Road planners embrace the diverging diamond interchange | Hacker News Reader

Road planners embrace the diverging diamond interchange

npr.org

108 pointsfrob149 comments

They never show the whole interchange in the article, for some reason. At least in the usual design there are two crossings, since you need one to get you back on the right side of the road again. https://en.wikipedia.org/wiki/Diverging_diamond_interchange#...

When I first encountered one in the wild (2017 in Utah I believe), it sure threw me off, but I navigated it fine by following the signs.

Noticed that as well. At first I wondered for how long they would be driving left, but then I remembered that highway crossings are often mirrored. An overview of the whole intersection would clear things up a lot.

Yeah. Maybe they sent the photographer out to take pictures but didn't really explain the story to them?

I always smile a bit when this particular topic comes up on HN.

The first time I saw it here, I noticed there was a lot of confusion about the traffic flow - so I drove to the one nearest me, popped up my drone, and got aerial video of a complete traffic cycle.

It's still in the article years later. It's weird to me to think that random thing I did on a lark is likely to be one of the more durable parts of my personal legacy.

Thank you for recording the video. I figured the intersection would need a duplicate on the other end to reverse traffic flow back into place, and your recording helped me confirm that was actually the case. Seeing it in motion, it's actually better than I originally thought it would be.

What's confusing is that most of our attention gets focused on "whoa, weird, driving on the wrong side of the road" and not on what it does to the on- and off-ramps (blue in the linked wikipedia image).

And what is that, in your perspective?

Provide the ability of on- and off-ramp traffic to merge without a signal.

Yeah, that’s a huge benefit for sure

They're also often far less confusing than people expect them to be. The interchange where you cross over is generally done at as close to a 90 degree angle as practical, so it "feels" like two one-way roads crossing at an intersection rather than switching to drive on the wrong side of the road.

We have had one of these locally for a while now. It does seem to work well but since they're rare and I don't normally drive around that area, when I do I really need to pay attention to the road signs to know what's going on and where to go as it's definitely not intuitive.

Ours seems like it was put in instead of any other option due to the available real estate. The diverging diamond doesn't seem to take up any more space than the existing roads and intersection did. The existing bridge work for the highway overpass didn't have to change afaict.

There is one in Ohio which I encountered when visiting family. At first, it is confusing and weird, but I imagine you get used to it if you drive through it regularly.

Seemed logical as it was a bridge over an interstate, so to make it easier for entrance/exit ramps. Still wonder how many accidents happened there the first month it was open.

They started putting in traffic circles about 10 years ago where I live at intersections where they didn't have enough traffic to need a light. it was the same thing. The first time people encounter them, they go a bit slow, but you get used to them very quickly.

I think we have at least 2 of them now in the Columbus area.

I read about them, and then I drove through one without even realizing it until afterwards. The walls were high enough that I didn't see vehicles to my right, and the crossings were close enough to square that it just felt like a normal intersection of two one-way streets. Once I realized, I actually turned around to go through it again for the novelty and my ADHD brain flipped off and didn't notice it again. I had to turn around again (to continue in my original direction) and that's when I actually paid attention.

I think people find them weird when viewed from above, but when you're going through them there isn't that much to them. If you're in the SF Bay Area and want to try one out, there's one in Manteca, a little more than an hour away.

> The walls were high enough that I didn't see vehicles to my right

Might it be deliberate? I imagine a lot of drivers might be spooked to see "wrong" traffic, especially if it's headlights at night and they can't quite see the layout.

It's because of the headlights - they bias right and would blind oncoming traffic.

That may be the reason in other countries, but parent-poster talks about an intersection in California, and as far as I can tell US regulations require symmetric (unbiased) low-beams, and high-beams are usually symmetric everywhere.

> When installed on a motor vehicle, the headlamps (or parts thereof) that provide the lower beam must be of the same type and provide a symmetrical effective projected luminous lens area when illuminated.

[0] https://www.federalregister.gov/documents/2022/02/22/2022-02...

> Ours seems like it was put in instead of any other option due to the available real estate. The diverging diamond doesn't seem to take up any more space than the existing roads and intersection did. The existing bridge work for the highway overpass didn't have to change afaict.

Yeah, this looks like it could be built pretty space efficiently, especially if you already had an intersection on each side of the bridge. I'm sure there's ways to make it use more space, but it seems like it wouldn't need to be a lot more space anyway.

