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Three things you will need:
- Latest version of NAM (windows), (Mac)
- Ground digger/lifter lots
- (Optional) BRF Tunnel and Slope Mod
First off, make sure you know where you want your highway to go. You can do this by placing numbered signs on the terrain.
Next, go into your transportation menu and find the ground lifter lot. Place one on the ground, and then run a road through it. After that, demolish the lifter lot, and place a single piece of road (or street) on what was the location of lot. Expand it out by placing pieces of road next to the original piece.
Now, here comes an optional part. 15m is bit high for this kind of highway, no? If you wish, you can lower the terrain slightly, and use the same road-placing technique I described above.
After that, at each sign you put down, Put three road tiles two tiles from the left of the already raised terrain. The three more tiles one tile from the raised terrain. Put three tiles in the center.
Once you're done with that, continue raising the terrain with the road tile technique in a straight line until you reach the three tiles. The three tiles should automatically raise up with the other tiles.
Now begin using the road tiles to raise the terrain, although shift the placement so that you're heading in the direction you want to go. Lather, rinse repeat.
Now, you're probably wondering "What about diagonals?" No worries. When you reach a an area where you want to go diagonal, lay out those three road tiles in the center, but keep shifting them one tile in the direction you want to. You can see how I did that here. Not that you may need to separate them a bit.
Once you're finished, the rest is a cakewalk! Observe the magic.
Now let's do some underpasses. Mark a good point where you would like the underpass to be. On where the road/avenue/street/one way will be, use the road tile technique to even out the terrain for 4x4 tiles on each side, as you can see here.
Once again, use the ground lifter lots. Make sure not to put the lots on the already raised terrain! Put road through them, even it out, and it should look like this:
On this section, we're going to smooth things out for a realistic incline. It's harder to do this without the slope mod (it can still be done, though). Select the rail. Demolish the highway around the area. From the road tiles, drag the rail down until you reach an end of the raised terrain (if there is a curve). Then, lay down more rail adjacent to the one you just laid down.
Get rid of the rails and the two road tiles on each side. Go into the highway menu. Select Custom Interchanges. Tab through the pieces until you find the one titled Straight Highway OnSlope. Place it on each end. Replace the highway on both ends.
Warning: I've found SC4 to be quirky with this part. If you can't seem to be able to place the ramps, try demolishing the Straight Highway OnSlope pieces, then place the ramps, then replace the OnSlope pieces.
Bam. An underpass. Just do the same here for streets and one way roads. Bear in mind, that for other forms of transportation, such as rail, the gap doesn't have to be as big because there is no need for ramps. For avenues, you'll need to make the gap 5x4, not 4x4, because the ramps are longer.
And finally, you certainly wouldn't leave the area vacuous now would you?
Hope this helped!