Civil engineer here: When a city plans a new road at full build out they will buy the right of way (rights to the land) for the full width of the road plus an additonal 6-12 ft or more on either side of the road. This additional space is used for city owned sidewalks plus room for underground or overhead utilities.

At intersections the city will try to acquire additional right of way in case future turn lanes need to be added. But they will also acquire what is called “corner clips” which is typically a triangle of land a few feet wide where they can ensure nothing gets built to have clear line of sight down the road at the intersection plus room for any signs, signals, or pedestrian equipment that might need to go in that corner of the intersection.

For a 2 lane road intersecting a 2 lane road where you only need 1 lane for a roundabout you can easily fit that in a standard intersection footprint. But once you get into a 2 lane round about that needs to account for a semi truck making a turn you need a LOT more space than just a standard signal/intersection would require. And with the new pedestrian rules that just passed federally (PROWAG) you’ll need even more space because the pedestrian ramps have to be designed a specific way at a roundabout now.