Pennsylvania's first "diverging diamond" interchange coming to Washington County (businesses, town)

[Copanut]

Wasn't sure, but that needs to be done, rather needs done. Seems like they've been working on those roads down there for years.

[Craziaskowboi]

Originally Posted by SteelCityRising

As an urban planning enthusiast I'm actually quite excited to see more of these diverging diamond interchanges spread like wildfire throughout our Commonwealth. They are a great space-saver when compared to the traditional "cloverleaf" interchanges, and traffic should flow much more efficiently.

Diverging diamonds and SPUIs are good at handling high volumes of traffic between Interstates and secondary roads in urban areas without taking up too much space. I've been an advocate of rebuilding I-376 with several SPUIs replacing some unnecessarily complex interchanges. Not only would they handle high volumes of traffic well, but they'd actually save space in the case of some interchanges, most notably the Wilkinsburg/Forest Hills interchange, which has too many strange ramp motions that take up too much space.

Stack interchanges are good at moving high volumes of high-speed traffic between two Interstates, although "cloverstacks" can work fine if the loop ramps are moving a relatively low volume of traffic. The interchange between I-79 and I-376 is a cloverstack, with flyover ramps handling traffic from I-376 WB to I-79 SB and I-376 EB to I-79 NB, and loop ramps handling traffic from I-79 NB to I-376 WB and I-79 SB to I-376 EB. It works because there's only one loop on each side of the interchange, so the acceleration lanes can be made very long to compensate for the speed lost in the loop motion.

In short, full cloverleaf interchanges (those with four loop ramps) are becoming dinosaurs.

[Craziaskowboi]

Originally Posted by Copanut

Wasn't sure, but that needs to be done, rather needs done. Seems like they've been working on those roads down there for years.

They rebuilt I-79 from I-70 north to the Allegheny County line back in the 1990's, and have had to make a few emergency overpass repairs on I-70 due to overheight trucks. Otherwise, the mainline of I-70 has been given nothing more than the occasional resurfacing on certain segments. What's about to happen to I-70 in the near future is a total reconstruction from the ground up, plus upgrading all design features to make the entire segment from Washington to New Stanton AASHTO-compliant, which it's currently not. This will basically eliminate all the deficiencies on one of Pennsylvania's pre-Interstate Interstates. And once they begin reconstructing I-83 in Harrisburg later this decade, that'll leave I-376 in Pittsburgh and the Schuylkill Expressway in Philadelphia as the only segments of Interstate in Pennsylvania that are not AASHTO-compliant. (Those will be the biggest projects of all, though, which is part of why they'll be the last to be reconstructed and upgraded.)

[ditchdigger]

That damn thing ain't gonna do nuthin yinz couldn't do with a well executed Pittsburgh left...

[jjs65]

I produced a report during undergrad for diverging diamond interchanges:

Traffic movements (red lines are red-lights, green lines are green-lights):


DDI rendering as an underpass:


Existing DDI (MoDOT):


Overhead showing traffic movements. Teal is eastbound, Orange is westbound:


Vehicular view going into the new layout. Shown in the picture are CMFs (Crash Modification Factors) with wrong-way arrows to reduce the likelihood of people going the wrong-way as well as a glare screen to reduce headlight glare:


The only negative with this type of interchange is that it is not beneficial for the minor road thru-traffic.

[Troubleshooter]

There are now 11 DDIs open to traffic in the US:

- Springfield MO - I-44 and MO 13 (the photo is of this interchange)
- Springfield MO - US 60 and National Ave
- St Louis MO - I-270 And Dorsett Rd (find a photo of this to see one with the freeway on top)
- Branson MO - US 65 and MO 248
- Kansas City MO - I-435 and Front Street
- American Fork UT - I-15 and Pioneer Crossing
- American Fork UT - I-15 and 500 E
- Lehi UT - I-15 and UT 92
- Salt Lake City UT - UT 201 and UT 154
- Alcoa TN - US 129 and Bessemer St
- Lexington KY - KY 4 (New Circle Road) and US 68

In each case, the first listed road is the freeway.

There are currently 85 other projects in various stages of planning, design, or construction.

One nice feature of converting a diamond to a diverging diamond is that often the existing bridge can be used.

The dotted lines are the guide lines for the cars through the intersection.

Driving through a diverging diamond is not as difficult as it looks. Just follow the signs and markings. It's one of those experiences where you drive through it first, and then think, "What just happened?" afterward.

[UKyank]

I wonder if they considered adding a small over/underpass to the diamond to eliminate the stop lights entirely.

[Craziaskowboi]

Originally Posted by UKyank

I wonder if they considered adding a small over/underpass to the diamond to eliminate the stop lights entirely.

Every over-/underpass adds to the cost. It's easier just to have a pair of traffic signals.

[Troubleshooter]

We just passed a milestone. In the US, there are now 100 diverging diamond projects in various stages of study, planning, and construction, with 11 of them already open.

[Troubleshooter]

Update on DDIs in the US:

There are now 18 DDIs open to traffic, 13 under construction, and another 92 projects in various stages of design or approval. 8 of the open DDIs are in Missouri, and 4 are in Utah.

The open ones are (in order of date opened):

- Springfield MO - I-44 and MO 13 (the photo is of this interchange)
- American Fork UT - I-15 and Pioneer Crossing
- Springfield MO - US 60 and National Ave
- St Louis MO - I-270 And Dorsett Rd
- Alcoa TN - US 129 and Bessemer St
- Lexington KY - KY 4 (New Circle Road) and US 68
- Lehi UT - I-15 and UT 92
- Salt Lake City UT - UT 201 and UT 154
- Kansas City MO - I-435 and Front Street
- American Fork UT - I-15 and 500 E
- Branson MO - US-65 and MO-248
- Atlanta GA - I-285 and Ashford-Dunwoody Rd
- Hanover MD - MD-295 and Arundel Mills Blvd
- Farmington MO - US-67 and MO-221
- Rochester NY - I-590 and Winton Rd
- Springfield MO - US-65 and Chestnut Expressway
- Reno NV - US-395 and Moana Ln
- Kansas City MO - MO-150 and Botts Rd

In each case, the first road listed is the freeway.