You want to add a bridge. How do you do it? Here's how.
1. If you have created a layer for your bridges, select it and make it active first. Next, select the kind of bridge you want to use from Game Objects.
2. First you have a section of Road that you wish to link to your bridge. Then you place your bridge next to it. Follow this with another section of road.
3. Using the Move Tool, grab the blue handle on the road piece and drag to the bridge piece until it clicks. The Blue handle should turn Light Blue when done right. This is similar to joining road pieces. If you don't know how, read this.
4. Now under Terrain sculpting tools. Select Valley Steep and create a river beneath the bridge. Be sure to leave the edges on dry land, otherwise the whole bridge will sink into the water.
That's all there is to it. Will work on all kinds of bridges.