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The Diverging Diamond
October 29, 2010 6:21 AM   Subscribe

"The Diverging Diamond reduced traffic accidents by a remarkable 60 percent. The only complaint is the strangeness of being shifted to the wrong side of the road."
posted by jefficator (77 comments total) 26 users marked this as a favorite

 
Kurumi's Field Guide to Interchanges
posted by exogenous at 6:25 AM on October 29, 2010


It's a French innovation, but it's starting to catch on in the states.

And that little tidbit of information will make sure that it doesn't.
posted by three blind mice at 6:28 AM on October 29, 2010 [12 favorites]


Ouch.
posted by mccarty.tim at 6:29 AM on October 29, 2010


Similarly the SPUI (which only has one signal on the surface road, for what it's worth) often faces resistance because when making a left turn, the driver is on the "wrong" side of opposing traffic.
posted by wierdo at 6:31 AM on October 29, 2010 [1 favorite]


Ok it took me a minute to get my head around it but I would describe it so:

Nobody has to cross traffic to make a left turn because ALL traffic crosses over - giving you the optional chance to make a left - and then crosses back.
posted by vacapinta at 6:31 AM on October 29, 2010 [6 favorites]


Pretty much any change to road infrastructure will initially result in fewer traffic accidents: road users are more careful when they're in unfamiliar circumstances. Unfortunately that makes it very hard to gauge the true benefit of an unusual configuration like this one, since it will remain permanently 'unusual' by comparison with the rest of the road infrastructure.

It sounds like the other benefits are worth having regardless of whether the design is really that much safer though.
posted by pharm at 6:33 AM on October 29, 2010 [13 favorites]


Great place to have a sobriety roadblock. You'll know pretty quickly who can handle it or not.
posted by AugustWest at 6:36 AM on October 29, 2010 [1 favorite]


in looking at it the single biggest advantage of this is the fact that it can cycle more cars per hour. that will mean a lot in these parts.
posted by lester's sock puppet at 6:38 AM on October 29, 2010 [1 favorite]


And that little tidbit of information will make sure that it doesn't.

Goddamn right. Next thing you know the ^#& liberals will want government to be responsible for public roads.
posted by nomadicink at 6:38 AM on October 29, 2010 [5 favorites]


single biggest advantage of this is the fact that it can cycle more cars per hour

I'm thinking this isn't so much an advantage in this day and age.
posted by The 10th Regiment of Foot at 6:41 AM on October 29, 2010 [1 favorite]


I've always believed the answer was in unmarked intersections with brush along the side of the road on all four approaches (with, of course, 80 mph speed limits for all).

Darwin is your friend in the long run.
posted by HuronBob at 6:43 AM on October 29, 2010


Pretty much any change to road infrastructure will initially result in fewer traffic accidents:

I recently read that traffic circles or rotaries, while perceived as more dangerous than controlled lighted intersections, are actually much less dangerous, because people perceive them as dangerous and thus exert more caution. Even over time when they are more familiar. Anything which causes a driver to focus more attention on his or her surroundings increases traffic safety and reduces accident rates, according to this thing I was reading, I can't remember where. Similarly, "traffic calming" tricks like painting sections of the roadway a different color also rely on this tendency.
posted by Miko at 6:45 AM on October 29, 2010 [2 favorites]


It's funny, you would have thought that the nation that is obsessed with cars and working long hours with little time off would be an innovator in traffic control, but instead it's the country with a 35 hour work week and mandatory 5-8 weeks of vacation time leave plus 10 national holidays. Or perhaps it makes sense if you see it as them wanting to spend less time in work mode.
posted by Rhomboid at 6:47 AM on October 29, 2010 [2 favorites]


Any new, unfamiliar system that makes drivers pay more attention to other vehicles also makes them less likely to pay attention to pedestrians. Traffic circles are especially bad for this, as drivers are either watching for cars in the circle, or driving with "right of way" impunity.
posted by rocket88 at 6:51 AM on October 29, 2010


This is scary, but has nothing on the magic roundabout.
posted by schmod at 6:52 AM on October 29, 2010 [6 favorites]


The 10th Regiment of Foot writes "I'm thinking this isn't so much an advantage in this day and age."

Of course it's an advantage; the less time cars sit idling at lights the less pollution they give off.
posted by Mitheral at 6:53 AM on October 29, 2010


I am all for the reduction in traffic-lighted intersections and 4-way stops. The places where roundabouts have been built around here have-in my own, biased study--significantly reduced traffic wait time and sure, ok, fewer accidents.

