I've had a few problems with the 3D viewer, because cars are not always displayed all of the time. Sometimes, they vanish from the displayed network, even though the flows all balance, so I'm, not losing traffic mid-block or anything like that.

I'm using a widscreen monitor, set to 1680x1050 pixels, 32bit, 256MB ATI Radeon XT1300PRO.

I'm not sure why this is happening, but I have noticed that it tends to happen when the 3D-viewer window is maximised. Reducing the size of the window, and the viewer works properly.