Variations on the DDI

[tradephoric]

It should be noted that the removal was done in conjunction with a major reconstruction of that portion of I-15. That included the installation of about three miles of one-way frontage road...which doesn't really work with a DDI.

New frontage road does throw a monkey wrench in it and makes the DDI impractical.  Just seems wasteful when a major interchange is ripped out less than 10 years from being completed.  Where was the planning and development?

[Tom958]

^ Yes, cost is still a deciding factor.  SPUIs are more efficient and higher capacity than DDIs but DDIs can be retrofitted into existing diamond interchanges much more easily.

Or not. The GA 400-Abernathy Road interchange reconstruction near Atlanta had been proposed as a SPUI since the '90s, but it's now being rebuilt as a DDI. The whole area is under massive reconstruction with the original bridges being removed and replaced, so it's unlikely that cost was a factor. Plus, the GDOT people at the public meetings I attended said it performed better in their modeling.

[Tom958]

That's basically like "El Sarape" interchange in Saltillo, Coahuila.

Might as well invoke this one in Aguascalientes. It's not nearly as spectacular, but it makes far greater use of the retained cuts and fills that the crazy-small footprint of the grade-separated DDI in the OP implies.

I really think this one's kind of brilliant, though other than the side-swapping roadways, it doesn't compare to a normal signalized DDI.

[froggie]

^ Yes, cost is still a deciding factor.  SPUIs are more efficient and higher capacity than DDIs but DDIs can be retrofitted into existing diamond interchanges much more easily.

Or not. The GA 400-Abernathy Road interchange reconstruction near Atlanta had been proposed as a SPUI since the '90s, but it's now being rebuilt as a DDI. The whole area is under massive reconstruction with the original bridges being removed and replaced, so it's unlikely that cost was a factor. Plus, the GDOT people at the public meetings I attended said it performed better in their modeling.

In that particular case, perhaps not.  But it's well-documented (and done elsewhere) that DDIs can be retrofitted into existing diamond interchanges without the need to replace or expand overpasses.

[tradephoric]

The DDI at I-85 and Jimmy Carter Blvd outside Atlanta seems too compact.  In addition the DDI reduced the travel lanes from 7 to 6 compared to the old diamond interchange while using the same width bridge deck (admittedly the pedestrian path was widened for the DDI but at the expense of a travel lane).  Every time Google updates their aerial it just seems like there is a massive traffic jam at this DDI.

[vtk]

Wait what's going on there with the southbound exit ramp? I see cars entering from both sides of the conflict. Isn't that junction signal-controlled?

[kphoger]

Wait what's going on there with the southbound exit ramp? I see cars entering from both sides of the conflict. Isn't that junction signal-controlled?

Tailback from SB-to-EB congestion?  That is, perhaps the EB light turned green while cars were still stopped in the conflict point?

[jamess]

That's basically like "El Sarape" interchange in Saltillo, Coahuila.

Might as well invoke this one in Aguascalientes. It's not nearly as spectacular, but it makes far greater use of the retained cuts and fills that the crazy-small footprint of the grade-separated DDI in the OP implies.

I really think this one's kind of brilliant, though other than the side-swapping roadways, it doesn't compare to a normal signalized DDI.

Problem is, American drivers cant handle something like this

https://goo.gl/maps/ZbFMGXbMHHQFXtPb7

[CoreySamson]

Problem is, American drivers cant handle something like this

https://goo.gl/maps/ZbFMGXbMHHQFXtPb7

If you put the interchange above ground instead of below it, I think that could work here.

[CtrlAltDel]

The DDI at I-85 and Jimmy Carter Blvd outside Atlanta seems too compact.  In addition the DDI reduced the travel lanes from 7 to 6 compared to the old diamond interchange while using the same width bridge deck (admittedly the pedestrian path was widened for the DDI but at the expense of a travel lane).  Every time Google updates their aerial it just seems like there is a massive traffic jam at this DDI.

The shoulders on I-85 seem substandard, too.

[jeffandnicole]

The DDI at I-85 and Jimmy Carter Blvd outside Atlanta seems too compact.  In addition the DDI reduced the travel lanes from 7 to 6 compared to the old diamond interchange while using the same width bridge deck (admittedly the pedestrian path was widened for the DDI but at the expense of a travel lane).  Every time Google updates their aerial it just seems like there is a massive traffic jam at this DDI.

I've been at this one at 95 and GA 21 a few times, and same thing.  While it doesn't show any traffic in the aerial, I've only encountered jams and long waits at this DDI.  Again, as above, there's a lot going on in a short distance.   I've been in this area because gas tends to be cheaper than elsewhere for a distance, and once I get off the exit I realize why I hate getting off the exit.   I really can't see the benefit of the DDI here.

[Anthony_JK]

The DDI at I-85 and Jimmy Carter Blvd outside Atlanta seems too compact.  In addition the DDI reduced the travel lanes from 7 to 6 compared to the old diamond interchange while using the same width bridge deck (admittedly the pedestrian path was widened for the DDI but at the expense of a travel lane).  Every time Google updates their aerial it just seems like there is a massive traffic jam at this DDI.

One problem I can see right off: the overpass structure is only 3 lanes in each direction (matching the original six-lane setup of the traditional diamond). Not enough space to handle the JCB through traffic and the left turning movements.

The solution would be to widen the overpass to add an additional lane or two in each direction to act as the buffer for weaving movements and allow for three dedicated lanes for straight on JCB traffic.

Also, I noticed that the through lanes in each direction drop a lane to the left turning movements before the switch back to conventional-side movement, meaning only 2 lanes go through before picking up the 3rd lane again from the right-turn entrance. That might be your choke point; need to widen to make it 3 lanes all the way through for consistency, and use acceleration/merging ramps at the on-ramp connection points for better merging.

Though, at this point, a SPUI with extended ramps would probably be more efficient here.

[vtk]

The whole point of a DDI is that it performs really well when the traffic entering and exiting the freeway is significantly greater than the traffic passing through on the surface street. Under such conditions, having three travel lanes each way reduced to two at the interchange should not be an issue. I can't tell from the photo whether it's a problem at this interchange, but either way, there's probably no better solution here without spending a lot of money on ROW acquisition and/or new structures.

[MASTERNC]

How about this DDI around a T-intersection north of Las Vegas?  Even more amazing is there are no traffic lights (only stop signs)

https://www.google.com/maps/@36.3812266,-114.8930064,465m/data=!3m1!1e3