Why isn't my connections to the highway valid in Cities: Skylines?

I have just bought my first neighboring area and want to hook into it.
I first tried with a large roundabout and then with a three-way intersection, but none of them works. I keep getting the "broken" icon and the cars just disappears once they enter the area.
What have I done wrong?
Best Answer
To reverse the direction of a one-way road, use the road upgrade tool and Right Click the section of road that you want to reverse. This will work even if it gives you the "cannot upgrade roads of this type" error when you hover over it. Be sure to select the right kind of one-way road first!
Pictures about "Why isn't my connections to the highway valid in Cities: Skylines?"



How do you control intersections in Cities: Skylines?
The "Junctions" tab allows toggling traffic lights and stop signs. The final tab, "Adjust Roads", allows you to adjust the road, primarily for the road name. A line will appear to highlight the road, with dots on either end that can be dragged to adjust the road.Why Isn't Lightweight Bike Tech Cool Anymore? | GCN Tech Show Ep. 239
More answers regarding why isn't my connections to the highway valid in Cities: Skylines?
Answer 2
To add on to Jeroen's answer, remember that one-way roads always go from where you click to start them to where you click to end them, independent of any other roads. Looking at the second picture specifically, my guess is that you built both of them by starting either from the end in the intersection or on the edge of the map, which means both sides of the road are going the same way, creating the issue that one side is considered incomplete.
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Aleksejs Bergmanis, Aleksandar Pasaric, zhang kaiyv, Nick Kwan