Diverging Diamond Interchange w/ Truck Bypass

[jakeroot]

Hi all. Did a fair amount of searching on this topic to ensure no duplication, as usual. Nothing popped up.

In Anchorage, a parclo (B4) interchange was recently replaced by a diverging diamond; Glenn Highway @ Muldoon Road (heads up: EDIT: Street View only ... no satellite photos).

I noticed after looking at the contractor's website, where photos have been posted, that a truck bypass was provided for overheight vehicles across what is a decidedly-wide median. Any other examples of this at DDIs? I can't recall any. Look for the red-colored pavement between the on- and off-ramps, which are blocked by collapsible bollards.

(more photos on contractor website here)

[jakeroot]

Unrelated: I'm still unsure of what I think about this interchange in general. DDIs are extremely good at handling traffic at junctions where most of the traffic along the arterial enters or exits the freeway, but not so good at junctions where most of the traffic continues along. I have no idea what the major flow is along Muldoon Road, but I'm sure the old interchange was pretty decent?

Of course, considering all the work they did for pedestrian movements, I'm sure the new interchange is much better for modes other than cars.

[hotdogPi]

What am I supposed to be looking at? I'm not seeing any bypasses in either photo.

[TheOneKEA]

What am I supposed to be looking at? I'm not seeing any bypasses in either photo.

The red-colored concrete on both sides of the freeway is the bypass. They're both blocked by collapsible plastic bollards.

[jakeroot]

What am I supposed to be looking at? I'm not seeing any bypasses in either photo.

The red-colored concrete on both sides of the freeway is the bypass. They're both blocked by collapsible plastic bollards.

Right. I updated the original post to make it clearer.

[UCFKnights]

There appears to be solid wood fencing blocking one of the paths too, no?

[jakeroot]

There appears to be solid wood fencing blocking one of the paths too, no?

I think that was a temporary fence erected during construction. I think.

Then again, I don't see any other evidence of construction, so it could be a permanent, collapsible barrier.

[tradephoric]

In the project purpose and needs, it listed the operational deficiencies of the existing Parclo B4 interchange:
1.  Left-turning movements on the ramps cause congestion that obstructs through traffic on Muldoon Road;
2.  Lack of gaps in opposing traffic causes extended delays and queuing of vehicles on interchange ramps; 
3.  Bridge clearance for the Glenn Highway is below the minimum height design standard of 18 feet -  6 inches, which impedes over-sized freight movement; and 
4.  Pedestrian facilities are only present on one side of the interchange and the existing sidewalk/trail follows a lengthy and circuitous route, which results in about 1/3 of the pedestrians using an undesignated roadside route.
http://dot.alaska.gov/creg/PDE/projects/54625_Glenn_Muldoon_Interchange/54625_CE.pdf

They didn't need to select a DDI design to address these issues.  A new bridge was needed regardless because the existing one was too low to meet minimum height design standards.  The new wider/higher bridge deck they constructed takes care of points 1, 3, & 4.  They added traffic signals at the new DDI design whereas the existing Parclo B4 design was unsignalized.  Simply adding traffic signals to the existing Parclo B4 design would have taken care of point 2.  I just don't see why a revised Parclo B4 design couldn't have been selected.  The DDI design they chose takes up roughly the same footprint as the old Parclo B4 design so i don't see what they gained with it.

[jakeroot]

They didn't need to select a DDI design to address these issues.

Without quoting everything, I agree with your points.

That said, if I had to think critically about it, I think it was a domino effect that caused the change from Parclo B4 to DDI:

1) There was a lot of left-turning traffic (and queueing), meaning that the loops may have been in the wrong place and that a DDI may actually be a good fit (not tons of through-traffic);
2) the bridge was too low...fine, but replacing it would likely necessitate reconstruction of the ramps to meet the higher bridge; and
3) the pedestrian infrastructure was poor; to make the interchange friendlier, the ramps would (again) have needed reconstruction for less "merging" effects (usually).

I think, when you add everything up, the current interchange would have needed so many modifications to work effectively, total reconstruction (probably the final alternative) was selected.

I don't work in construction, but if I had to surmise an added benefit of total reconstruction, it's that the entire interchange can be [justifiably] closed while construction occurs. Closing Muldoon at Boundary and the Tikahtnu Commons allowed them to negate the need to "work around" traffic, completing the work faster. Changing a diamond into a DDI doesn't require anywhere near the number of modifications, so it's harder to justify complete closure of the interchange while work occurs (case in point: I-5 @ Marvin Road in Lacey, WA).

[jamess]

In the first photo, it appear that pedestrian access from the trail to the overpass is via (covered) stairs.

I dont see an ADA path?

[jakeroot]

In the first photo, it appear that pedestrian access from the trail to the overpass is via (covered) stairs.

I dont see an ADA path?

In the first photo, you can see a stretch of pavement following along the edge of the road, that eventually goes around in a loop to go under the road. You can see this loop in the third photo as well.

[johndoe]

Interesting, thanks for sharing.  I can't think of other "through bypasses" but if you want to explore, this map has most (if not all) the DDI:
https://www.google.com/maps/d/view?mid=1MMZKi0RdQqjZjtBxmSTznUVnurY&ll=29.329761645703215%2C-76.21513359999997&z=2

Someone just posted a similar treatment for a roundabout ramp terminal.

Some DDI end up where you could go "through" depending on crossover location; I know some engineers prefer to have curb/islands prevent that so drivers are clear they must turn left into the closer lanes.

Another interesting thing is the walls in the islands.  I wonder if they're to get oncoming drivers' attention.  Like "hey don't go straight" and it prevents potential head-on crashes and headlight glare. Like fixed objects in center of roundabouts it could be a problem though...

Agreed, Im not sure why DDI is superior to the old parclo (or maybe a signalized version of it).  Note the DDI still needs 6 bridge lanes.  I know some engineers sell that pedestrians can get from one side of arterial to other with DDI (middle peds), but with the grade separated crossings I'm not sure it matters here. 

Forgot to mention: technically the oversized loads couldn't use the previous parclo either.  Interesting that they thought it was important enough to build the "through bypasses" but not important enough to raise the profile a little more.

[jakeroot]

Another interesting thing is the walls in the islands.  I wonder if they're to get oncoming drivers' attention.  Like "hey don't go straight" and it prevents potential head-on crashes and headlight glare. Like fixed objects in center of roundabouts it could be a problem though...

I didn't notice that before. Very interesting. Seems like needlessly-added danger, but I doubt there's much research on the effects of solid objects in pork-chop islands to begin with (so who would really know better?)

[Scott5114]

So, same question I had the last time one of these truck bypass ramps was posted–when someone needs to use the ramp, must they contact ADOT&PF ahead of time to have them remove the bollards? Or is that not necessary, and the truck can just run over the bollards and it's fine?

[vdeane]

All states have height limits beyond which permits are required, so the permit process would likely include scheduling the removal of the bollards.

[jamess]

In the first photo, it appear that pedestrian access from the trail to the overpass is via (covered) stairs.

I dont see an ADA path?

In the first photo, you can see a stretch of pavement following along the edge of the road, that eventually goes around in a loop to go under the road. You can see this loop in the third photo as well.

I see it, now. Thats a good design.