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Old September 2nd, 2009, 05:46 PM   #221
NCT
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Roundabouts are ideal for medium to large unsaturated junctions where there's ample space, especially where there is a dominant flow. Compared to a signal controlled junction less time is wasted waiting for a green light; compared with non-signal controlled junctions traffic in non-dominant directions have a better chance to move (imagine sitting on a minor road waiting for traffic to clear on the main road in both directions).

Provided there's ample space, at a saturated junction, a signal controlled roundabout is the best option (where traffic volume does not warrant a grade-separated junction of course). All you need is two stages in each sequence of traffic lights. I drew those diagrams for a junction in Shanghai which I think is suitable for a roundabout, so apologies for the wrong language. Other than that I hope they are self-explanatary:



The segragated lanes on the very edge of the roads are cycle lanes, which are not relevant for all countries. Traffic lanes bordered by a thick continuous line are bus lanes.

Traffic-light sequence - stage one:



Stage two:



This is more efficient than flat junctions which would need 4 stages in each traffic-light sequence, straight and turning for each direction. There's the added bonus of visual friendliness for there isn't a massive patch of black tarmac at a roundabout junction.

Now some say roundabouts with through-roads defeat the purpose, like this one:

http://maps.google.co.uk/maps?f=q&so...05472&t=k&z=18

Actually these hybrid junctions are even better than conventional signal controlled roundabouts, if you consider where turning traffic can be easily stored.
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Old September 15th, 2009, 09:58 PM   #222
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Quote:
Originally Posted by nerdly_dood View Post
On another note, here is a roundabout in Spain (Madrid i think) that entirely defeat the purpose of a roundabout. (Do other countries murder their roundabouts this way, or is it only Spain?)

We have one at Piața Romană, Bucharest, Romania:

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Old September 15th, 2009, 10:37 PM   #223
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what's wrong with a simple stop light?

so much more straight foward
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Old September 15th, 2009, 10:40 PM   #224
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That Bucharest roundabout reduces annoying left-turns. Those are capacity reducers at traffic lights...
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Old October 19th, 2009, 02:24 PM   #225
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Some stuff from Czech Republic:

Demonstration of widespread roundabout plague - this is in Novy Jicin:


This weird thing in Most:


And two-and-a-half roundabout-ish awesomeness in Liberec (the middle one isn't full circle, as it wouldn't make any sense at all):
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Old October 19th, 2009, 02:35 PM   #226
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That center picture is called a "dog bone roundabout". We have a few of them too in the Netherlands.

[IMG]http://i34.************/10xg5ls.jpg[/IMG]
N11/N459 Bodegraven
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Old October 19th, 2009, 03:22 PM   #227
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It's not entirely the same though. The leftmost Czech roundabout is a complete circle.
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Old October 19th, 2009, 03:34 PM   #228
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I was referring to the picture above that one.
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Old October 19th, 2009, 03:50 PM   #229
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Old October 19th, 2009, 03:54 PM   #230
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Jakarta, Indonesia

Bundaran Hotel Indonesia (Hotel Indonesia Roundabout)
from flickr

image hosted on flickr


image hosted on flickr


image hosted on flickr


__________________
Reticulating splines...
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Old October 19th, 2009, 04:13 PM   #231
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Quote:
Originally Posted by ChrisZwolle View Post
I was referring to the picture above that one.
Oops.
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Old December 11th, 2009, 10:26 PM   #232
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I am a big fan of roundabouts. Yes, I am American. Yes, I got my driver's license in a town of 1300 people, 29 miles from the nearest stoplight. No, I didn't learn about them in driver's education. But it seems like common sense to me: 4-way stops require ALL traffic to stop, which is the antithesis of efficiency; stoplights require sudden stops, which increases the risk of serious collisions.

One thing I've become convinced of over time, however, is that roundabouts should never be constructed in proximity to stoplights. See this example in Belleville, Illionois:
http://maps.google.com/maps?f=q&sour...,0.004823&z=18
At least three of the four surrounding streets have stoplights, and Illinois Street is one of the busiest in town. What happens is that one or more of the stoplights turn red, traffic queues up onto the roundabout, and then NOBODY can move.
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Old December 12th, 2009, 12:20 PM   #233
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Moroccan roundabouts, unfortunately - Morocco has followed the example of France and implemented like thousands of roundabouts lately.

Some examples:

[IMG]http://i45.************/33ark45.jpg[/IMG]

[IMG]http://i47.************/2zyezif.jpg[/IMG]

[IMG]http://i49.************/2dw7r4i.jpg[/IMG]
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Old December 12th, 2009, 12:30 PM   #234
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Regular concentric circular lanes - how NOT to design a roundabout.
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Old December 12th, 2009, 03:43 PM   #235
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Quote:
Originally Posted by kphoger View Post
I am a big fan of roundabouts. Yes, I am American. Yes, I got my driver's license in a town of 1300 people, 29 miles from the nearest stoplight. No, I didn't learn about them in driver's education. But it seems like common sense to me: 4-way stops require ALL traffic to stop, which is the antithesis of efficiency; stoplights require sudden stops, which increases the risk of serious collisions.
That's why we have a lot of roundabouts in Europe. They're practical and reduce traffic jams. As far as I'm concerned, we can eliminate all traffic lights and install roundabouts.

This picture is old, new road section has been built since then. Notice 5 roundabouts and 3 small ones before the bridge.

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Old December 12th, 2009, 04:38 PM   #236
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Quote:
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As far as I'm concerned, we can eliminate all traffic lights and install roundabouts.
No, you can't.

You cannot control the traffic flow within a network of intersections with roundabouts. Another issue is that roundabouts don't work well on high-trafficked intersections, especially if traffic volumes are over 15,000 for single lane roundabouts and 20,000 - 25,000 on multilane roundabouts.
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Old December 12th, 2009, 05:09 PM   #237
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Traffic lights don't work in that case either. This:



You don't need traffic lights at all.
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Old December 13th, 2009, 12:08 AM   #238
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In most built up places there's no space to stick a junction like this or a roundabout. You'll have to make do with a conventional crossroad.
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Old December 13th, 2009, 01:09 PM   #239
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You can always make an underpass.
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Old December 13th, 2009, 05:14 PM   #240
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You still need lights for left/right turns and the cross traffic.
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