs containing people, people sitting on the sidewalks, the occasional cluster of "street markets" (aka: combination flea market and stolen-goods-market), and -sometimes- people grilling with charcoal grills.
> ...the gangs ran checkpoints.
This one requires a bit of explanation:
In the heart of the TL, some corners have drug and/or sex salesmen advertising their wares to passersby. Some streets will have clumps of salesmen, and some corners and section of sidewalk will be crowded, simply because a whole lot of folks in the neighborhood socialize outside... so they're hanging out on the sidewalks. (Why not hang out in parks? Because the cops will push them back into the TL so that they don't make tourists, techies, and other such milquetoast folks uncomfortable.)
Never, _ever_ have I run into anything remotely like a "checkpoint". Crowded sidewalks? Sure! Folks doing more than hawking wares at me as I pass by, unimpeded by anything other than folks that are on the sidewalk minding their own damn business? No way.
The crowds and dealers certainly can be intimidating (and were definitely so for me for my first few months in the neighborhood)... but once you figure out that they don't behave like they're portrayed in the movies... that they're disinterested in starting shit and are just out there hanging out and running a business, you treat them like any other salesman hawking his wares.
> we've ceded the Tenderloin to being a known unsafe part of the city
That's its reputation, sure. It is the site of the city's open-air drug and sex markets.
Based on my time in the neighborhood, I've come to the conclusion that the City acknowledges that these sorts of things _will_ be sold, and that it's better to have an arrangement with the sellers and other "market participants" that ensures that everyone works together to prevent the violence that can often happen when such markets are pushed deep underground.
Because these are open-air markets, if the City wished to, they could drive by every day, rolling up dealers, pimps, prostitutes, and customers. I see the patrol cars rolling by on a regular basis, and _everyone_ who has been in the area for more than a few weeks can spot the folks that would be rolled up. They don't... and -as far as I've seen- have no intention of doing so.