The article doesn't mention the numerous downsides to pedestrians for diverging diamond interchanges. Four crossing points means a lot more conflict points and a lot more chances for death.

Yeah, that's why it's illegal to walk on the side of highways.

We have an intersection like this just south of Santa Fe, where NM14 intersects I25.

NM14 is not a highway.

But you may want to walk across the highway which is almost always allowed.

Making walking illegal is how you create the car-first culture that we now know the downsides of.

Not everyone agrees that car-first design is a problem.

Not everyone who smokes cigarettes agrees that they are a problem

Amen!

Even without car first culture, you're going to have big roadways meeting at some points, and it doesn't seem particularly smart to have pedestrians mixed in with that.

It's completely legal to walk on the side of one of these two highways (US-15, a.k.a James Madison Highway) there's even an asphalt MUP for just such a purpose.

Wouldn't the crossings themselves be safer because the cars are coming from fewer directions? I'd rather have four crossings, each with only one direction to look in, than two where I have to scan a whole intersection.

I dunno, they seem better to me (from experience actually walking across [1], not just looking at pictures on the internet)

The crossing are controlled, distances are relatively short and the cars are only coming from one direction. It's a couple of crossings to get all the way through, but they're easy crossings.

[1] https://maps.app.goo.gl/6SwfTPrnXkUtTyym8?g_st=ic

But each conflict point is crossing only one direction of traffic, and there are no cars turning left or right into the crosswalks (which is huge cause of pedestrian impacts). So even though there are 4 crosswalks, maybe the aggregate danger is less?

It’s just extremely unpleasant as a pedestrian to be in the middle of 55MPH (88km/h) traffic, even if you are separated by concrete barriers, due to all the noise and particulates.

Each individual crossing is also wider if you are staying on the same side of the road. In a traditional diamond you’d be crossing one-lane on and off ramps, not half of one of the major roads each time.

Roadguyrob has a video detailing this issue. https://youtube.com/watch?v=xlnz04EZpMQ

I don't see why this is unique to a divergent diamond as it only applies to non-light controlled points of conflict of which there are at most two in both designs...

Four crossing points (two with traffic lights, in this case), whereas for a "normal" interchange, you'd get...also four crossing points.

diverging diamond: https://www.openstreetmap.org/directions?engine=fossgis_osrm...

cloverleaf: https://www.openstreetmap.org/directions?engine=fossgis_osrm...

There is one ten mins from me. It is definitely safer for pedestrians and bike riders. It wasn’t built for golf carts (this is Florida) which really chaps people here. But once they got the lights synchronized, the design works really well. There are little to no accidents. We also have two traffic circles that people call murder circles. The main one has high incident of traffic accidents albeit very very minor in terms of damage and almost no injury.

There’s actually fewer crossings involved, so safety for pedestrians should improve as well. But notably these interchanges are often in extremely pedestrian-hostile areas anyway, but it seems more likely that a newly constructed interchange will make at least some attempt to accommodate pedestrians, whereas interchanges built 30+ years ago often ignore pedestrian concerns entirely.

This specific interchange looks superior to both a traditional diamond and a cloverleaf for pedestrians. All crossing points are controlled by a light (unlike a cloverleaf), and you don't have to worry about cars taking an unprotected left-turn into you:

https://www.google.com/maps/@38.8171139,-77.6412514,3a,75y,3...

[edit]

The westbound on-ramp crossing appears to not be protected by a light[1], so would be similar to a cloverleaf onramp; still it's "not bad" as far as non-human-scale interchanges go.

1: https://www.google.com/maps/@38.8191922,-77.6383384,3a,75y,2...

Anywhere with an intersection this large seems like it is inherently hostile to pedestrians to begin with.

It seems OK as pictured in the article with a dedicated MUP on one side of the formation. There's probably no interchange design for this volume of traffic that will work all that well for pedestrians and cyclists, though.

Looks like it takes up a lot of space, unlike the most of the US and Canada, where I live the roads are all small, a large road is two lanes with room for parking on one side, so I guess it could be called 3 lanes.

To build these intersections you would need to tear down many buildings/houses. So I doubt I will ever see that where I live.

I remember years ago, I once visiting a place in the middle of the country, I could no believe how wide just regular roads were, some were 3 lanes on each side. To me, that is a highway, but it was in a residential area.

