Cities: Skylines is incredibly robust in its city building simulation. Roads are an integral part of any city, and you’re going to wind up having to deal with a lot of traffic issues if you don’t plan ahead. One of the best parts of easing up traffic in certain areas (besides rotundas, of course) is upgrading your roads. This will lead to them potentially allowing more cars on them. Or, of course, you can use the “upgrade” to make the road smaller.
The tool allows you to make whatever road you’re highlighting into whatever different road you want it to be. Simply hit the Roads icon as though you’re going to build new ones, choose which road type you want the upgrade to be, and then click on the blue Exclamation point icon.
This will make your mouse into an upgrade sort of tool that you simply hover over any road, click, and presto it is now upgraded however you want it to be. Keep in mind the amount you’re spending while upgrading! It’s easy to go click happy with this tool all over the place. And also bear in mind that you could potentially demolish surrounding buildings (except unique ones) with the upgrade if it’s bigger than the road that was in place before it.
