

Nifty Freeway Interchange Design - frankus
http://www.npr.org/templates/story/story.php?storyId=120403340

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kevinskii
This is exactly the type of article I like to see on HN. It presents a simple
solution to an obvious problem that I hadn't even really recognized, and it
reminds us to keep looking at things in different ways.

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jonah
In [un]related roadway news: [http://www.fastcompany.com/1660258/traffic-
report-how-to-swi...](http://www.fastcompany.com/1660258/traffic-report-how-
to-switch-to-the-other-side-of-the-road-without-causing-a-70-car-pileup)

One of the most vexing aspects of traveling between mainland China and Hong
Kong is the car travel: People in the former drive on the right side of the
road; people in the latter drive on the left (a vestige of the British
empire).

So to quell confusion at the border and, more importantly, to keep cars from
smashing into each other, the Dutch firm NL Architects proposed a brilliant,
simple solution, the Flipper bridge.

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jarek
I'm a pretty big fan of Ontario's parclo design
(<http://en.wikipedia.org/wiki/Parclo>).

The most standard design (called A4 in the wikipedia article) has the same
amount of traffic lights as this design. There is no driving on the left, no
weaving, and there are no left turns with oncoming traffic.

Access onto the highway is a right turn for all directions. Access from the
highway is light-controlled right or left turn. The highway has more capacity
than the intersecting road, so while dumping traffic from the road onto the
highway at a maximum rate using the easy right turns is fine, the light at the
exit off the highway serves as rate limiter if necessary so that the road is
not overwhelmed with traffic.

~~~
chancho
To be sure, those interchanges are the most pleasant to drive through, but at
a cost of land. The interchange design presented in the NPR article is much
more feasible in urban centers where you might be constrained by existing
structures. The accel/decel ramps can run parallel to the freeway, leading to
nearly-right angle turns at street level. (It's not drawn this way in the
article, but it doesn't take much imagination to picture it.)

~~~
jarek
That's definitely a good point, but in this particular case the picture posted
elsewhere in the comments by raquo [1] looks like it could fit or almost-fit a
parclo (possibly in a slightly elongated/compressed arrangement). The more
hippie side of me is tempted to suggest that if you can't afford the land, you
shouldn't be building an expressway there :)

[1] [http://www.inside-lane.com/wp-
content/uploads/2009/11/Diverg...](http://www.inside-lane.com/wp-
content/uploads/2009/11/Diverging-Diamond-Springfield-MO1.jpg)

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usaar333
I'd be interested in seeing more analysis of this.

It is pretty close to a cloverleaf interchange
(<http://en.wikipedia.org/wiki/Cloverleaf_interchange>), but with the addition
of stoplights on the road.

It seems that this will still suffer the weaving problem that plagues
cloverleaf intersections - note how on the bridge cars first enter the road
(from the freeway) and then shortly afterward, leave.

Perhaps this could be mitigated by placing stoplights on the offramps (which
can be conveniently anti-synchronized with the lights controlling the bridge
entrance), but I imagine space on the offramp could be a huge issue.

~~~
dminor
The main idea is that it's a much cheaper upgrade to the diverging diamond
from a standard diamond interchange with left hand turn lights, but it's still
a fairly substantial upgrade for traffic flow. The cloverleaf eliminates all
the lights, but requires a lot more construction and takes up way more land.

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PidGin128
Obligatory: <http://en.wikipedia.org/wiki/Diverging_diamond_interchange>

Makes it easier to understand, seeing it from above. I also suspect I have
seen this on an infrastructure blog which I can't recall offhand.

Instant edit: Could the submitted title be made clearer?

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kikibobo69
Seems like a lot of lights, compared to a roundabout. I don't get why people
don't like roundabouts, they seem to work quite well, at least until traffic
backs up (at which point nothing really works). They also avoid the left (or
right, where they are most common), but typically with fewer lights. The
second light in this system seems like it will really slow things down, but
maybe it works. I'd love to see video footage of it in action during a busy
period.

~~~
_delirium
I have a sort of visceral negative reaction to roundabouts, but I think
because most of the ones I've seen in Europe, when the road is more than one
lane wide in each direction, seem to end up with a crazy ad-hoc-lanes thing
going on in them, where it's as if you drove onto a huge plaza with no lane
markings, where everyone is driving in all sorts of criss-crossing diagonal
directions trying to get from their entrance road to their exit road across
multiple lanes of traffic. I imagine some countries have more orderly
versions, but that's how the Paris ones all felt to me.

edit: video of an example, around the Arc de Triomphe ->
<http://www.youtube.com/watch?v=lay8aZlsbB0>

~~~
hugh3
I approve of one-lane roundabouts and wish America would replace all its four-
way stop signs with them. Four-way stop signs make no sense and just make
everything painful, _especially_ for cyclists.

Two-lane roundabouts are pretty dangerous, though -- while I've never had an
accident in one I've always suspected that I might. And anything more than two
just seems to be crazy.

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lmkg
I think the biggest problem with such a design is simply people not being used
to it. Once you're used to occasionally being on the left side of the road for
thirty seconds twice a day, there's no downside. The first took a long time,
the next few will come slowly, and then they'll become just part of the
terrain like cloverleafs (bleh). I also expect their acceptance will vary
geographically, like roundabouts (still bleh for me, YMMV).

