A single centerline can be used for situations where the speed limit is different in both directions by having different attributes to account for the different speeds. For example, here at the City we have centerlines with a "To From" Speed Limit and a "From To" Speed Limit, which refer to the speed limit for each direction based on the line directionality. We also have fields for "To From" Minutes and "From To" Minutes that are populated with the time it takes to transit that segment based on the speed for that direction. Part of the reason it was set up this way is so we can maintain a Network Dataset.
We also do have "divided highways", where the travel lanes are defined as one-way, but those are in specific cases where there is a central barrier divides the road into essentially 2 one-way streets.
So I guess one question back to you is, what is the context that goes with your question? Are you looking to do Network analysis?
One caveat - we are not doing the Local Government Information Model (I noticed you tagged this post with LGM, so I am guessing you are using that).
Chris Donohue, GISP