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Comment by mcv | Hacker News Reader

A better solution for high traffic flow is the turbo roundabout[0] which is getting increasingly popular in Netherland. Before you enter the roundabout, you select the lane for the direction you want to go. You only cross traffic when entering the roundabout for the first time. After that, you simply follow your lane and it delivery you where you want to go.

It looks complicated from above, but it's very simple once you're on it. It can handle a much higher traffic flow than other kinds of roundabouts. The only potential downside is that you need to know where you want to go before you enter the roundabout. There's no possibility to keep going around until you figure out where you want to go.

[0] https://nl.wikipedia.org/wiki/Turborotonde (In Dutch, but with images. "Rotonde" is Dutch for roundabout.)

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I don't understand how the turbo roundabout is any different than a typical two-lane roundabout. It looks like a regular two-lane roundabout from that picture in your link.

I think it just is a regular two-lane roundabout? We have one of these nearby me in Cleveland, it works pretty well.

https://www.google.com/maps/place/41%C2%B027'58.2%22N+81%C2%...

The inside lane changes into the outside lane. This is usually done by traffic analysis. Meaning: it doesn't always immediately change the inside lane into the outside one. It works quite well for roundabouts with heavy uneven traffic (close to an Ikea :-P).

This is a very elaborate version of the turbo roundabout, checking out streetview might help.

https://www.google.com/maps/place/Energieweg,+Nijmegen/@51.8...

I'm not seeing the difference between this and two lane roundabouts. We have a bunch of these in the southern suburbs of Minneapolis/St Paul, and they work well even on roads with moderately high traffic. My only issue with them is that I find them somewhat scary as a pedestrian during winter.

The first few times I encountered a two lane roundabout, I had some concerns about whether you had to cut across lanes to exit in some cases (especially when going straight across the roundabout). But after using it a bit, it's clear that it works pretty well, and that it's no worse than a 1 lane roundabout.

https://www.google.com/maps/@39.9561738,-86.1623079,150a,35y... is a "standard" two lane roundabout. Roundabouts like https://www.google.com/maps/place/Minneapolis,+MN/@44.883386... are more similar to a turbo roundabout but does not force traffic out after 3 exits or less, the inner lane could be followed indefinitely.

On that second link, I don't see any roundabout at all. It looks like a regular intersection to me.

Maybe the aerial imagery is out of date? It looks like a regular intersection on the satellite view, but Google Maps has it marked as a roundabout.

This is interesting, I seem to be getting newer aerial imagery than either of you https://i.imgur.com/tC6fsQm.jpg

If you switch to globe view, the image changes.

Yep that's it, globe view has the newer image and legacy mode has the older image.

Took me a second look to see it, look at the diagram in the Wiki link. The inside lane physically becomes the outside lane at two points, meaning you don't have to cross a lane to exit like you would sometimes need to in a "normal" two-lane roundabout.

The difference is that, on a turbo roundabout, when you're on it, you're not allowed to go straight when on the right lane. Right lane is only for exiting the roundabout, while left lane is for either exiting or going forward to the next exit. This contrasts with the regular roundabout, where if you try to exit from the left lane, you are risking collision with someone on the right lane who's not exiting.

> This contrasts with the regular roundabout, where if you try to exit from the left lane, you are risking collision with someone on the right lane who's not exiting.

That's how the roundabouts are built where I am from. You can be in either lane and exit from the middle of outer lane. The right lane must exit the inner lane can go around or exit. Although some people in the outer lanes try to turn left and crash into the people in the inner lanes.

I get the difference now, that's how they should be here there would be fewer accidents.

In a traditional 2 lane roundabout you aren't forced out of the roundabout. An example would be https://www.google.com/maps/@39.9561738,-86.1623079,150a,35y...

Are you just talking about being able to make a lane change within the roundabout? Because the outer lane cannot take the third exit in your example (at least according to the lane markings), they must exit before then.

In Ontario anyway, the general rule for roundabouts is that you aren't allowed to make lane changes, though they still use a dotted line so as to not make you think you're prevented from entering the inner lane.

Edit: I see it now. Took me a while to see how the inside lane stops at two spots. Interesting design. It ensures that you're only crossing traffic when entering, not when leaving, at the expense of it being less flexible (note how in two directions the right lane cannot be used to go straight through, so signing clearly in advance is much more important).

On these "turbo roundabouts" there are also kerbstones between the lanes, not just lines. You can cross them but it'll be obvious that was not the intended use.

I hate driving them. Somehow every one I drive on seems massively overengineered, and could have been done with a completely normal single lane roundabout. But it's usually not rush hour when I drive there.

Follow a lane around and you'll find that the inside lane becomes the outside and the outside lane exits.

On a regular two-lane roundabout some people don't use the inner lane when they want to go left, as they are afraid of not being able to merge to the outer lane. This design makes it easier to use the inner lane to go left.

We have one of these close to where I live. The idea is that the inside lane automatically becomes the outside lane after half way around, and when you are on the outside lane you are automatically forced to exit soon after.

Isn't a two-lane roundabout one that simply has two lanes going round? The thing about the turbo-roundabout is that there are actually no lanes going round. Each lane just takes you to your exit. There's no option to stay on the roundabout at that point: you're forced off. It's very restrictive in that sense, but that also means that traffic flows smoothly: other drivers also have no option to merge wrong or try anything weird.

> other drivers also have no option to merge wrong or try anything weird

If they're following the markings, yes. We have a 1.5-lane roundabout in my area[1] (two lanes on the north side of the circle, one lane on the south side) fed on the southwest by a two-lane road; the right lane is supposed to be a bypass for vehicles turning right, and does not actually enter the roundabout. Some vehicles still make the attempt, ignoring road markings, adjacent vehicles legally entering the roundabout from the left lane, and even the impending concrete barrier.

[1] https://www.google.com/maps/@42.0332849,-91.6029348,100m/dat...

In turbo roundabouts, the markings are usually delineated by raised curbs.

The one you linked should've physically separated the right turn lane from the rest of the roundabout if that was the intention. You don't need all that much curb to do it.

The lanes spiral outwards as they go round.

On a plain 2-lane roundabout when you're driving the outside lane you are not forced to leave it at the 2nd exit.

Sometimes there are even no signs/arrows on the road that require you to do it. It is expected of you, but if you happen to be one of the drivers that suddenly change their minds or you don't know how to drive on roundabouts you could continue driving around in the outside lane.

So a typical situation which makes roundabouts dangerous is when you're driving in the inside lane and you want to take the second exit (on a 4-exit roundabout that's going straight). If a driver next to you (in the outside lane) carries on driving until the third exit, they'll crash into you.

If it happens to you in Poland the situation even more stressful, because according to law, you are (partly) the responsible party, because you were the one who changed lanes!

IMHO, that's why turbine roundabouts are a great idea. Too bad that there aren't many of those or they are implemented incorrectly. See [0]. The outside lane ends in only 2 of 4 exits. Is it 50% safer though?

[0] https://www.google.com/maps/@52.2547072,20.9822988,188m/data...

> a typical situation which makes roundabouts dangerous is when you're driving in the inside lane and you want to take the second exit

If you want to take the second exit, you should be in the outside lane to start with. In the UK, for a standard two-lane roundabout, unless indicated otherwise by road markings and/or signs the outside lane is for taking the first or second exit, and the inside lane is for taking the third and subsequent exits. You move from inside to outside as you pass the second exit.

There are also rules for indicating correctly that help smooth the traffic flow, but unfortunately many drivers seem oblivious to (or just ignore) them. Also, on smaller two-lane roundabouts more than a few drivers will cut across the inside lane (taking the shortest path) with no consideration for vehicles beside them.

In a regular two-lane roundabout, this difference is arbitrary. Your second exit is the third exit for cars entering before you and the first exit for cars entering later. If someone is entering from your exit, you may still be in trouble when you try to move to the outside lane after the second exit. Hence the turbo roundabout.

It's a two lane roundabout with an integrated instruction manual that makes it much easier to use.

> I don't understand how the turbo roundabout is any different than a typical two-lane roundabout.

See:

> These designs require motorists to choose their direction before entering the roundabout, thereby eliminating many conflicting paths and choices on the roundabout itself ...

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

Basically you have to pre-choose your lane before entering as you are not allowed to change lanes once you're inside. With regular RAs you can change lanes if you wish.

This style exists in the UK too: near where I group is https://maps.app.goo.gl/hjhi3gQijUGnuXha7 and a bunch of others like it: five roads in and out, with lanes that naturally flare out (though with the option to cut over right to continue further round). Quite intimidating when you're learning to drive.

That said, my recollection is that the default in the UK is that lanes flare out from the center, so that once you get into the lane you don't have to switch to another to exit.

Having lights there defeats a lot of the roundabout. The turboroundabout definitely does not have traffic lights. Look at this[0] implementation, the one you linked qualifies as a "turboplein".

[0] https://www.google.com/maps/place/Energieweg,+Nijmegen/@51.8...

That satellite view describes it perfectly. Especially how it is differentiated from a standard 2 lane roundabout.

In the UK there are road markings that indicate a similar flow, but this turboroundabout makes it physical.

* Raised curbs force the outside lane to exit.

* After each exit, there are only 2 lanes left.

* After each entry, there are 3 lanes, a new one starts from scratch on the inside.

> Having lights there defeats a lot of the roundabout.

You're not wrong, but roundabouts degraded the busier they get:

> As at other forms of unsignalized intersection, when traffic flows on an approach exceed approximately 85 percent of capacity, delays and queue lengths vary significantly about their mean values (with standard deviations of similar magnitude as the means). For this reason, the analysis procedures in some countries (Australia, Germany, and the United Kingdom), and this guide, recommend that roundabouts be designed to operate at no more than 85 percent of their estimated capacity.

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

1-, 2-, and 3-lane RAs each have different capacities, and it may not be easy to expand one size to the next larger one depending on what else is in the area, so signals can allow for the general architecture of RAs to be extended without major changes besides putting up some poles and wires.

That's neat. How does it handle more than 4 junctions? The Magic Roundabout was devised as a means to efficiently handle the junction of 5 or more roads.

I tried searching for that, but couldn't find a very good example. I don't see why that wouldn't work, though: just split into more lanes before you enter the roundabout.

Although it appears that Dutch traffic design tries to avoid intersections with more than 4 directions. Better merge a couple of them before the intersection.

https://www.google.co.uk/maps/@51.7309167,-0.3021986,152m/da...

I grew up by this roundabout; the south exit joins to a village. Note it has 5 exits, and is slightly tweaked: there are two lanes that go off most of the exits, except the village exit has one lane and actually going off is optional. Every major entrance to the roundabout has four lanes so you get in the lane you want for the exit you want. The minor entrances have fewer lanes so you just sort of race into the lane you need.

Oh, and it has traffic lights as well.

> There's no possibility to keep going around until you figure out where you want to go.

I assume that traffic on the loops always has the right of way over traffic trying to enter the loops?

In that case, why can't you keep going around? At the place where the northbound inner loop becomes the southbound outer loop, it crosses the start of the southbound inner loop. The cars coming from the south trying to enter the southbound inner loop have to yield to the cars on the northbound loop, so it should be perfect safe for those cars to turn onto the southbound inner loop.

Note that allowing this maneuver is necessary if you want to allow eastbound or westbound travelers to make U-turns. If it is not allowed, then only northbound or southbound travelers can make a U-turn on this.

You're not allowed to change lane once you've entered this type of roundabout. In practice the lanes are separated by a curb. Of course you still can change lane at the points where traffic joins the roundabout, but it's a sharp 90 degree turn and you'll discover real soon that it's a mistake to try.

There's one of these on the junction of the A3 at New Malden. https://www.google.co.uk/maps/place/A3,+New+Malden,+UK/@51.3...

Here's one in Helsinki, Finland: https://goo.gl/maps/NoWE44KLKukEeq4d9

I also disagree that it is simple while you're on it although I have to admit that I'm not a very confident driver at the best of times so maybe that's just me.

There's one in Espoo, Finland (Suomenoja). It's apparently the "roundabout from hell", although I don't personally get it. https://www.hs.fi/kaupunki/espoo/art-2000005975273.html

I would consider myself good with following direction signs and being in traffic in general. However, I find that roundabout very confusing.

What makes it confusing is, I think, the overhead signs in the roundabout. It would be easier without them. Because the arrows pointing up also points towards the incorrect exit at the same time due to the round nature of the road :)

The only "downside" with these is in case of an emergency vehicle entering the roundabout.

In a "traditional" roundabout, all motorists could perform an extra round and thus clear the roundabout fast and easy. This doesn't work that great in these kind of roundabouts, I guess.

Works just fine in the real examples of these roundabouts, this[0] is a better example which is widely implemented in the Netherlands. The bollard thingies are really small, but don't encourage driving over them. Emergency vehicles can go over just fine.

[0] https://www.google.com/maps/place/Energieweg,+Nijmegen/@51.8...

This[0] is a very elaborate version of the turbo roundabout, checking it out in streetview might help. But basically the roundabout functions more like a standard junction with room to choose a direction before turning (I don't think there is an English translation for "voorsorteren", or sort before...).

[0] https://www.google.com/maps/place/Energieweg,+Nijmegen/@51.8...

"presorted" "prefiltered" would be adequate translations, but sort is a bit of an odd word to use for traffic in English.

> A better solution for high traffic flow is the turbo roundabout

Q: Is such a construction grade-separated and/or are the separate flows in controlled by traffic lights? The diagram isn't that clear...

https://en.m.wikipedia.org/wiki/Roundabout#Turbo_roundabouts English link

I try adding it to the language list for turbo roundabout, but Wikipedia doesn't allow that. Different languages organising their content in a different way hurts the ability to make these kind of cross-language links.

The problem with The Netherlands is that in some cities (I'm looking at you, Nijmegen) they've built massive roundabout which aren't actually roundabouts.

It looks like a roundabout, it feels like a roundabout but the priority rules are reversed so if you treat it like a roundabout it only ends badly..

> A better solution for high traffic flow is the turbo roundabout[0]

See also sub-section in the main EN page:

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

there's one channeled roundabout here, it might be great for residents but if you happen to be channeled in the wrong lane it's a 3km detour before you can get back where you wanted to go.

Another potential problem is that it's not always possible to make a U-turn at a roundabout like this. Sometimes you can, but not always.

I don't like it. From the bottom, if you want to go straight you enter on the right lane. From the right, if you want to go straight you enter on the left lane. This lack of symmetry means you need to be very familiar with the path in advance, and not get confused about which orientation you're in. For the signage to be far enough from the roundabout to get your bearings in time, it will be too far for most people to even be looking for it. This design will add to the late merge asshole problem.

Late merges are optimal, and the fact that so many people disagree with this practice (or take it as a personal affront) is one of several gross failures of US driver education programs.

However, yes, complicated roundabouts that require perfect knowledge of what lane you need in order to take an exit you can't see in advance do suck. If you find yourself installing a roundabout with more than one lane, you probably should be installing a traffic light instead.

> late merge asshole problem.

Early mergers are the assholes. You're SUPPOSED to merge late. If you see a sign saying "Construction ahead 3 miles, left lane closed" or something, and you immediately start trying to merge, you're the one doing it wrong.

Early merging creates unpredictability in traffic flow. Zipper merges are safer and lead to shorter merge queues. https://en.wikipedia.org/wiki/Merge_(traffic)#Zipper

There's a difference between late merges and late lane changes, which is what OP meant. When all vehicles are merging into one lane, sure, the zipper merge is superior. But when you realise you're in the wrong lane right before an intersection and try to force your way over, that's the problem.

> There's a difference between late merges and late lane changes

Starting to go off topic, but I'm starting to feel I'm the only person the never refers to a lane change as a merge when it isn't a part of a lane ending. IMO, a "merge" should only refer to lanes ending. Otherwise, fighting about "late merging" becomes ambiguous.

Correct. I mis-spoke. I guess I picked up on 'late merge' term from someone else and that's how I call it. But indeed I meant a last-minute lane change.

Folks commenting on this, thanks for the correction but also you should have picked up from the context (of a tornado roundabout) that I was referring to a lane change.

We don't have many, and the few we do are pretty new, but several of the roundabouts near me in the US work like this.

Might be wrong, but I remember this existing (albeit with traffic lights) in the 90s near Amsterdam Amstel Station.

I expect most are better given that "In 2009 it was voted the fourth scariest junction in Britain."

Looks like it works with 4 roads, how about 5 like this one?

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