I still can't believe the number of nimrods who feel they have to come to a complete stop even when there is NO other traffic at all on the roundabout. And don't get me started on the one local interchange that replaced a whole ton of lights & stops with a series of 4 roundies, 2 on each side of the freeway and local access roads. Add to the problems with this one that the majority of traffic is from non-urban drivers, and while I am sure that the traffic moves more smoothly, the percentage of blue haired farmers' wives who can barely see over the steering wheel of the Buick just add to my sense of thrill & excitement. GAH!

Sorry, that is all.
posted by beelzbubba at 6:53 AM on October 29, 2010 [1 favorite]


I'll grant that it's clever, but gosh, the land footprint for that local road is almost bigger than that of the freeway that it passes over (and connects to via what appear to be the longest onramps possible).

In terms of encouraging walkable/bikable infrastructure, this fails pretty spectacularly. There are no sidewalks, and I suspect that it's considerably more dangerous to bike through than a normal intersection.
posted by schmod at 7:08 AM on October 29, 2010


A traffic engineer friend once referred to the "balance of terror" inspired by traffic diversions as a factor in reducing accidents.

Her case in point was the city put in a cross walk after long agitation for it and there immediately was the first fatal pedestrian accident at that location because the crosswalk lowered the "balance of terror."
posted by warbaby at 7:10 AM on October 29, 2010 [3 favorites]


This post gives me hope that I'm not the only person here who regularly goes on a browsing expedition through Wikipedia and the like, just looking at intersection designs and other traffic geekery.

I also have a copy of some traffic engineering tools1 that are fun to play with by designing intersections, traffic volumes, signal timings, and so on. There are plenty of real-world examples where you're sitting at a light or something, and you wonder "why don't they just make the left turn arrow stay on a few seconds longer and there wouldn't be this huge back up?"

And then you simulate the same intersection and you can try it for yourself, and you find out that if you make the left turn arrow 5 seconds longer, it causes a huge backup in every other direction, for example.

Or as another example, there are plenty of rural roads around here which are one lane in each direction, but a second lane opens up before every traffic light and disappears again after it. I used to get mad at the people who'd move into that temporary lane when they know damn well they're just going to have to merge back in again. Why don't they just wait their turn in line instead of trying to pass everybody at the light?

But after you mess around with these intersection simulations, you realize that the challenge on a busy road is trying to get enough cars through the intersection on the green light to prevent congestion. Since the light's green less than half the time, every lane can deliver more cars to the intersection than can actually get through it on the green light. So those people sneaking through in the extra lane are actually increasing the capacity of the intersection and preventing huge backups from occurring when the road is busy. Backups that you can see occurring if you remove the extra lane from the simulation.

1: A legally downloaded demo version of some of the TrafficWare tools, direct from their web site. The free demo versions have some restrictions, but in version 5 you could still design your own layouts with the demo. Versions 6 and up unfortunately seem to limit you to the example scenarios supplied with the demo, but might still be fun anyway.
posted by FishBike at 7:12 AM on October 29, 2010 [15 favorites]


Heh. And St. Louis/Missouri makes it onto the blue again, albeit in a roundabout (heh) way; Missouri actually has three of the country's four first diverging diamond interchanges (the other is in Utah). And there are plans to build several more here within the next few years.

The latest one, at Dorsett Road and I-270, just opened here a little over a week ago. Here's a MoDOT video of a driver navigating the new interchange; here's a rendering video of how it works.
posted by limeonaire at 7:12 AM on October 29, 2010 [2 favorites]


"...and connects to via what appear to be the longest onramps possible..."

And people in Utah still won't make it up to freeway speed by the time they're merging.

Gah, don't get me started.
posted by mr_crash_davis mark II: Jazz Odyssey at 7:18 AM on October 29, 2010


FishBike wrote: "This post gives me hope that I'm not the only person here who regularly goes on a browsing expedition through Wikipedia and the like, just looking at intersection designs and other traffic geekery."

m.t.r represent! ;)
posted by wierdo at 7:21 AM on October 29, 2010


Mitheral: "The 10th Regiment of Foot writes "I'm thinking this isn't so much an advantage in this day and age."

