Ok, I see what you mean.
Those cars are are not coming from our scenery. In fact, we haven't placed any road there, for the precise reason to prevent that problem, since default ground traffic cannot pass under bridges. So, they must coming from another scenery ( something with Vector data ) in conflict.
In theory, if KORD it's placed on a higher layer in the Scenery Library, it's supposed to exclude everything beneath it, but if layers have been set explicitly, either by another addon installer or by add-on.xml utility, the layering might not be default anymore, which means "the last installed scenery goes on top".