Well this may be a glitch that just happens to me, but when i go extremely fast eg. 208mph - on a tunnel or anywhere that has a part that sticks out in the road, i smashed into it (pillars in tunnels) and my car fell through the floor to reveal a hidden map - the road is gone and its just sky - if you fall through the floor then you can see the road above you - bearing in mind your still on that track except this time there is no road - but barriers are still there but not visible.

Another way this happens is if your head of the tracks spawning - notice when you go really fast you can see the sky but no buildings? The track is building, if you get ahead of the tracks building - you will be driving in the sky