This is a pretty bad little issue with the game that I encountered today myself.
 
At some point during a fight with the enemies all the civilian vehicles and people disappeared in my game. Only enemies were around. Resetting and reloading doesn't fix it. BUT! If you start a race, then abort the mission, it should reset everything. If you didn't know that you'd probably drive around for an hour trying to fix it like I did....

Weird, I had one time when I destroyed a car and then there was just nothing around, but it was just a quirk I guess, I parachuted along for a bit and cars started spawning.

Yeah this was a little different. Enemy vehicles spawned and you could pull out more raising the Heat Level. But you'd otherwise be driving on barren roads and empty oceans. If the glitch happened it would be the perfect time to do some races, had that not turned out to be the solution!

I actually experienced this last night, and used your fix to sort it out too.  It happened for me as/after I completed the faction mission to blow up a statue of baby and chopper its head down off a mountain.

I just had that problem in the PC version. In my case, it was triggered during or after the "Red or blue!?" faction mission in the north-west corner of the map. Luckily, your solution worked for me just fine, too.