Tutorial:Multi Highway
A Multi highway is a highway network in SC4 that involves an inner and outer lane highway system.
Lakeyboy's Multi-Highway Guide
lakeyboy has written a guide uploaded to the STEX (here) explaining how to make a Multi-Highway. This guide shows how to expand the Maxis ground highway from a 2 tile, 6 lane network to a 4 tile, 12 lane network.
Recently however, the images he posted are no longer found in his guide, making it difficult for first-time users to understand how to make it.
A photo tutorial for a multi-highway setup. Dragging one type of highway essentially overrides the other, creating a 1 tile highway in one direction. A Multi-Highway can be made without any mods installed, including the NAM.
Multi-RHW
In addition to the Multi ground highway, a similar concept can be applied to the RHW network. This creates a minimum 4 tile network, containing at least 8 lanes. The concept was first discovered by Haljackey and a guide was written soon after. Prior to the v.20 release of the RHW network, the Multi-RHW was extremely difficult to construct. With the release of v.20, Haljackey rewrote the guide because the concept was changed, making it much easier to construct Multi-RHWs.