Plus those roads were very dangerous for walking and bicycling. It made me feel way insecure for driving.

I will take my narrow, winding, intersections were multiple streets meet at all different angles instead of these square wide grid roads any day :)

Roundabouts take up space too. But, they have no stoplights and are inherently load-balancing. I'd be interested to see why anyone thinks these are better.

The only double diamond I routinely encounter (and just realized that’s what it was) is around an interstate interchange. With the scale of the interstate traffic, roundabouts would be difficult to use.

Difficult, but not impossible… I’ve also used roundabouts over highway traffic, but not with the same throughput of traffic that interstates have.

Don’t take this the wrong way, but Americans seem to have an issue with roundabouts. I’ve seen several go in around LA and then promptly get turned into wacky stop signs. Btw I am American, though I’m living in New Zealand and I prefer roundabouts to stop signs.

Roundabouts have low capacity

Hence the turbo-roundabout.

Good luck putting one of those on an interstate.

The diverging diamond is also not _on_ the interstate, but over.

It's a matter of max throughput. The diverging diamonds is great when we are talking many lanes in all directions anyway, like in highway interchanges. The roundabout wins when max throughput is not the goal, and some of its side properties, like some mild traffic calming and better interactions with pedestrians come into effect.

They aren't really in competition with each other under most circumstances. It's like comparing bicycles and trains for transportation.

In Spain they managed to make roundabouts with traffic lights. It's not uncommon but having traffic lights on small roundabouts can make roundabouts very hard to navigate.

Lots of roundabouts have traffic lights here in the U.K.

See here for e.g. for a large one that intersects several major roads: https://maps.app.goo.gl/51sQeVa4ozSQgW9U6?g_st=ic

To be pedantic; roundabouts are a subset of traffic circles that don't have traffic lights or other flow control in them. Don't think I'd ever seen the term "traffic circle" until I learned that distinction though.

Roundabouts are not inherently load-balancing. If you drive on the right, so counter-clockwise around the roundabout, and there's a dominant south to north flow, you won't get a fair share when coming from the east.

East can only enter the roundabout when there's a gap, and if the south to north traffic is coming from an event, or a roro ferry, it's going to a lot of waiting. A traffic signal that's in event mode isn't going to give the east entrance much time either, but it will have some limit, most likely before the capacity of the ferry or the ballgame parking lot.

They're not better, they serve a different purpose: primarily for highway access from an arterial. You're not going to put a roundabout on your highways, and both roads can expect to have the highest level of demand on throughput.

for regular roads, sure. But this diamond style is used more at expressway interchanges. It’s especially useful in places where a full cloverleaf interchange would not fit.

These are for land rich countries like USA and Canada as you have pointed out and that too for expressways, interstates etc. These are not meant to be build in cities.

Well, it's a service interchange it's meant to connect a highway to a street road, so you'll need to build it in the city.

https://maps.app.goo.gl/DEFdtvFKev54LW5M7

Not necessarily.

All service interchanges take a lot of space. Europe has a lot of system and service interchanges too. DD doesn't take a lot more space than a regular cloverleaf, which is commonly found all over Europe. It is however, less flexible, as the loops in cloverleaf can be shaped to fit the area.

It is capable flowing quite a large amount of traffic, so it's not suitable for building anywhere.

I love the giant grid.

I’ve seen driveways in Calgary that are wider than national highways in Europe.

The grid has advantages:

- dead easy to navigate

- lots of room for proper sidewalks, not sure what you mean dangerous for walking. On narrow roads pedestrians end up walking in the road

- lots of room between opposite directions of traffic

- lots of room for my giant SUV

For the space that these diamond intersections use, wouldn't a roundabout also fit in that space and be a better option?

Like this:

https://i2-prod.examinerlive.co.uk/news/local-news/article24...

Having said that, as someone who has some historical familiarity with these sorts of intersections, but doesn't live in a country that has them, they are pretty scary too when encountering them only rarely.

For context, that's a very specific kind of roundabout that you find near the junction of a motorway (the M1) with a primary road (the A630). The roundabout is situated on the A-road and accessed from the motorway by a slip road. That way, motorway users who don't care about the junction can just keep cruising forwards and drive over the top of the roundabout. Nevertheless, drivers can approach from any direction and exit in any direction (including back the way they came). It doesn't look like this is the case in OP's article; I'm not sure how someone approaching from the left can continue in the same direction and drive off to the right.

