I'm using data from the Transportation section of the National Map (US). They provide three layers for roads: Trans_RoadSegment, Trans_RoadSegment2, and Trans_RoadSegment3. I seem to recall this scheme from other GIS data--the same road is split into multiple layers.
In the example above, the road is split into three layers. Some of the segments overlap others, some take a slightly different route, and some do not overlap at all. In any case, if you were to display only one of the layers, there would be gaps in the road. Why is this done? Why isn't the road data all on a single layer? The National Map provides a data key poster, but I don't see any information about this split there.