Of course it's an advantage; the less time cars sit idling at lights the less pollution they give off.
"

Yea but making the highways faster will just encourage sprawl in that general direction and ten years later the new road is just as contested as the previous one.
posted by octothorpe at 7:27 AM on October 29, 2010 [1 favorite]


That looks like it would be fine in the day, but really awful to encounter for the first time at night in the rain.

Like everyone keeps saying, it wasn't designed for bicycle or pedestrian access, but then I've never seen a freeway on/offramp set-up that was, either.
posted by Forktine at 7:31 AM on October 29, 2010 [1 favorite]


This kind of makes my brain feels funny, like that proposed bridge that links Hong Kong and China. The two countries drive on the opposite side of the road, and this figure-eight design swaps them over.
posted by ukdanae at 7:32 AM on October 29, 2010 [1 favorite]


This will improve things quite a bit for the last ten months of the existence of fossil fuels.
posted by shakespeherian at 7:34 AM on October 29, 2010 [2 favorites]


That figure-eight bridge is cool, though it would be even neater if it doubled or tripled the figure-eight, such that you kept swapping sides three or four times in a long DNA-style coil. Probably hell for traffic safety, but spiffy to look at.
posted by Forktine at 7:40 AM on October 29, 2010


Neato!
posted by feistycakes at 7:42 AM on October 29, 2010


The one shown in the link is just a few miles from my house. My friends and I have noticed that local acceptance of it pretty much falls along party affiliation.
posted by sourwookie at 7:46 AM on October 29, 2010 [2 favorites]


Whether or not it's better for pedestrians is unclear.

No, it's pretty clear: Not good for pedestrians. You can pretty much count on any change that's geared at streamlining traffic flow being bad for pedestrians.

But no one will notice in these instances, because they're in such an inherently pedestrian-hostile space that it doesn't matter.

And I agree that the proof of the accident-reduction pudding is in the 10-year-old pudding: When this thing's been up there for 10 years, let's look at the accident rates again.

Personally, my initial reaction is that it's insanely user-hostile, which I suppose could prove a point about usability: Sometimes bad usability can be good, if it forces you to pay attention to something you otherwise wouldn't. IOW, a highly usable product (like a traffic interchange with explicit traffic controls in the form of signal lights) may encourage you to ignore some poor design features (e.g., autonomous agents you can't control who can do lethal damage to you). This poorly-usable design forces you to pay attention to those actors (at least, at first), potentially reducing the instances of injury from that source.

Where you will likely have a problem with this strategy is when you introduce people who are not familiar with the pattern into the system. For very high-throughput commuter areas, that might be quite rare (so rare that the commuters establish a traffic pattern that's obvious and visitors can just go with the flow); but for areas or times where there's no set pattern, it could be more dangerous.

And it's got no business being used in an area where there's expected to be foot traffic. This is a pedestrian-hostile design if I've ever seen one.
posted by lodurr at 7:57 AM on October 29, 2010


Two roundabouts would do the trick too.

Vaguely related: Traffic lights switched off in small UK town- Empathy ensues.
posted by marvin at 8:04 AM on October 29, 2010 [1 favorite]


Miko: "I recently read that traffic circles or rotaries, while perceived as more dangerous than controlled lighted intersections, are actually much less dangerous, because people perceive them as dangerous and thus exert more caution."

I assume that statement referred to some place like the U. S., where circles are not commonplace?

Last spring I (from the U. S.) spent a bit over week driving around Ireland. How I got us out of the airport unscathed the first day is anyone's guess, but after the week was over, I was a thorough convert to roundabouts. They make perfect sense in a place with relatively low speeds, lots of little towns and lots of manual transmissions, because you rarely have to completely stop.

And I even began to perceive them as less dangerous. The beauty of a roundabout, once you can re-train your Yankee brain, is that the Trouble is only coming from one direction.
posted by Rat Spatula at 8:05 AM on October 29, 2010


I just came here to give my very insightful comment: cool!
posted by Mental Wimp at 8:11 AM on October 29, 2010


They built this in the state where people think the diamond lane is a passing lane?
posted by Brocktoon at 8:14 AM on October 29, 2010


my initial reaction is that it's insanely user-hostile

We must have different definitions of hostile. I find it hostile when I am presented with a left turn in which I must compete with heavy oncoming traffic in the opposite direction, where I feel the pressure of a long column of impatient cars behind me urging me not to be cautious but to turn as soon as possible even if I don't think the gap is quite big enough. This is the kind of situation where you find people so anxious to get out of it that multiple drivers will advance into the intersection while waiting to turn so that they are guaranteed to be able to escape even if it means that they are still in the intersection once the light has turned red and the other direction wants to move. I vividly remember during driver ed that this was one of the most stressful situations I ever encountered.

