The easiest, quickest and most straight forward way of making a road crossing for a culvert or canal on flat terrain is to simply place them like this:

However we can do a little better…

We need the Mayor Mode (MM) Menu, to find the terrain tools, but more importantly it gives us another useful terrain editing tool…   the compass.    How does the compass help us?   When we use the terrain editing tools, it applies its effect to the ‘North West’ corner.

Turn on the grid by pressing ‘G’.
Select the MM raise terrain tool.  Make the tool as small as possible by pressing Shift + 1, and move the tool circle, to the NW corner of the target grid square.

Once and only once left click the mouse.   This will raise the terrain slightly like this…

Now move the raise terrain tool to the right, to the next NW corner of the neighbouring grid square.

“Verify range to target, one ping only…”  

Oops…  I am getting distracted…
…click…  I meant one click only…

Rinse and repeat this procedure around the target grid square.   This will give us a small hump on our terrain.

If we wish, we can give the hump a completely level top by placing one road tile on its crest.

Now we drag our street or road over the hump, we just made.

Next we place a culvert either side of the road,  leaving a gap of one grid square.

Now we can add in some terrain detail…   go to the MM flora menu and select your flora detailing brush of choice…

Detail the terrain, as you like.

The raised hump can easily be made longer, for larger items like the canal.

God terraforming in mayor mode mod – download here