Roads have to be specifically configured by the asset creator to carry moving cars - it is a simple one line addition to the config.txt file of the road asset. Not all roads on the DLS have this addition.
The road itself does not have any cars included in the asset, it just has the "permission", if you like, to allow road traffic. The route creator, who will usually be someone else, then identifies the cars that will appear on those roads. This means that the same road can have totally different road traffic in different routes.
The region asset that is attached to a route has the list of assigned road vehicles.
To further complicate the issue, the Environmental Tools in Surveyor contains a setting that will switch road traffic on (enable) or off (disable) for ALL roads in the route regardless of the other settings.