The diverging diamond does away with all that nonsense. There is only a single light and its only function is stop or go forward. There is no turning against traffic, there is no competition. Turning either direction means taking an exit ramp. Sure, it means driving on the wrong side of the road for a bit, but that's simply following the lines painted on the road, something that you do every day and is completely natural.
posted by Rhomboid at 8:21 AM on October 29, 2010 [10 favorites]


This gets my vote. Throw in a few gentle line-of-sight obstructions and you'd never even notice you had crossed to the opposite side of the flow.
posted by scrowdid at 8:34 AM on October 29, 2010 [1 favorite]


There's a diverging diamond interchange at the intersection of I-695 and Reisterstown Road, near Baltimore. I've always found it pretty easy to navigate. Follow the signs and traffic lights and you'll be just fine.

I will admit that I'm hard-pressed to think of a setup that could be less pedestrian-friendly. I'd hate to be navigating that intersection on foot.
posted by cheapskatebay at 8:40 AM on October 29, 2010


And it's got no business being used in an area where there's expected to be foot traffic. This is a pedestrian-hostile design if I've ever seen one.

It's not wonderful, even though there are sidewalks and pedestrian crossings there, mainly because you have to cross a couple of single lane on/off ramps without a signal to stop the traffic for you. Instead there's either a "yield to pedestrians" sign for the cars, or a "wait for gap" sign for pedestrians, which in practical terms are about equivalent. At the signalized road crossings, pedestrians get to cross with a light and no turning traffic to potentially hit them, which is nice.

The same design can be built with signals on the ramps to give pedestrians a safer way of crossing those ramps, though this one doesn't seem to have been built that way. But in that sense it's not really worse than a lot of other designs that also have these pedestrian crossings of ramps (e.g. the parclo style interchange that's common in Ontario).
posted by FishBike at 8:42 AM on October 29, 2010


What about a footbridge or subway (in the UK sense of a tunnel under the road for people to walk through) for the pedestrians? More expensive than a regular crossing to implement I guess but if you're re-building the junction anyway...
posted by EndsOfInvention at 8:49 AM on October 29, 2010


My town placed a traffic circle with ped crossing right beside a senior's center.

I don't know why my city council wants to kill seniors. Maybe bed space was getting low.
posted by five fresh fish at 9:06 AM on October 29, 2010


I'm from Springfield. I freaked the hell out the first time I went home for a visit and had to drive through it -- my parents' house is about three minutes north of that interchange. Could not understand why they did it before I read this.

They're right that it's kind of troubling for pedestrians, but there is very little foot traffic in that section of Springfield anyway, or for that matter anywhere outside of downtown.
posted by middleclasstool at 9:15 AM on October 29, 2010


I assume that statement referred to some place like the U. S., where circles are not commonplace?

They're commonplace where I live, but that doesn't mean people don't perceive them as more dangerous. I thnk they work really smoothly too, but people are more on the alert - you can't do them on autopilot.
posted by Miko at 9:23 AM on October 29, 2010


Here in Pasadena, we solved the traffic circle confusion by installing stop signs...at intersections that were four-way stops before being converted to traffic circles.

I wonder how we could screw these up...perhaps a few "no left-turn" signs?
posted by malocchio at 9:36 AM on October 29, 2010


I'm inexperienced with roundabouts, so can someone explain what the correct way to navigate would be in this situation? -

You are driving in one direction on a three-lane road and there is a three-lane road going the opposite direction. At an intersection, where there's a roundabout, you need to turn left.
So you would go around the roundabout about 270 degrees and then turn right.
The question is, which lane should you stay in?

  • The left doesn't make sense, since you would end up on the wrong side, when it would be time to take the right turn
  • the right lane doesn't make sense, since as you turn around, you might cut off people in the center lane who want to go straight
  • so the center lane it is - but isn't it dangerous to have to quickly switch from center lane to right lane in such a short time (i.e. it seems that you can only switch after you have passed the 180 degree mark)?

  • posted by bitteroldman at 9:43 AM on October 29, 2010


    There are no sidewalks

    Sure there are. Take a close look and you'll see that pedestrians walk down a sidewalk that runs down the middle of the bridge. Kinda clever.
    posted by zsazsa at 9:48 AM on October 29, 2010


    There's a diverging diamond interchange at the intersection of I-695 and Reisterstown Road, near Baltimore.

