In order to help ease traffic congestion, you can manipulate the land in order to manually create an overpass. The basis of the technique relies on the automatic terraforming that results in placing and extending parks .

Without the technique, you have a four-way intersection that causes traffic to stop in all directions; but with the overpass, you have two roads with no breaks. Creating an overpass reduces the number zones in the immediate area since the overpass itself is off the ground and unable to be zoned, but this can help with traffic flow and can help emergency vehicles get to their destinations quickly without running into a traffic jam. Even better, if you use this technique exclusively with high density streetcar avenues, you can create an unbroken figure-8 pattern to better serve your commuters (and so the AI of the streetcars don't get confused).

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Video: How to Build an Overpass in SimCity

In order to pull it off, you'll need a city site that has a large altitude range. This works especially well near the water for that reason, as sea level is typically below the majority of your land. However, there are several plots in several regions where it won't work, most notably Cape Trinity and the Horizon Archipelago, due to the flatness of the city plots. For the following screenshots, we're at Pinewood Hills in Titan Gorge.

This technique can be pretty expensive, so you'll either want to do it early and plan around it, or make sure you have a solid profit and tens of thousands in the treasury before attempting it.

First, draw a road parallel to where you want the overpass. The road should be significantly uphill from the overpass, but still within the same screen when the camera is zoomed out halfway. The road is temporary, so stick with a low density dirt road to minimize the cost.

Second, lay down a long park in the middle of the road. We've found the best one is the Simple Walkway, which is under the “Plaza” category.

Third, click the park, then click the Edit button in the popup box. Add another Simple Walkway or two to extend it, then add two more turned the other direction. In other words, you want to form an elongated T-shape of the parks.

When we exit Edit mode, you'll se a severe cliff if you did it right. If a cliff didn't form, you probably need to choose another location or another city site.

Now, select the road or rail you want to use for the overpass. You can use any type, but this technique is significantly easier with rail than roads. If you do use roads, it's significantly easier with streets rather than avenues. We'll be using a medium density street for this example.

Start your road a short distance away from your cliff. Draw it parallel to your original road, and drag it up the the cliff. You'll see the road try to match the hill, and if the hill is sharp enough, the very edge of the road will change to show that it wants to form the beginning of a bridge. The road will seemingly be cut off, rather than rounded, if you did it right.

Note that this is the hardest part of the technique, and it's difficult to do consistently. If it doesn't seem to be working, cancel the road placement and try it from a different location, perhaps even the other side of the cliff. It seems to be easier the farther you are from the cliff, but this naturally adds expense since you're having to lay down more road. Observe where the road started in the screenshot below.

Now, grab your bulldozer and destroy the parks. Although most of the cliff disappears, the road section remains. You can also destroy your first road (the dirt road) while you're at it.

Finally, extend the road, and a bridge will form! It may be lopsided, but your Sims won't mind.

If the top the bridge was high enough, you'll be to run another road perpendicular underneath it, and you'll have yourself a brand new overpass!

Assuming you use a wide enough T-section, you can create an overpass with avenues. You can then later upgrade them to streetcars; because streetcars have issues with intersections, you can still use the overpass technique to create an unbroken figure-8 to let the streetcars deliver riders with maximum efficiency!