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svl | 1 year ago

No discussion about roundabouts can be complete without a mention of the (mostly) Dutch "turbo roundabout", where the lane you take going into a multi-lane roundabout depends on where you want to exit, and you can't / are not allowed to switch lanes while on it:

https://wiki.openstreetmap.org/wiki/Tag:roundabout%3Dturbo

https://en.wikipedia.org/wiki/Roundabout#Turbo_roundabouts

https://www.arcadis.com/en-us/knowledge-hub/blog/united-stat...

discuss

order

asib|1 year ago

This is literally just how you're supposed to go round any roundabout. Right lane if you're going straight ahead or right (to any extent), left lane otherwise. Anything else _will_ cause crashes, because vehicles will necessarily have to cut across each other to exit the roundabout.

The "turbo roundabout" might make this explicit, but it's not different.

Etheryte|1 year ago

This is hugely oversimplified and doesn't really correspond to real life. Not all roundabouts are symmetric and not all have four entry-exit pairs. Many roundabouts have two lanes on some entries, but a single lane on others, similarly for exits. In scenarios like this you will inevitably have to switch lanes in some scenarios. It isn't really as big of a problem as you make it sound though, since roundabouts naturally have everyone go slow, crashes are very rare so long as the layout is clear.

zamadatix|1 year ago

A turbo roundabout is directionally biased while "any roundabout" doesn't have to be. A turbo roundabout also does not allow u-turns which becomes quite the limitations for road systems wanting to utilize medians for left turn control.

E.g. a standard 2-lane by 2-lane roundabout intersection may just as well look like this https://i.imgur.com/jqhMxW4.jpeg. Note the entrance markings allowing all lanes to go straight with 1 alternative turn direction per lane choice, the exit markings allowing dual lane exits in all directions, and internal markings allowing u-turns (the roads in this case have medians farther out). It has some of the downsides you mention but also some upsides in exchange for allowing slightly more lane flexibility. Regardless, you're definitely not supposed to follow the turbo's rules in that roundabout.

Now you could "no true Scotsman" it and say all the other roundabout types aren't roundabouts because they are supposed to be like turbo roundabouts to be so... but that still leaves needing the distinction in types, for which everyone calls one a turbo roundabout and other variations different types of roundabout.

tda|1 year ago

It's only the difference between CI enforcing code style vs manual PR reviews that have a checkbox for code style. They accomplish the same, but one is infinitely better.