    That intersection is a SPUI, mentioned upthread. /pedant
    posted by zsazsa at 9:52 AM on October 29, 2010


    The beauty of a roundabout, once you can re-train your Yankee brain, is that the Trouble is only coming from one direction.

    You're kidding yourself. My town got rid of a large rotary because there were so many rear-end accidents. People did not understand that they had to make sure the car ahead of them has actually gone before they lurched out to catch that gap in the traffic already in the rotary. All it takes is one indecisive driver and one behind him, looking over his own left shoulder, and BANG!

    I do miss that rotary. They replaced it with a nightmarish set of light-controlled intersections, where you almost always enter in the wrong lane for where you need to exit.
    posted by Kirth Gerson at 9:54 AM on October 29, 2010 [1 favorite]


    So Kirth Gerson, the question is, are there now fewer accidents or fatalities overall?
    posted by Miko at 9:55 AM on October 29, 2010


    I have been unable to get any data. All those rear-enders were pretty low-speed, so I don't think any of them were fatal. It's aggravating enough that I now go far out of my way to avoid the place when I can.
    posted by Kirth Gerson at 10:01 AM on October 29, 2010


    It's a French innovation, but it's starting to catch on in the states.

    FREEDOM DRIVES
    posted by Debaser626 at 10:01 AM on October 29, 2010


    Rhomboid: We must have different definitions of hostile

    Yes, we do. I would argue that left turns with explicit signal lights are user-friendly, because they tell you exactly what you're supposed to do and (theoretically) give you the space to do it.

    This, by contrast, gives you counter-intuitive, non-explicit cues about what to do that eliminate the space in which you are accustomed to doing what you want to do. Hence: User-hostile.

    I'm not arguing that user hostile = bad. I'm arguing that user friendliness of design is subjective and dependent on your criteria, and even then can have unexpected results. This is a traffic example; another example could be automated spam and virus filters for email: By creating a false sense of security, they may increase the incidence of phishing success. (This is a hypothetical; I have no idea whether they actually that, but it's a plausible extrapolation.)
    posted by lodurr at 10:36 AM on October 29, 2010


    FishBike: "This post gives me hope that I'm not the only person here who regularly goes on a browsing expedition through Wikipedia and the like, just looking at intersection designs and other traffic geekery.
    "

    Oh hell no you're not the only one. The DDI Limonaire mentioned above - two weeks old, now - is awesome. The first day it was open, I made a detour just to travel it in all directions!
    posted by notsnot at 10:36 AM on October 29, 2010


    Or put another way: Sometimes user hostile = good, depending on what you're trying to accomplish. If for example, you want to irritate people into doing something, you can only call that 'user friendly' using a Newspeak definition of the term.
    posted by lodurr at 10:38 AM on October 29, 2010


    bitteroldman: the UK Highway Code on roundabouts.

    You'll need to mentally flip left and right to adapt it to the US, but basically: if you're making a three-quarter or full circuit around the roundabout, you should take the inside lane around the hub.

    Like you, I always found this more problematic in practice on busy multi-lane roundabouts where actually getting from the inside lane to an exit could be a challenge.

    It kind of ducks the question on bigger roundabouts:
    When there are more than three lanes at the entrance to a roundabout, use the most appropriate lane on approach and through it.
    "You're on your own there, mate."
    posted by We had a deal, Kyle at 10:51 AM on October 29, 2010 [1 favorite]


    UDOT had to give an education/awareness seminar about this when they put it in. Add to that, the area of Salt Lake I live in has an obscenely large number of roundabouts, I feel stabby whenever I drive. It's not because of the new road patterns, but because of the people who fail to use them correctly. No, do not stop in the roundabout to make your turn. If you miss it because you failed to merge into traffic successfully, that is not my problem. Make another pass please.
    posted by msbutah at 11:42 AM on October 29, 2010


    You are driving in one direction on a three-lane road and there is a three-lane road going the opposite direction. At an intersection, where there's a roundabout, you need to turn left.
    So you would go around the roundabout about 270 degrees and then turn right.
    The question is, which lane should you stay in?


    As We had a deal, Kyle posted, strictly speaking, the innermost lane is the correct one.

