Straight to the point, this is a minor glitch that is easy to activate and get rid of. Here, I will explain everything I know about this, since I couldn't find a video or topic of it.
What you need: A mission that triggers a cutscene.
How to make it work: Enter an interior, then exit it in an "unorthodox" way, which means, without using the door. There are a bunch of ways you can do that. The easiest one, in my opinion, is glitching through the bathroom wall of the Apartment 3C. Another way to do this is the "bike trick", as shown here:
If done right, you will be standing in what is commonly referred to as the "Ghost World". From here on, try and navigate your way in the dark all the way to a mission marker. A helicopter can help a lot.
The Ghost Town: After the mission cutscene is over, you will find yourself in a city where no traffic spawns, and has a darker background, giving it a "creepy" vibe, akin to a Ghost Town, hence its name. Pedestrians, parked cars, and Police roadblocks during a high wanted level still appear, though. A side-by-side comparison can be seen here:
Getting rid of the Ghost Town: Simply enter an interior again, get Wasted/Busted or trigger another cutscene. The traffic and lighting will revert back to normal. Your game is not affected by this glitch in any negative way, as far as I know.
After 100%: This needs some more investigation for the time being. Since there are no missions left that trigger a motion cutscene after non-glitched 100% completion, I only know of one way to bring the Ghost Town back, by glitching the Hotring Racer out of the Hyman Memorial Stadium, and entering the Shooting Range. More details on how this is done in the Special Vehicle guide, in the FP/EP Hotring Racer section.
Anyone is welcome to post more regarding this investigation, so if you find anything new, please let us know.