Many people use highways for roundabouts, as there's no other 3-lane one-way road in vanilla, or use 80km/h highway ramps for small residential roundabouts as the only 1-lane road in vanilla.

But the roundabout going faster than the connecting roads is both unrealistic and less efficient -- having the speed limit lower makes it easier for traffic to merge onto the roundabout (less time accelerating means they can fit into a smaller gap, and the same distance of gap becomes a longer-in-time gap) and allows for more traffic to be on the circulating roadway at a time.

So it would be nice to have an option for the roundabout auto-setup to also set an appropriate speed limit based on the size of the roundabout.

How fast should it be? Well, the FHWA's roundabout technical summary suggests speeds of 25-50km/h, for different sized roundabouts, and we know that a=v²/r, so it seems plausible for the speed to be linear in the square root of the radius.

Playing with the numbers a bit, setting the speed limit to 4√r (with r in meters and speed limit in km/h) seems plausible

3 20
5 25
7 30
10 36
14 42
20 50

So here's a concrete suggestion for how this could work, as a probably-close-enough approximation:

• Estimate the radius of the roundabout as average distance of the nodes from the centre of mass of the nodes
• Toss it in the formula above to get a speed limit
• Just in case, clamp it above 13 km/h [aka 8mph] so at worst the round-to-5mph signs will show 10mph and the round-to-10km/h signs will show 10km/h. (But the roundabout would need r<1.3u for that to matter, so it shouldn't ever come up really.)
• Don't set any segment above its natural speed limit. (Just in case the roundabout is huge -- the normal two-lane road shouldn't be a 100km/h roundabout just because someone's making a ring city and it has a 80u radius. It would also protect intentionally-slow streets.)

Other estimates for the radius could be 1/2π of the length of the roundabout, or anything else that's easy for you get and about-right for anything that's about-circular. (If the speed limits are a bit off for a "wonky-about" I wouldn't really care.)

CitiesSkylinesMods/TMPE