    Which one you actually take depends a bit on the current traffic density and how quick on your feet you are. If the roundabout is very busy, it may be difficult to get to the innermost lane and to get out of it again to take your exit. No worries if you don't manage though, just go around once more :)

    Also, if you're driving a large, slow vehicle with limited vision, such as a truck, you tend to stay in the outermost lane whichever exit you need.

    You have to get the hang of them, then they're fine. Driving instructors use them to terrorise their students with great success.
    posted by Djinh at 11:58 AM on October 29, 2010 [1 favorite]


    My kids make me drive through that interchange in the picture every time we are up in Springfield, MO. It is very helpful, but I go through it knowing exactly where I need to start and end. I would hate to navigate through it blind.
    posted by shinynewnick at 12:05 PM on October 29, 2010


    So basically this is the next best thing to a full four leaf clover, but taking up less space along the axis defined by the surface road?
    posted by BrotherCaine at 12:17 PM on October 29, 2010


    BrotherCaine wrote: "So basically this is the next best thing to a full four leaf clover, but taking up less space along the axis defined by the surface road?"

    Better, from the controlled access road's perspective. Cloverleaf interchanges induce a large amount of weaving, which is both dangerous and reduces throughput. They also limit the available space for entering traffic to accelerate, slowing things down even further.
    posted by wierdo at 12:53 PM on October 29, 2010 [1 favorite]


    What about people simply trying to travel east -> west? It looks like they hit the diamond and are going north/south whether they want to or not.
    posted by Cranberry at 1:09 PM on October 29, 2010


    What about people simply trying to travel east -> west? It looks like they hit the diamond and are going north/south whether they want to or not.

    You might be misinterpreting the picture. The east-west roads with dark pavement are the on and off ramps, so nobody would be going straight through on those. If you look a little farther away (towards the top and bottom of the picture), you'll see another set of east-west roads that do go straight through. The one at the top even has traffic lights over it.
    posted by FishBike at 1:16 PM on October 29, 2010


    I would argue that left turns with explicit signal lights are user-friendly, because they tell you exactly what you're supposed to do and (theoretically) give you the space to do it.

    You only find signal light intersections intuitive because you're used to them. Imagine for a minute that you're an alien and I'm going to describe to you two different ways of handling an intersection.

    Here's how you describe a traditional intersection: Red means stop, unless you're turning right, in which case it means stop then go if you can, unless there is a sign posted that says you can't or the light is a red arrow. (If the cross street is a one way street going to the left then switch right for left in the previous sentence.) Green means go, except if you're turning left in which case it means stop and then wait until there's an opportunity to turn, optionally advancing into the intersection to wait if you're the first in line, unless it's a green arrow in which case you don't have to stop.

    Here's how you explain a diverging diamond: To turn left take the exit ramp to the left. To turn right, take the exit ramp to the right. Stop when you see red, go when you see green.

    Now, which one of those is intuitive?

    This, by contrast, gives you counter-intuitive, non-explicit cues about what to do that eliminate the space in which you are accustomed to doing what you want to do. Hence: User-hostile.

    It's only hostile because it's not what you're used to. If that is the metric you're going to judge something by then it means you can never improve anything because change always means something different.
    posted by Rhomboid at 1:21 PM on October 29, 2010 [2 favorites]


    DC has its fair share of terrifying traffic circles. Most of them have multiple traffic lights in them, which combined with DC's ban on overhead stop lights (ie. traffic lights are usually only on the sides of the road), makes some of these circles terrifying to navigate, as you can never be 100% sure that the oncoming traffic has seen the red light. The timing of the signals is also usually such that drivers try to speed their way through the circle as to not get caught in the next light.

    Things got so bad that the city eventually tunneled under a few of the circles so that drivers on one of the roads passing through could opt to bypass the circle entirely. This produced a number of unfortunate traffic patterns in and of itself.

    Dupont Circle wins the award for the worst circle, because the roadway within the circle is partially divided with a median for no discernable reason, and the circle makes extensive use of flashing yellow arrow signals. Unlike yellow lights everywhere else on the planet, drivers do not have the right of way when they are facing a flashing yellow arrow.

    Ironically, the circles with fewer/no traffic lights are far easier to navigate.
    posted by schmod at 4:31 PM on October 29, 2010


    You only find signal light intersections intuitive because you're used to them. Imagine for a minute that you're an alien and I'm going to describe to you...

    OK, stop right there: I am not an alien. I'm an earthling, and what's more a native of the state of New York in the USA. I already know how lights work. That is the context in which this exists. Asking that it be interpreted in another context misses the point of user-friendliness.

