[_zamorano_]

In Spain we use roundabouts heavily.

They're perfect for crossings with low to middle traffic. No useless waiting time, good visiblity and no need for traffic lights.

For heavy traffic, though, they're useless as the car flow of one entry/exit can take over the roundabout and block all other entries.

Also, it seems hard to understand for many drivers that regulations regarding changing lanes don't change in a roundabout. So more than 3 lanes are not advisable in my opinion.

[severino]

> and no need for traffic lights

Inside cites, those traffic lights in intersections were usually also semaphores for pedestrian crosswalks. So when you replace the intersection with a roundabout, you usually keep the traffic lights for the crosswalks alone. At the speed a car exits a roundabout, a non-regulated crosswalk could be quite dangerous.

[andrepd]

One of the downsides, of course, is the space. Car infrastructure is already an horrendous waste of space as it is.

[throw0101a]

> For heavy traffic, though, they're useless as the car flow of one entry/exit can take over the roundabout and block all other entries.

Not wrong, but traffic lights can be added to roundabouts as well ("signalized roundabouts")

* https://www.youtube.com/watch?v=XCrAPcUzOdw

So if an intersection is (e.g.) four-way, and one particular entrance monopolizes things than it can be given red at times and the others green (and than one-green/three-reds).

See (e.g.) "Performance and Safety of Roundabouts with Traffic Signals" by Tracz and Chodura:

> The paper presents designs, advantages and some drawbacks of various layouts of roundabouts with traffic signals, used in urban arterials on crossings with large traffic volumes. The authors analyze signal settings with alternative phasing used at intersections that can provide the intersection capacity of about 6500 veh/h in 2x2 lanes arterials or even 8000 veh/h in 2x3 lanes arterials. Accident statistics for roundabouts with traffic signals in Krakow are also given and discussed. In addition, presented are practical examples of changes of signalized roundabouts geometry; reconstruction into a form of turbo- roundabout and its effects regarding traffic safety and good performance.

* https://pdf.sciencedirectassets.com/277811/1-s2.0-S187704281...

And §8.1 Traffic Signals at Roundabouts:

* https://www.fhwa.dot.gov/publications/research/safety/00067/...

Also:

* https://www.access-board.gov/research/prow/literature-rounda...

[m463]

wow this stuff is like factorio, where you learn signals.

too bad you can't go underground.

[crote]

You can!

In The Netherlands there are several multi-level roundabouts, with one level for cars and another for bikes and pedestrians.

[m463]

wow.

Somehow I'm reminded of those old pneumantic mail tubes interwoven with the fabric of the building...