I had them just completely disappear yesterday throughout the whole map. Was kind of nice except I needed to get the threading the needle in the Forzathon, but did make getting the Ultimate Speed one way to easy. I think they need some work, as I will cross over to oncoming traffic and a ton of cars will appear, then go back to the other side which is now completely empty, and then they start showing up there and disappear on the other. (Wide streets/roads)