I only saw this once, but it was an interesting one. The high LOD zone did not load when I was in the city, even though I walked right up to the zone. So I was left with the low LOD zone. Here are some pictures: Cars were blocked by the low LOD zone and so were piling up when they hit it. Johnny Gat also got stuck here.
Someone else reported a similar issue on the Steam forum here: http://steamcommunity.com/app/304530/discussions/0/1473096694448640882/
this is a common issue throughout the ctg engine, from my testing its relative to the camera or something along those lines.
Well, LOD (Level of Detail) is supposed to be relative to the camera such that closer objects are high LOD and further objects are progressively lower LOD, but in this case I was close and the high LOD zone did not load like it's supposed to. Looking at it again, it doesn't even look like the low LOD textures loaded properly either. And once it got into this mode, walking far away and coming back didn't force it to load the high LOD either. I had to restart the game.