Roundabouts generally look more like this in the UK (without any overpassing roads): https://www.google.com/maps/place/Howarth+Ln,+Rotherham/@53....

I picked that one because it matches the sort of intersection covered in TFA.

Also, the motorway passes under the roundabout, not over it.

Oh I see! Thanks for the clarification. I think my point still stands that the motorway is not interrupted in the case of the roundabout, whilst it is in the diamond intersection.

Nope, it's not :)

Look at TFA again. Just like the photo I linked to, the motorway (interstate in this case) passes under the 2 bridges.

The local version of this (NM14 & I25) has the interstate passing over on a single bridge, the double diamond is on the "ground level".

You're right; I hadn't scrolled down to the wide-view shot. I stand corrected.

I live in a Canadian province that has enthusiastically embraced roundabouts, so I’ve been able to get used to them for years. Their efficiency is very dependent on drivers being able to anticipate the actions of others.

These are things like watching the angle of the front wheels, the speed at which other cars are taking the intersection (faster = going straight through; slower = taking the turn), but also how much time it takes for a stopped car to start moving so you can judge whether you have time to enter the intersection.

This is a learned skill, and not everyone in the general population has the cognitive capacity to reliably pick up on these cues.

I’m not saying that I’m Nigel Mansell, but I genuinely think that the inability to pick up on these cues is why many are terrified and opposed to them.

Given that countries like the UK use roundabouts incredibly extensively, and they have never been shown to be a cause of increased accident rates, I suspect that your suspicion about this is probably wrong.

It depends on the country. Familiarity with roundabouts would be a necessity in the UK. In parts of the world where roundabouts are only recent, there are many drivers who are too old to “learn new tricks”.

I’ve got older family members that have self-excluded from driving to the provincial capital because the roundabouts are overwhelming.

I have older friends who self-exclude from driving at night. That doesn't make me conclude that night-time driving is inherently a problem (even though it is, based on statistics, more dangerous than driving around a roundabout).

It's asking for a lot of detailed attention in a lot of different places.

With right hand driving, I need to look left to consider if that traffic will be exiting or continuing. I need to look right to consider if that traffic has slowed and I need to slow. I also need to look hard left and hard right for pedestrians, who likely want to cross near the intersection (where it's most convenient) regardless of where traffic engineers are encouraging them to cross. Oh, and I better keep an eye on my mirrors, because I may need to take evasive action if the guy behind me isn't looking and I need to stop for traffic.

It's pretty hard to look at five different places at once.

I find it hard to believe that the diverging diamond interchange from the link is less scary than a roundabout. Roundabouts are all the same, from the smallest (https://www.google.com/maps/@40.2258039,-3.7709757,219m/data...) to the large (https://www.google.com/maps/@40.488386,-3.65609,436m/data=!3...), unless the designer felt the need to "improve". Arrive, yield, turn right, drive until your exit, leave.

Are you afraid of the inner lanes? Drive exclusively on the outer one, nobody cares. Do you need to go slower because you are in doubt? No problem, take even a second turn around if you need to get extra-sure.

I need to go through 16 roundabouts everyday, and after the 1000th (2 months) you gain familiarity enough.

What is this bullshit? https://www.google.com/maps/place/Madrid/@40.4965508,-3.6484... . That monstrosity, that you need to use carefully until you memorize it, is probably unique with all the mental toll that means. First time I used it, I made a bet on a lane and prayed that it went to where I wanted. A few kilometers south of that, you have this sane roundabout: https://www.google.com/maps/place/Madrid/@40.4297331,-3.5975..., doing the same 4-way joining in a more transparent and forgiving way.

Must the the Spanish influence :)

https://www.google.com/maps/@35.1052356,-106.6298892,1177m

>I find it hard to believe that the diverging diamond interchange from the link is less scary than a roundabout.

a (good) diverging diamond is really not scary. my first time driving through one i didn't even realize i was driving through one. if you came up to this intersection, would you be able to tell you were entering a diverging diamond? or would you think it was just a normal instersection of two one-way roads?

https://www.google.com/maps/@50.9043408,-114.0639993,3a,75y,...

