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Old 11-23-2020, 06:48 AM   #1
cheez
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New Intersection Type: Diverging Diamond Interchange

I've seen these before but they are starting to become more common in their use; like a roundabout, if you haven't encountered one before they can be a bit confusing. I figured a post on BARF to encourage discussion and raise awareness wasn't a bad idea, being as the one on 120 near Manteca is about to open (or already has, I'm not sure.)

A Diverging Diamond Interchange works to decrease the amount of traffic which crosses other traffic. In a normal four-way interchange, there are 12 points of conflict which can arise- 3 directions of travel for each of four directions. The right turn traffic is merging with a traffic flow and is generally low risk for severe injury/accident as a result. The straight traffic is generally well controlled and has delays between each direction of travel change, allowing traffic to come to a stop. Left turns, however, introduce significant conflict with oncoming and cross traffic, and are the single most dangerous type of turn by severity of accident produced.

By eliminating the left turn conflict in the DDI interchange, the risk of severe injury accidents is decreased. As drivers approach the interchange, they have the option to turn right at the near-side road. They then proceed through a stoplight which crosses them over to the other side of the roadway, allowing those who need to turn left to do so in a protected manner resulting in a similar merge to the right turn. Traffic then crosses back over under another control signal, returning to 'normal' operation. This reduces the number of conflicts where traffic crosses other traffic from 8 to 4 by eliminating the left turn conflicts.

That's a lot of words. The video may explain it better.


youtu.be/-wnNi2O-Vos

(Mods, if this post belongs in a different forum, I humbly request you rehome it.)
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Old 11-23-2020, 07:11 AM   #2
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Old 11-23-2020, 08:22 AM   #3
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“If you’re an attentive driver and you understand the rules of the road, you should be able to navigate this no problem, but you need to abide by the rules of the road so to speak,” Shackelford said.



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Old 11-23-2020, 08:53 AM   #4
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Reno has a left turn lane leading to a 580 on ramp that is to the left of oncoming traffic. Stoplights of course but the layout seems odd the first few times. I’m all for traffic flow.
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Old 11-25-2020, 05:27 PM   #5
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Originally Posted by cheez View Post
eliminating the left turn conflict
This.

As an erstwhile moto safety professional and all-around nerd, I approve completely.
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Old 11-25-2020, 07:50 PM   #6
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That's a lot of words. The video may explain it better.


youtu.be/-wnNi2O-Vos
That's a really nice video. to the City of Manteca.

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Originally Posted by bikewanker View Post
Reno has a left turn lane leading to a 580 on ramp that is to the left of oncoming traffic. Stoplights of course but the layout seems odd the first few times. I’m all for traffic flow.
Here it is. Same layout, except the surface street passes under rather than over the freeway. Google street view is fully functional, so you can follow any path.




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As an erstwhile moto safety professional and all-around nerd, I approve completely.
The worst kind of conflict, left turns, are eliminated. But there is a potentially troublesome merge conflict--WB to SB, for example, because you have to cross the NB to WB traffic to get to your onramp. That could be a problem for a motorcyclist whose speed or position sets him up for a cut-off.
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Old 11-26-2020, 07:16 AM   #7
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That's nice, seems a little expensive in real estate.
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Old 11-26-2020, 08:58 AM   #8
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In Texas now at the daughters. Takes some time to get used to their on and off-ramps on the interstate. You may drive a 1/8 mile on a frontage and bam there is the ramp, you will drive by them a few times before you get used to it. Also the far left lane on the frontage road coming to an overcrossing maybe a freeway U-turn. Lane will become walled and you have nowhere to go except back from where you come from.
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Old 11-26-2020, 09:02 AM   #9
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In Texas now at the daughters. Takes some time to get used to their on and off-ramps on the interstate. You may drive a 1/8 mile on a frontage and bam there is the ramp, you will drive by them a few times before you get used to it. Also the far left lane on the frontage road coming to an overcrossing maybe a freeway U-turn. Lane will become walled and you have nowhere to go except back from where you come from.
I call that "The Texas A&M school of road design." One-way frontage roads, underpass/overpass intersections over highways with protected U-turns in both directions, and then a normal box intersection in the middle.

My favorite is where the business is just past the exit ramp of a road. So you'll see the parking lot you want to be in from the exit ramp as you're exiting, you'll have to take the frontage for the 1/4-1/2mi down to the protected U-turn, take it back up the 1/4-1mi to the other protected U-turn, and then make the right into the parking lot you passed 100yds from a mile and a half ago.

If you've ever gotten onto the freeway via a Texas two-way frontage road where you have to cross oncoming traffic and hope they are stopping at the stop sign, you'll accept that this costs fewer Texans their lives.
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Last edited by cheez; 11-26-2020 at 09:07 AM..
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Old 11-27-2020, 06:34 PM   #10
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Roundabouts work well where traffic is light to moderate. The best part is you usually don't have to stop at all.
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Old 11-28-2020, 01:07 AM   #11
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Roundabouts.... a curse from the UK. Basta!
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Old 11-28-2020, 01:00 PM   #12
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Roundabouts work well where traffic is light to moderate. The best part is you usually don't have to stop at all.
hell yeah! I love this roundabouts! Especially double-lane ones!

What's wrong with cloverleaf exchanges? No lights or waiting needed! Tully used to be great! Then they added lights! Horrible wait now.
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Old 11-29-2020, 11:08 AM   #13
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I call that "The Texas A&M school of road design
Heh. I’ll have to pass that along to my dad, who lives in Austin.

Whenever I visit, I always have to recalibrate my understanding of what a “boulevard” is... in Austin at least, it could be a twelve-lane road (including frontage roads) that’s a quarter-mile wide, including all the medians.
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Old 11-29-2020, 12:49 PM   #14
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I call that "The Texas A&M school of road design."
Yep. Talk about a design that wastes real estate. I used to visit northwest Houston regularly, and the intersections were unbelievably wide. Very hostile to pedestrians and cyclists.

As someone who's ridden in Europe quite a bit, I would much prefer that we adopted roundabouts. They also get rid of the left turning conflict, and keep traffic moving much better than four way stops or even most traffic light controlled intersections.
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Old 11-29-2020, 01:46 PM   #15
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Roundabouts work well where traffic is light to moderate. The best part is you usually don't have to stop at all.
I’ve seen a few here where the entrance to the roundabout is indeed a stop sign, that defeats the whole purpose.
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