    It's only hostile because it's not what you're used to. If that is the metric you're going to judge something by then it means you can never improve anything because change always means something different.

    You don't seem to be bothering to understand the rest of my argument: That "user friendly" is not synonymous with "better."
    posted by lodurr at 12:36 PM on October 30, 2010


    lodurr, familiarity and user-friendliness are just synonyms. There is no such thing as user friendly. Discoverability is about the closest you can get. Driving is none of those things.

    Besides, shouldn't we make the roads better for future users who have not yet been trained in our backwards ways?
    posted by wierdo at 12:52 PM on October 30, 2010


    I lied. Driving is familiar to drivers who reside in a given jurisdiction.
    posted by wierdo at 12:54 PM on October 30, 2010


    Well, then, Wierdo, substitute "familiarity" for "user-friendly" if it makes you feel better. Both points still stand.

    In any case, you're actually not right about that. I'm familiar with the argument that you're making, and it's mistaken: There is such a thing as user-friendliness, and it is not the same as familiarity. If you want to say they're the same thing, you need to make a case, and Don Norman, Edward Tufte and a few other folks I could name have written books that argue for the existence of design principles that make things easier for people to use that are not synonymous with user-friendliness.

    Now, if you'd said that "intuitive" was synonymous with "familiar", then I'd probably be with you. I can't tell you how many arguments I've had over whether something was "intuitive." You'll notice I didn't use that word -- my ergonomics professor broke me of that habit in 1993.

    This is a distinction with a difference: "Familiarity" means it's what you expect; "user friendly" means you can figure it out easily based on the information that's available to you. In theory, that will be easier to do if it's what you expect, though you could probably pick out many examples between your desk and your car of cases where familiarity makes it harder to figure out what to do (because it leads you down the wrong path of reasoning). So clearly, they're not the same.
    posted by lodurr at 7:09 AM on November 1, 2010


    As for whether we should change things for future drivers, I would suggest that you, also, consider the second part of my argument: That "user friendly" isn't equal to "better."

    But in afterthought, also consider: posted by lodurr at 7:13 AM on November 1, 2010


    You only find signal light intersections intuitive because you're used to them. Imagine for a minute that you're an alien and I'm going to describe to you two different ways of handling an intersection.

    Here's how you describe a traditional intersection: Red means stop, unless you're turning right, in which case it means stop then go if you can, unless there is a sign posted that says you can't or the light is a red arrow. (If the cross street is a one way street going to the left then switch right for left in the previous sentence.) Green means go, except if you're turning left in which case it means stop and then wait until there's an opportunity to turn, optionally advancing into the intersection to wait if you're the first in line, unless it's a green arrow in which case you don't have to stop.
    This reminded me of a driving incident late one night on the north side of Chicago in the mid70s, coming home from a show at the Brown Shoe or Ratzos. We were sitting in a left turn lane with a red arrow light under the Kennedy Expressway. The left turn would allow us to get on the Kennedy, and the light was taking forever.

    The guy behind the wheel was a nervous sort, but nearly as fidgety as the shotgun, who was mindlessly chanting: "Right Turn on Red--Left Turn on Green" over and over and over, until the driver kicked it into gear, and turned left. On Red. In front of oncoming traffic and worse yet, The Chicago Police Traffic Enforcement patrol.

    The two in front were yelling at each other about whose fault it was, and the three of us in back were trying to look like new fallen snow. By the time the policeman came to the window, the driver was pleading that it wasn't his fault, and shotgun kept babbling "Right Turn on Red, Left Turn on Green" as if he were unaware that he was acting the fool. The policeman let us go, with a warning for BOTH in the front seat.

    Traffic lights can be a wonderful thing, but they can be misused by the weirdness of friends.
    posted by beelzbubba at 7:28 AM on November 1, 2010


    lodurr wrote: ""user friendly" means you can figure it out easily based on the information that's available to you."

    By that definition, I fail to see how a diverging diamond is not user friendly, presuming it is as well signed as the examples we've seen thus far.
    posted by wierdo at 8:59 AM on November 1, 2010


    Well, then, we're at an impass, because by that definition it seems like it's pretty hard to figure out -- asking you, as it does, to do something you've been trained not to do since you first got into that driver ed car.