Maybe it's that I never used one of them, so forgive me if I'm wrong: can I still go go the right it I missed the exit, and how? Can I do an U-turn and how? And finally, there are still traffic lights, and thus the potential to jump a red and t-bone someone.

I'm not really saying they are scary, but they are not less scary than a roundabout IMO.

Most roundabouts are the same, and then there are magic roundabouts.

https://www.youtube.com/watch?v=6OGvj7GZSIo

You don't have to predict other drivers on a diverging diamond.

What you can't see from the aerial view is that the road-level indicators, slope, and guard rails enforce the diverging part. They're quite simple.

Roundabouts feel adversarial. I don't like them.

They are not simple. You need traffic lights. You need road-level indicators, slopes and guard rails for them to work.

Meanwhile, I've seen roundabouts that are just a circle painted in the middle of the crossing, and that's it: everybody knows how to use it. See for example the main image of the linked article below.

USA people is so against roundabouts it hurts. According to Wikipedia, there are 160 diverging diamonds in the world, 150 of them in the USA. France built two in the 1970's, and it was so good that today they have... two. Meanwhile there are at least 300 roundabouts per million of habitants in Europe (France has 1,000 per million). Heck, even the US has at least 5,000 roundabouts. Some people call them "ugly", but even they say:

"Mélanie Cathelin, who runs a toy store on one of Abbeville’s main shopping streets, said she now finds traffic lights in other cities jarring, whether in neighboring Amiens or on a recent vacation in Florida."

"To cut down on the noise, traffic jams and fender benders occurring at one intersection, Mr. Dumont decided in 2010 to turn the troublesome spot into a roundabout. It solved the problems. Ten more traffic circles followed. In October, the city’s only remaining traffic light was sawed down."

So much for an adversarial structure...

src: https://www.nytimes.com/2019/12/25/world/europe/france-round...

the primary problem with roundabouts is their throughout. the only way to raise throughout is to increase the radius since roundabouts with more than 2 lanes don't really work. roundabouts are great for low speed roads with moderate congestion but as the speeds and number of cars increase, they make less sense

> roundabouts with more than 2 lanes don't really work.

Strange how there are so many of them in the UK then.

Where I am in the US, you run into them a lot. I find them easy to deal with, but I learned to drive on them.

But it is funny watching non-locals deal with them and I can see why they dislike them. They work great when only people use to them drive through them.

I think it depends on the specific traffic expectations, a roundabout interrupts the traffic flow of everything that feeds into it... the double diamond can prioritize a more uneven flow in one direction (so might be better in cases where lots of people travel in one direction, like rush hour)

Might also be worth mentioning that Americans seem to be really bad at using roundabouts because they're not common in much of the country.

France uses roundabouts a lot and even has multi-lane roundabouts, the most famous one being around the Arc de Triomphe[1].

[1] https://maps.app.goo.gl/fSzgiWabGtwFhh4b8?g_st=ac

Paris under Anne Hidalgo has been undoing a lot of them because they are so hectic and pedestrian unfriendly.

Place de la Republique: https://www.lejdd.fr/Societe/La-place-de-la-Republique-se-re...

Same in the UK, I find it kind of funny that some North American transport planners are starting to put roundabouts everywhere at the same time as they're being ripped out in London.

They're still pretty much the default junction outside of towns and cities because they do work well but large roundabouts are no fun at all to cycle round

The type of roundabout being installed in America is generally one or two lanes wide, not some of the multilane monsters prevalent in the UK though.

I've also heard that traffic is so chaotic around the Arc de Triomphe that insurers always assume 50-50 fault for accidents within it.

That is an old-style traffic circle, not a roundabout, which is specially designed to improve safety, by requiring cars coming into the circle to approach at an angle, as opposed to 90 degrees, and requiring cars outside the roundabout to yield to cars inside. As someone who's actually driven around the Arc de Triomphe, it's quite dangerous - it is always congested, and has unmarked lanes inside the circle (between 4-5 depending on the whims of the drivers at that particular time). Modern roundabouts are a reaction to dangerous intersections like the Arc de Triomphe.

No. The entire point of these is to funnel the traffic onto and off the highway with out crossing any oncoming traffic, and to reduce the total number of traffic crossings overall.

Here’s a YouTube video that does a much better job of highlighting these benefits. https://youtu.be/A0sM6xVAY-A?si=qrk2ZFFYPA2-6rAS

Roundabouts don't have any traffic crossings, and sometimes don't even need traffic lights either so no one has to stop/give way

Not necessarily.

