The entire highway can be made one-way. Thus, you may have 6 lanes one direction by laying down three parallel highway lengths. Leave the middle one normal. Overlay the wrong way parts of the other sides with a one-way road for the length of the highway.
In this image, a 12 lane freeway meets an eight lane freeway and converts going east to a four lane highway with four lanes of service roads on each side as bridges are necessary and bridges cannot be overlaid.
Two eight lane highway bridges. Four lanes each direction.
How to make:
1. Lay down two parallel highway bridges next to each other. Four separate bridges will be made.
2. Remove the center two bridges.
3. Lay down a highway bridge between the two remaining bridges.