
Designing a safer intersection with geometry (2013) - whiskypeters
http://www.theatlanticcities.com/commute/2013/01/could-these-crazy-intersections-make-us-safer/4467/
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LukeWalsh
It's going to be really interesting to see what intersections look like once
we no longer allow human drivers behind the wheel. We can only hope this will
happen during our lifetime.

~~~
joshuaheard
There's a link to that in the article.

[http://www.theatlanticcities.com/technology/2012/03/what-
int...](http://www.theatlanticcities.com/technology/2012/03/what-
intersections-would-look-world-driverless-cars/1377/)

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dnautics
"And we all know how that's gone in America with the simple roundabout."

Roundabouts are not unpopular, and exist in cities like DC with no one
complaining. The reason why roundabouts are unpopular is because city meddlers
often put them in places where their purpose is not to mitigate accidents but
to attempt to limit speeding through side streets, with negligible (if not
negative) effect on traffic safety.

~~~
peatmoss
Some terminology about circular traffic features:

Big things that you can travel quickly into, which are most commonly found on
the U.S. East Coast (e.g. Washington, D.C.) are called rotaries, and are
generally considered to not be particularly good or safe.

Mid-sized things with splitter islands preceding the entry, and that have an
angle of entry that prohibits entering at speed are called roundabouts, and
generally are considered a good thing from a speed / efficiency perspective.

Small things with no splitter islands preceding, that are usually put on quiet
residential streets (common in Seattle among other places), are called traffic
circles. Confusingly, rotaries are sometimes called traffic circles too (see
Dupont Circle).

I disagree with your assessment about traffic circles being placed by city
meddlers, as well as the assertion that they don't have a positive effect on
safety. First, in Seattle and elsewhere, they are typically requested by
residents of the street (not us meddling urban planners), and are built where
a speeding problem is confirmed, as resources become available, and according
to some geographic equity criteria. Second, though I haven't done an
exhaustive review of traffic circle literature, this summary review article is
bullish on traffic calming features generally, and specifically on traffic
calming circles as well:
[http://jpl.sagepub.com/content/23/4/347.short](http://jpl.sagepub.com/content/23/4/347.short)

~~~
dnautics
Thanks for the terminology clarification. I grew up in the DC area, so to me a
'traffic circle' is all of the above, but thinking of places like dupont
circle as the archetype of the 'working' circle.

I didn't say urban planners, I said city meddlers, i.e. select residents of
the street. And all the complaints I have ever heard about the (small) traffic
circles have been from the residents themselves. There could be a sample bias:
I'm a lyft driver; the complaints come from passengers getting dropped off at
their homes, usually from nights of drinking.

~~~
judk
You are hearing complaints from the troublesome speeders and drunk drivers.
Thank you for Lyfting them out of there own cars!

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meshko
This reminds me of the great transformation of Drum Hill Rotary into Drum Hill
Square:
[https://www.google.com/maps/place/Drum+Hill+Rd/@42.6225607,-...](https://www.google.com/maps/place/Drum+Hill+Rd/@42.6225607,-71.3666283,17z/data=!4m2!3m1!1s0x89e3a35cf6c32ab7:0xfc19e146f5ee894e)
The resulting design is ridiculously confusing and unintuitive. The first time
I was driving to it I thought: "WTF were they thinking? How do I get into the
right lane?" Then it changed to "Woah, I got lucky, I'm sure I will get
confused the next time" and then "OK, somehow I keep getting it right" and
"Wait, I am no longer stuck in traffic here?" "Not even at 5pm?" "...woah?"

Yes, designing the traffic lanes works.

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jonas21
It's interesting to note that much of the gain in automobile safety over the
past 50 years has been offset by an increase in miles driven due to increased
sprawl [1].

It seems like this sort of intersection, while possibly being safer, would
encourage even more sprawl due to its scale, and pedestrian-unfriendliness.

[1]
[http://www.planetizen.com/node/68200](http://www.planetizen.com/node/68200)

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bsder
I also note the the "lights" in the simulation are cycling _WAY_ faster than I
have ever seen a traffic light cycle in the US.

A whole lot of intersections in the US would likely improve with that simple
"innovation".

People are less likely to crash a yellow-to-red light when they know that
their turn is coming again in 30 seconds. Make that 5 minutes and streams of
people are going to crash that light.

~~~
CodeMage
That sounds (to me) like an excellent idea, as long as the intersection
doesn't have any pedestrian crossings.

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arh68
I used to live near the Mixing Bowl [1] (not exactly an inspiration in city
planning), so I just wonder how these would hold up under congestion. The
mixing bowl proves you can do anything with ramps (& no lights!), but building
flat intersections seems much harder.

Building freeway ramps isn't really that hard, though, so I don't see the
point in restricting dimensions. It's good not to let it get out of hand like
VDOT, but concrete is pretty cheap. Maybe if you've got a lot of 45mph roads
connecting you'd want good lighted designs. Where I live now it's all 40s
connecting to 55s connecting to the interstate, so it'd be pretty moot.

Planning for road speeds & traffic rates seem far more important than keeping
things flat.

[1]
[http://en.wikipedia.org/wiki/Mixing_Bowl_(Springfield)](http://en.wikipedia.org/wiki/Mixing_Bowl_\(Springfield\))

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vitd
I used to live near a road that was designed similar to the "Continuous flow"
model. It wasn't quite that elaborate, but Palatine Rd. just north of Chicago
has "local" and "express" lanes. (Or was it Dundee Rd.? I forget.) I have no
doubt it flows more efficiently than other roads, but I consistently had
people drive into the wrong lane directly at me. I've never had that on other
streets. So the question is - do these trade efficiency for basic safety?

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bsder
That "diverging diamond" would terrify me.

Seeing a car moving at high speed on the right in a parallel trajectory is a
deeply ingrained alarm bell that I'm in the wrong lane.

US Interstate 35 in Austin at 51st Street used to (may still have) have an
overpass lane like this and it took a _LONG_ time to get used to without it
causing "Danger Will Robinson" reactions.

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user080
_Continuous Flow_ model reminds me of divide and conquer sorting algorithms.

Most elaborate roundabout( with traffic lights ):

[http://i193.photobucket.com/albums/z202/keber1/razno/tomaevo...](http://i193.photobucket.com/albums/z202/keber1/razno/tomaevo3-pzr.jpg)

~~~
nkoren
That is far from the most elaborate roundabout. In the UK they have one that's
fractal:

[http://en.wikipedia.org/wiki/Magic_Roundabout_(Swindon)](http://en.wikipedia.org/wiki/Magic_Roundabout_\(Swindon\))

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judk
In Continuous Flow, how do you ever get _in_ to the Left turn lane?! The video
crops that part out, as if it is a prank design.

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dnautics
amazing. I've always fantasized a "diverging diamond" for the I-5/Genessee
exit in San Diego. There are space constraints (the freeway is in a canyon)
that prevent more traditional designs, and the exit is a nightmare, causes
atrocious traffic snarls.

I had no idea that someone had actually made one.