Roundabouts lock up if the major flow of traffic has to go 270 degrees. (A left turn in an RHD country)

Diverging diamonds do their best at handling large left turn flows since that is the type of movement that has the most reduced number of conflicts. To get any lower you would need to start building flyover ramps.

If they used bridges instead of intersections where the roads cross over each other there would be no need for traffic lights and it would further reduce the chance of someone driving into oncoming traffic. The bridges could run on top of each other and further reduce the area needed for the interchange.

I'm sure bridges are more expensive than roads but it seems the obvious choice versus stopping four lanes of traffic at a light.

That only makes sense if both sets of traffic don’t interact. Most bridges over highways that have off-ramp traffic have traffic lights.

That is called a stack interchange, it's much larger and much more expensive to build and have no peds or bike access.

We have one of these and it’s awful. Everyone just waits a long time at each intersection.

It’s basically introducing a bunch of traffic lights where none would be needed with other designs.

The truth is that the standard cloverleaf is way superior but they’re cheaping out on land or construction costs and trying to pass it off a better design.

As they say in the article, it’s intended to be better than the pattern where people have to turn left across oncoming traffic to enter or exit the highway. That often also has stoplights.

A cloverleaf requires more width to fit. The highway right-of-way may not be wide enough, or the width may have been “eaten up” by widening the highway. The more lanes going through, the less room on the sides for exit ramps.

The cloverleaf doesn’t need any stoplights so in theory it should have the best continuous throughput. However it does require mixing lanes, which can lock up more easily under heavy traffic than separated sequential exit and entry lanes.

Cloverleafs also suffer a lot more when merging traffic becomes backed up, since there's no guarantee that the traffic merging on/off will be at even close to the same speed as the traffic that isn't merging.

It turns out that engineers select different interchange designs based on the expected traffic patterns, and there is no one-size-fits-all method.

As a sidebar, my dad was part of the (huge) team of engineers that helped implement the first few diverging diamond interchanges, and so its a fun reminder of him, even if I'm not on a road he had any hand in designing.

I'd be interested in the story of how these designs got developed, but there's not much media centered on traffic engineering.

there are a lot more conflict points in the cloverleaf, the diverging diamond reduces locations where a crash may happen. and there are a lot of busy interchanges that don't have the land for a full cloverleaf. it may feel very similar to the driver; having to wait at one light, and then the next one. but we have a few diverging diamonds in my area and they work way better handling the higher traffic volumes we're seeing with recent development push.

Original title: When driving on the wrong side of the road is the right way to speed up traffic

There are a few Diverging Diamond intersections around here and they seem to all work rather well from a vehicle perspective. I can't speak about the pedestrian side of that equation, as I've never been on foot.

Compares to a roundabout placed over a highway, it uses 1 bridge instead of two, but otherwise I don't know which flows best.

The photos in TFA show 2 bridges.

And looks like they also intentionally separated them by a fair distance to break the "wrong side of road" sense

It can also retrofit a single bridge diamond, if needed.

Yes, though they are close to each other (a bit too close to make a roundabout).

All the ones around where I live are on a single bridge; but they usually need to widen said bridge when they convert the intersection.

That's clever. They had just better be 100% on top of maintenance. I can see that interaction at crossover being non-fun with say faded markings.

How hard would it be to just build a pedestrian tunnel underneath it and avoid the problem altogether?

In case this isn't sarcasm:

1. Based on the article, and the parent comment here, it's not about the pedestrians (in this case).

2. Even if it were, it'd be much more difficult. Tunnels and bridges must be 10x - 100x more red tape and cost than surface-only structures.

There's an interstate underneath, which is the reason there's a complicated intersection at all. Building another bridge level might be safer for pedestrians, but also more expensive in construction and maintenance, plus now you need to worry about clearance. Perhaps building a footbridge over the interstate a bit down the road could help, but then it adds a detour...there's a tradeoff in each of the cases.

I think you would want to go over, not under. It seems easier to build a 30' bridge that can support foot traffic than to dig a 30' tunnel that needs to support semis driving over it.

