I'm running into troubles planning a city ahead, because you don't get highways until 2600 population, and by that time, I kind of just expand into the area where I wanted my highway to be. At first, I thought I'd run it underneath the city, but after a couple failed planning attempts, I learned that I need to dezone the way. Can I lay down a one-way road ahead of time, and then upgrade it to a highway later?
Cities Skylines – How to Upgrade Two-Lane One-Way Roads to Three-Lane One-Way Highways
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Best Answer
Yes, as HorusKol says, you can simply upgrade them by using the "!" button (in amongst "straight", "curved" and "freeform").
However, when laying out the city, take special care to leave space around the roads you plan to upgrade, and don't attempt any close/tight intersections. The 3-lane highway is quite a bit wider than a 2-lane road, and this will have 1 of 3 effects when you upgrade: