Magic Roundabout: Circumnavigating the World’s Most Complex Intersection | Hacker News Reader

Magic Roundabout: Circumnavigating the World’s Most Complex Intersection

99percentinvisible.org

50 pointsmisnamed38 comments

As a systems guy I love roundabouts. They make it so much easier to get across intersections and they completely eliminate the variable cost of left hand (right hand if you're British) turns. That is because traversing them and exiting simply a linear function of traffic and not based on whether or not there is a special light for cross traffic turns which may or may not be timed appropriately. Thus it is always the "fastest" way to get from point A to point B by travelling along the highest flow routes.

That differs in the US where travelling a slower, but less controlled side street can be faster than using boulevards with left turn lights.

Roundabouts are awesome --- self-organising, self-managing, power-free traffic junctions! But they do have a failure mode, which is that they tend to starve low-traffic loads if the traffic is too asymmetric.

For example: consider a T-junction with a roundabout at the junction. (Very common in the UK.) Most of the traffic is travelling across the T in a straight line. But I'm entering the junction via the cross road.

The rules are that traffic from the driver side has priority over me, but that I have priority over traffic on the passenger side. So I stop at the line and wait for a gap.

With balanced traffic, I'll be guaranteed a gap because the oncoming traffic will have to stop to wait for someone else, which means I can move onto the roundabout ahead of the vehicle they're waiting for. But with unbalanced traffic, there won't be a gap, and I'll be stuck.

Typically what the traffic planners do at this point is add traffic lights, which is basically giving up, and defeats the whole point of having a roundabout in the first place.

...I used to live near this monstrosity, which had lights at every stop line. Blech. https://www.google.co.uk/maps/@51.436168,-0.8971696,200m/dat...

A big issue is motorists failing to stop for bicycles already on the roundabout. This is particularly problematic with small roundabouts where traffic does not always come directly from the right. A well designed roundabout should make people slow down.

Also, those big signalised roundabout are terrible for pedestrians. It is ridiculous to make someone wait to cross every single feeder road.

West Palm beach, fl had large round abouts installed about 10y ago and I thought that was great, except with little traffic at the time it seemed wasteful in time navigating and more importantly in space. I think the idea is great though, it's safe and usually gives everyone a fair shot to get moving without much delay.. I'd be interested in knowing how the diameter of the round about is decided on, specifically is it some algorithm based on predicted traffic congestion numbers.

If you're curious: https://goo.gl/maps/so6EDBad15K2

South Florida has quite a few round-a-abouts. There's a few in Hollywood and Lake Worth as well.

Roundabouts have a 'window of efficiency' where light-to-moderate irregular traffic works wonderfully well. But when an area grows and traffic increases, it can move out of this window. When you have regular moderate to-heavy traffic, it can be impossible to get onto a roundabout when facing say commuter traffic in the other direction.

This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.

Rural and city are opposites. A city is a large urbanized area.

Villages and small towns are installing roundabouts though.

The USA uses 'city' to refer to a district --- it's a unit of administration rather than strictly referring to an urban area. It's quite possible for a city to contain rural areas.

Not really. A city is an administrative area, yes, but depending on where you are in the country the role you are describing might be a "township." In addition, there are often qualifiers to be a "city" in most states. Most cities end before reaching rural areas and you get into a township or "just a" county; we generally refer to "rural" as not in a city in casual conversation. Urban is different and not necessarily the direct antonym of rural; there's a middle ground.

California seems to be an exception where my GPS will report a city quite far away as "where I am now" when I'm in the sticks, but that might be my GPS. California is also less particular about administrative division naming than, say, Michigan.

City, town, village, county, parish, and township all have particular meanings that vary across the country. This is not an area where one can say "the USA" and be correct, as with many things in our really-50-countries patchwork. One example is that Wisconsin dubs "towns" what Michigan calls "civil townships."

America is confusing, even before you get into the debate over New Urbanism.

California is quite peculiar. California has 482 municipalities. Whether they call themselves cities or towns is local preference, although fewer than two dozen call themselves towns. The largest municipality is Los Angeles with 3.7 million people. The smallest is Vernon with 112 people. I live in the unincorporated town of Cambria with a population of about 6000. We are about 20 miles from the nearest city, but we have no local government at all. The closest we have to a government is the community services district which runs the water district, the sewerage treatment plant and the fire department.

Cambria is in San Luisbo County which has an area of 3600 square miles and a population of over a quarter million but only 5 cities. California has a law that makes in almost impossible for a new city to incorporate. The new city must reimburse the county for any lost tax revenues caused by incorporation. If your GPS reports a city some distance away, it probably reporting the county seat of an unincorporated area.

I still think my comment reasonably captures the parlance, even if it doesn't account for the sea of legal definitions in the US.

Ah yes, the Magic Roundabout... indelibly written in my memory, as my first encounter with it was on a freezing January morning in thick fog. I had no idea what on earth was going on.

The one at Hatton Cross is pretty good as well (https://www.google.co.uk/maps/@51.4679146,-0.4230654,172m/da...) especially since a traveller may come across it, jet-lagged, and having just picked up a hire car...

There's one close to me as well - not as complicated to drive on as you would think looking at it from the top - https://www.google.co.uk/maps/place/Hemel+Hempstead,+Hertfor...

I've been on the one in Hemel on my bicycle. There's also one in Colchester: https://en.wikipedia.org/wiki/Magic_Roundabout_%28Colchester...

In addition to magic roundabouts, Britain also has gyratory systems, which are like normal roundabouts, except much larger and possibly with buildings in the centre, reachable on foot via underpasses (which are called subways in England, just to confuse Glaswegians and New Yorkers). The most famous one is at Hanger Lane in London, which has a tube station in the middle.