~~~
tkaemming
I live in Springfield, and so far it actually seems like the most dangerous
parts are the right hand turns from MO-13 on to I-44 (step 6 on the
interactive diagram). A lot of people forget that although they have bypassed
the traffic signal they still have to yield to the oncoming traffic from the
_opposite_ side of MO-13 that they would typically be looking at to check
right of way.

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bgrins
I found a video that helped me to visualize this design better than I could
from the article. It is a 4 minute long animation of the interchange in action
from the Missouri Department of Transportation found on this page:
[http://www.modot.mo.gov/springfield/major_projects/Greene/I-...](http://www.modot.mo.gov/springfield/major_projects/Greene/I-44andRoute13.html).

And here is a direct link to the video (13MB):
[http://www.modot.mo.gov/springfield/major_projects/Greene/DD...](http://www.modot.mo.gov/springfield/major_projects/Greene/DDI_13-01Med_Prog.wmv)

There are also some more pictures:
[http://www.modot.mo.gov/springfield/major_projects/Greene/Di...](http://www.modot.mo.gov/springfield/major_projects/Greene/DivergingDiamondPhotos.html)

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ComputerGuru
Am I the only one that's bothered by the left-lane merges? That's really
difficult to implement, esp. in the way it's drawn in the diagram in TFA (no
lengthy merger lane).

In the left lane, people go fast. It's dangerous and difficult to merge from
an exit ramp onto the left lane.

~~~
raquo
See image: [http://www.inside-lane.com/wp-
content/uploads/2009/11/Diverg...](http://www.inside-lane.com/wp-
content/uploads/2009/11/Diverging-Diamond-Springfield-MO1.jpg)

The roads that merge both have only one lane. The one that comes from the
right has a yield sign. It also has a rather sharp bend just before the merge
so 1) the cars are unlikely to go at high speeds, and 2) the drivers can
easily see each other before the merge, so it's actually good design.

~~~
jarek
I believe ComputerGuru was talking about the merge into the left lane of the
intersecting road while exiting the highway and turning left. It's not the
most intuitive of arrangements, especially when combined with the left-hand
exit following shortly. Hopefully the drivers on the intersecting road will be
sufficiently surprised by the arrangement to break the usual expectation of
the left lane being a through lane with higher speed.

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malvim
Great article, very innovative design.

Also, something else caught my eye: City's name is Springfield, and the name
of the woman that works at the gas station is Faith FLANDERS. :)

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russell
This looks like a good idea. Picture your typical urban freeway interchange
with left turn lanes to get onto the freeway. This design eliminates the left
turns by moving each direction to the left on the bridge: English on the
bridge and American on the approaches. There is a diagram that makes it all
clear.

~~~
duck
I agree, looks like a pretty good idea. Any type of change like this always
gets negative opinions, same thing happened when they put roundabouts in and
now everyone loves them and want more of them.

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stretchwithme
Love this kind of stuff. It can be trippy driving through these things though.

How long before traffic systems start broadcasting the schedule for the next
minute or two so cars can plan ahead and always get there when the lights
green?

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pvilchez
The article ran in November of last year, so the interchange has (presumably)
been used for half a year now. Surely Missouri's been collecting data about
how the new system's been doing - I'd love to see it.

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mikerhoads
I'm sure I understand why the stop light/intersections need to be there at
all. Why can't the orange road pass over the red road on one side and then
pass under on the other side of the underpass?

~~~
jarek
Building overpasses isn't exactly cheap and the article said no bridge
construction was necessary when putting this layout up. It can definitely be
done if the traffic level warrants shelling out extra for not having two
traffic lights, but once you're going for a three-level interchange there are
a lot of existing designs out there already (and some don't have the entrance-
exit weaving pattern).

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vyrotek
I'm pretty sure this is what they started building in American Fork, Utah. -
<http://www.udot.utah.gov/pioneer> (video)

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robinduckett
This wouldn't be needed if they drove on the correct side of the road.

~~~
robinduckett
Downboats for being British. Not my fault you people don't have a queen
anymore, get over it.

