If you are adding a new road whch will be classified as mH or higher and is wide enough for median segments at intersections to be >5m, you need to create two one-way segments. You can open Advanced Preferences and uncheck the box for "Draw new roads as two-way" to help with this.
Most highways whidh are already on the map are already drawn properly. If you find an already-drawn highway that needs to be divided , create two one-way segments from each 2-way segment (preserving the traffic data in both directions). If you are not familiar with the technique for preserving traffic data in both directions, please work with an editor who is. We follow the [https://docs.google.com/presentation/d/1m8jgfYzGK_SxauaRtcmJvo5ScCfc81-TIbH23YmoxSU/edit#slide=id.p4 guidelines in this presentation on dividing roads]
==== Solve URs and MPs ====