As an American, I honestly think I'd just put my car in park and sob uncontrollably if I came across this.

We are currently experimenting with other strange interchange designs in America. I present to you the diverging diamond:

https://en.wikipedia.org/wiki/Diverging_diamond_interchange

We have one near where I live that opened a few months ago. There appears to be an adjustment period for our drivers. I have spoken to people who drive the diverging diamond near us on a daily basis and they say at least once a week a motorist who is not used to switching from the right to the left hand side of the road goes the wrong way, risking a head-on collision with oncoming traffic.

Roundabouts can be intimidating a first but I love them now.

I had just landed in Edinburgh after a 12 hour flight with an 8 hour delay and picked up my rental car which has the steering wheel on the wrong site AND it was standard shift, which I hadn't driven in 10+ years. I managed to pull out of the lot without hitting anything but nearly broke into tears when I hit the roundabout that was 3 lanes deep and a figure 8 to get on the A8. If my wife hadn't been there I probably would have just gone home.

Looking back, the scariest part of this seems to be that I had to go through 2 other roundabouts to get to that figure 8 monster and I don't remember either of those.

You only see one bit at a time, and the actual circles are small. You yield to anyone already on the roundabout, unless they're exiting and won't cross your path. If it's empty, you yield to someone waiting to your right.

Big, normal roundabouts with multiple lanes are much scarier...

I'm from Australia, and have been living in Canada/USA for 10 years.

Yesterday I flew to the UK, and drove around with a friend today. Immediately one of the first things I noticed and commented on was how much better traffic flows. We hardly stopped, and just kept moving through roundabouts.

Is there any world where four way stops make sense?

My manager insisted we should rent a car and just drive everywhere on our work trip to the UK. As the cab entered one of these monstrosities, he turned to me and said "you know, maybe you were right and it's best we didn't try this ourselves".

Still, it seems very efficient...

Huh, I didn't know these were a) a 'thing' (as in, more than one across the UK, and b) officially called 'magic roundabouts'.

I've encountered one in Hemel Hempstead; it was a little scary (istr you can see traffic coming 'towards' you), but the article makes me want to go back and try it out again.

Edited to add: Between this and the horrific Welwyn Garden City one-way system, I guess Hertforshire must have (or have had) some of the maddest traffic planners in the UK...

I think Birmingham may have Hertfordshire beat, particularly with the infamous spaghetti junction...

http://www.telegraph.co.uk/motoring/news/6662562/Birminghams...

https://en.m.wikipedia.org/wiki/Gravelly_Hill_Interchange

From your second link: >>>The junction provides access to and from the A38 (Tyburn Road), A38(M) (Aston Expressway), the A5127 (Lichfield Road/Gravelly Hill), and several unclassified local roads. It covers 30 acres (12 ha), serves 18 routes and includes 4 km (2.5 mi) of slip roads, but only 1 km (0.6 mi) of the M6 itself. Across 5 different levels, it has 559 concrete columns, reaching up to 24.4 m (80 ft). The engineers had to elevate 21.7 km (13.5 mi) of motorway to accommodate two railway lines, three canals, and two rivers.

Truly an inspiration to those of us who grew up with Erector Sets and Tinker Toys.

Spaghetti Junction (Gravelly Hill) looks horrid when you see a photograph, but it's well signposted and really easy to drive through so long as you're comfortable merging on a motorway (and if you're not, you really shouldn't have merged onto the motorway in the first place!)

Some of the intersections within Birmingham itself are pretty awful, though, and it's not helped by the redevelopment the city centre is going through. Last time I passed through it took me three attempts to find the short stay car park at New Street station...

FTA:

> and has been adapted to create other ring junctions around Britain:

That's a GIF recording of Cities: Skylines, not a roundabout in Britain. Purple pollution in the upper-right gives it away.

I've traversed it and it's not that bad. Google Maps on an iPhone is horribly useless in roundabouts (I've mentioned it on HN a few times). Apple Maps is a little better. I found our 6 year old TomTom to be the best at giving directions. Having a bit of spacial awareness helps a great deal.

It would make sense that the Apple version would be at least fairly reasonable with roundabouts then; as they currently get at least a portion of their data from TomTom.

Apple seems to rely on a mixed bag of suppliers that are not Google for their map data. I'm not surprised it's TomTom and that may be why it's gotten so much better after release.

In the US, Carmel, Indiana has the most roundabouts. They just installed over 100 with many more on the way.

Ironic as the first automated traffic light was also installed in Carmel.

http://www.carmel.in.gov/index.aspx?page=123

Roundabouts are pretty bad for pedestrians. Imagine trying to cross the magic roundabout on feet from one block to the opposite one. And adding walkways might be expensive (especially if you want to accomodate wheelchair users).

You would just cross each street that feeds in/out, right? No cutting across the islands.

This is how it works. (British)

Yes they often have grass and plants/trees in the middle, which both discourages people from walking across and stops you seeing ahead when you are driving, which makes you drive at a more cautious speed.

Or they can be huge and have underpasses like the massive (now demolished) 'hole in the road' in Sheffield, or this beauty in East London (1m25s in) https://youtu.be/Smbfm4unjh8

Bristol's bearpit is like that, there are underpasses that take you under the road and into the "pit" in the middle, so you can cut straight across- much more convenient and less confusing than attempting to cross the feeder roads.

Bit scary at night, mind.

My home town's claim to fame! Swindon used to be called roundabout city by some because we have so many.

Roundabouts are good in an urban setting. they are a pain in the arse on a freeway/dual carriage ways.

however, fuck the magic roundabout. Its a hangover from the 60s/70s urban planning where people decided it'd be great to carve up large amounts of a town and shit out a huge road in its place. Surrounded by baron lifeless concrete.

Frankly that seems more like a whirlpool.

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