    As I've noted, you seem to be responding very strongly to the idea I put forward that this is not a user-friendly design, and you haven't responded at all to the idea that user-friendly design can be bad. Why is that?
    posted by lodurr at 9:26 AM on November 1, 2010


    It's not hard to figure out at all, though. You cross through intersections as normal. You use ramps as normal. The only thing that's odd about it is that you happen to be on the "wrong" side of opposing traffic. I don't see what can be simpler than following the usual traffic control guides and devices, aside from having your car drive itself.

    It is true that easy to grok designs can be a problem when they invite you to pay less attention than necessary to the task at hand, but I fail to see how that's relevant to the diverging diamond interchange.

    I can see your argument when it comes to the SPUI, as there are usually large expanses of uncontrolled and unmarked pavement within. But the design we're talking about doesn't have that disadvantage. You follow the lines, just as you do on any marked road.
    posted by wierdo at 11:47 AM on November 1, 2010


    I don't see what can be simpler than following the usual traffic control guides and devices, aside from having your car drive itself.

    Well, in all fairness, the few times I've found myself in this situation (ignoring the driving I've done in Jamaica, Ireland, and New Zealand), it has felt weird and a little disorienting.
    posted by Mental Wimp at 12:38 PM on November 1, 2010


    You follow the lines, just as you do on any marked road.

    If all you're doing is following the lines, that's some pretty bad driving. (Which takes the discussion in a rather different direction again.) I would rather not believe that's what you're doing if you use one of these things.

    Anyway: You seem to me to be arguing that because this is a rational control device, it's therefore good. I would respond that your analytical concept of what's rational doesn't have any necessary relationship with the perception of a road-user. I can't say for certain that I've ever been through a Divergent Diamond intersection (if you've only been through it once you probably wouldn't know that's what you're experiencing), but in Buffalo one time about a year ago I passed through a traffic interchange that shunted me back and forth across traffic in such a way that I was driving on the left side of the road for a bit. I can tell you that I found it a bit alarming. I paid excruciatingly close attention to what everyone else was doing -- but since I was actually trying to get somewhere I'd never been before, I was also simultaneoulsy trying to pay attention to where I was going to be going. As an experienced commuter passing through that area, I would have just zipped through, but as a first timer (even on a Saturday morning) I was slowing people down enough that they moved to pass me. (I probably slowed to 35 in a 45 zone.) If traffic were at full weekday pace, I probably wouldn't have slowed down -- I would have gone on through and accepted that if where I needed to go was in that section I wasn't going to get there by that route.

    If what a divergent diamond intersection does is make me pay attention, that's arguably a good thing. But from what I can see, that's not the purpose. The purpose is to streamline traffic flow for regular users. For non-regular users who don't expect the controls it presents them with, the most likely result is increased agitation. That, it seems to me, would more likely serve to condition them strongly toward passively following rules and controls -- not to paying attention.

    This is the kind of design that looks cool and rational and great when you put it on paper, and it may even function well and without accidents in practice -- but overall, it strikes me as just another way to streamline the flow of people according to rules and controls. As I get older, I'm less and less in favor of that kind of approach to anything. If that's not what a divergent diamond intersection does, then fantastic -- but I don't want a rational flow diagram that demonstrates that argument, I want empirical evidence, because my experience tells me that rational flow diagrams don't predict traffic behavior. If they did, we wouldn't have gaper's blocks.
    posted by lodurr at 7:46 PM on November 1, 2010


    You're kidding yourself...

    I dunno, maybe. Allow me to spaz out in my own defense:

    1. Roundabouts the size of the monster you linked to were invariably signal-controlled when I encountered them; I'm inclined to agree with you that free love doesn't work with something that big.

    B. You yourself said you miss that rotary.

    III. I'm not saying they make driving trivial; but I think most of the dismay they're greeted with in the States derives from unfamiliarity rather than serious criticism; in the typical European milieu of low speeds and frequent intersections, it seemed to me like they flow better than four-way-stop intersections. That said, in the part of the country I live in, speeds are pretty high and intersections are more rare. And in fact, when we got out in the sticks of Ireland, plain non-circular intersections were more common.
    posted by Rat Spatula at 11:45 PM on November 1, 2010


    I was saying you're kidding yourself that rotaries limit the directions you are in danger from to one. I do miss that rotary. Once I learned (the hard way) that some people would start to enter, then stop, I had no problems using it. It never choked up with traffic to the extent that the new abortion does regularly, and when traffic was light, it worked very smoothly. The new system never works smoothly, even when there's only one car using it.
    posted by Kirth Gerson at 8:09 AM on November 2, 2010


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