One thought, if there isn't a high volume of foot traffic, is to just the fact that these are typically paired (one to swap lanes, another to get them back to the "right" side). Put a pedestrian crossing in the middle, and you can stop traffic from both directions at once. Bonus points that cars are supposed to stop pretty far away, so pedestrians could see someone running the light from much farther away rather than having to wait til they're 15 feet away to tell they aren't going to stop.

I just commented about this. We have one of these, and it's new, but the layers and layers of recent roadwork make it extremely confusing. I've seen people driving in the wrong lanes around the intersection. Thankfully there are plenty of businesses nearby (it's not a highway) so I'm sure those errors are corrected quickly, but it's dangerous.

That said, the intersection on average seems to be more efficient than it was before.

There's one of those north of Las Vegas which is probably the most poorly designed intersection I've ever seen. One direction has a stop sign for the crossover while the other way doesn't and if you aren't familiar with the concept it's kind of hard to figure out exactly what you're supposed to do.

All the other ones I've encountered are OK, lot better than roundabouts IMHO.

Is it the one at the crossing of Cheyenne Ave and Las Vegas Freeway ? The instant I dropped the little Maps guy in the middle of it and saw the street view, the "I'm in danger" meme started playing in my head.

Nice article!

This part got me thinking: "Chlewicki is [...] experimenting with some new "mutations" of the diverging diamond, as he puts it, combining them with roundabouts and other innovative traffic design."

Shoreline, WA (just north of Seattle, WA) is using exactly this to revise a highway overpass. They're putting in a roundabout ( coughrotarycough ) on either side of I-5 [1]

[1] https://engage.shorelinewa.gov/145corridor - scroll down to the first image in the article.

I was kinda wondering where they got the idea from :)

[2] Direct link to image: https://s3-us-west-1.amazonaws.com/ehq-production-us-califor...

> I was kinda wondering where they got the idea from :)

Parts of the country have been using roundabouts in highway interchanges for years. Western Colorado has a number of them.

This one is close to 20 years old: https://www.google.com/maps/place/39%C2%B037'37.8%22N+106%C2...

Another one that is basically five roundabouts in a row: https://www.google.com/maps/place/39%C2%B038'12.9%22N+106%C2...

I know of at least two others along that same stretch of highway.

Thank you for these links!

The phrase "5 roundabouts in a row" is nightmare fuel for sure, but the actual map makes it look a lot more reasonable :)

The German Wikipedia has a list of such DDIs in various countries (primarily France since they apparently invented them in the 1970s):

https://de.m.wikipedia.org/wiki/Anschlussstelle_(Autobahn)#D...

We have many of these where I live (at least 6) and contrary to what seems to be the prevailing opinion here everyone I know prefers them to the alternatives. Sure they can be confusing upon first encounter, but they are noticeably way faster than the traditional interchanges around here.

The history of this traffic pattern is quite fascinating. It's common to see claims that it was "invented" by somebody in the US, yet it had been in use since at least the '70s in France already [0].

I guess it's a case of "parallel discovery" - but some ~30 years later. For whatever reason, the more modern "reinvention" has taken off in a way the original did not.

[0] https://en.wikipedia.org/wiki/Diverging_diamond_interchange

We have one of these and it's extremely confusing. I don't drive the area much, but based on a few data points before and after, it might just have improved things.

That said, I've seen folks end up driving on the wrong side of the road there in one of my few drive-throughs. I think ours has been executed poorly with not nearly as drastic curves in the road, and really bad painted lines due to layers and layers of other roadwork on top of it.

I'd love to see the data. The photos in this article represent what seems to be a great example: obvious curves in the road, clear lines, plenty of space, etc.

There is one near here I use about once a week, and they've gone to lengths to address many of those challenges. Every single lane is flagged well in advance with what it leads to (Interstate North, Street North, Street South, Interstate South), and they also have high barriers so you basically can't see any of the traffic the other ways as you're moving through it.

Still was a few incidents at the start, but the results have been dramatic.

> I think ours has been executed poorly with not nearly as drastic curves in the road, and really bad painted lines due to layers and layers of other roadwork on top of it.

I find the line work on these more difficult to follow than usual roads. The dashed, monotone, criss-crossing lines are somewhat difficult for my eyes to follow. I feel confident I'm going to the right road, but much less confident that I'm in my own lane than normal.

I think these might be more visually obvious if the lines were different colors. I.e. northbound had red lines and southbound had blue lines (adjusted for color blindness and maybe overloaded meanings). I think my eyes could follow a set of differently colored dashed lines much better.

My town has one of these and it seems to work great. I don't love it as a cyclist, but the bike path just goes underneath it, so I absolutely never have to go through it.

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

https://divergingdiamond.com/history/

This will be fun in the dark and ideally rain. Suddenly you see lights on your right side. Happened to me in Las Vegas, NV. There is an exit where you are suddenly on the left side of oncoming traffic. First time this happened I almost had a panic attack.

The one I drive on frequently has high enough walls that you don't really see the other side.

Probably not an issue here, as there is as much space between the two directions as between a main road and a carriage road: https://www.google.com/maps/@38.8174153,-77.6405252,3a,75y,3...

Put some of the reflectors for lane lines that are white when you're going the proper way, and red when you're going the wrong way.

It looks like a roundabout (rotary) intersection would be a more optimal solution.

Roundabouts are great when there's one lane in either direction. They start to suck once there's more than one lane, since the inner lanes have to cross multiple lanes when entering & exiting the roundabout.

There is a diverging diamond interchange across I25 in Cheyenne, WY, and the next exit just a couple miles south is a double roundabout. It's almost like WYDOT is conducting intersection a/b testing.

God forbid a bicyclist needs to traverse this hell ...

Why not just use a roundabout?

This design seems odd to move traffic onto the wrong side of the road and make people come to a complete stop? Over-complex?

diverging diamonds handle much larger volumes than round a bouts. plus they're typically useful for interstate highway interchanges with large collector roads, so you can't really expect the high speed highway traffic to slow down and maneuver around a circle.

large traffic flows from one direction turning left will dominate a roundabout blocking access for everyone else. Especially at rush hour, you'll get people moving bumper to bumper leaving a city blocking traffic in all other directions. I ran into this in Indiana just north of Indianapolis. They have a few of the suburbs with large amounts of roundabouts. One of them was at a large industrial complex where everyone evidently left at 5. I swear 15 minutes of cars left there turning left and stopping the rest of the traffic.

Like all traffic infrastructure, there's variants that work for such situation.

Roundabouts can have bypass lanes, a protected (right) turn that allievates that issue.

In the UK you see these, and also designs where the through traffic bypasses the roundabout

That intersection should be an overhead pass.

my town has one. it seems to have helped the intersection be less horrible

> "We did see the crashes reduced somewhere in that 40 to 50% range, pretty much instantaneously," she said.

Yeah but as far as i know that happens every time something major changes in local traffic rules. People pay attention for a while, then get used to it and accident rate raises back up. Maybe not to the original levels?

There were those articles about a small town that removed all traffic lights...

Relevant xkcd

https://xkcd.com/2929/

What a colossal waste of land. We've emitted record amounts of CO2 this year alone (and it's only barely June), and yet we're still full speed ahead on cars. Anything to avoid smaller setbacks and bike lanes, I guess. THAT would be horrible.

Bikes have their time and place, but they're not the perfect cure all.

I've been injured and unable to use a bike. I can't fathom being elderly or pregnant and having to bike in the rain to pick up groceries.

Driving and cars are amazing sometimes too. Where I live I can drive to the mountains and the ocean. I can go hiking, camping, and bring my dogs with me. I can go off-road and set up my telescope in the rural night sky. I can haul equipment and furniture myself. I love the distance I can cover. It makes the world accessible.

How dare you desire to or actually exercise personal autonomy!

I'm sorry one injury invalidates an entire mode of transit. Better luck next planet, I guess.

> We've emitted record amounts of CO2 this year alone

Per captia CO2 generation in the US has been falling steadily for the past 30 years and is 25% lower now than it was in the 1990s. Single year maximum statistics are not useful for generating salient policy.

> and yet we're still full speed ahead on cars

They are very useful devices.

> Anything to avoid smaller setbacks and bike lanes, I guess

It's nice to have these things but it's pointless to compromise other important design elements just to tick these feel good boxes.

A lot of people died of heat stroke in India today.

Yeah I am gonna try this in cities skylines 2, when they fix it.

There are a few vids of people trying it in the first cities. The consensious was pretty 'meh'. Cities traffic is 'interesting' as its priorities are not the same in real life. Usually a roundabout and playing with the priority tool worked better. In the second one the alg is very different though so it may help some. Getting one setup and the lights timed correctly would defiantly be a challenge.

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