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View Full Version : SPUI vs Diverging Diamond


BostonUrbEx
02-02-2013, 12:30 PM
Does either have a particular use? They're both used in preference to a regular diamond under incredible demand and space constraints, but do they have particular uses which would make one preferable to the other?

Commuting Boston Student
02-02-2013, 01:29 PM
Does either have a particular use? They're both used in preference to a regular diamond under incredible demand and space constraints, but do they have particular uses which would make one preferable to the other?

From the Ask me about being a Traffic Engineer! (http://forums.somethingawful.com/showthread.php?threadid=3177805) thread over on Something Awful:

http://i30.tinypic.com/3rntf.png

Here is a DDI! It has the same number of signals, but each one only has 2 phases, making them more efficient. Additionally, there are no left turns. The weird part is that, on the overpass, traffic's driving on the "wrong" side of the road. Pretty cool, eh?

http://i26.tinypic.com/28iua7s.png

Single Point Urban Interchange, or SPUI for short. They're becoming quite popular due to their need for only one signal, and the efficiency of pushing through pairs of left turns at once. Unfortunately, they require a very wide bridge.

My takeaway from this is that you use an SPUI any time you can afford to piss money into a super-wide bridge but can't afford an extra signal for some reason?

Or, I guess if you're super concerned about drivers freaking out due to being on the left in a DDI. I don't know. The SPUI looks terrible and worse in literally every respect to a DDI.

PS: Buy a Something Awful account. It's the best $10 you'll spend today, plus you'll be able to ask questions like these directly to the source! You won't regret it!

Ron Newman
02-02-2013, 02:31 PM
Which of these designs is better for pedestrians walking along the smaller street?

kz1000ps
02-02-2013, 03:25 PM
Silly Ron, Trix are for kids!

Commuting Boston Student
02-02-2013, 05:00 PM
Which of these designs is better for pedestrians walking along the smaller street?

None of the above.

BostonUrbEx
02-04-2013, 09:35 PM
Which of these designs is better for pedestrians walking along the smaller street?

Very good consideration.

I would presume the SPUI. Either one would require pedestrians to cross traffic four times. However, only the SPUI can provide a dedicated signal period for pedestrians. But, keep in mind that this potential crossing period would still only be for just two of the four crossing points.

Commuting Boston Student
02-04-2013, 10:16 PM
Very good consideration.

I would presume the SPUI. Either one would require pedestrians to cross traffic four times. However, only the SPUI can provide a dedicated signal period for pedestrians. But, keep in mind that this potential crossing period would still only be for just two of the four crossing points.

It really isn't a very good consideration, because by the time you even need to consider breaking out an interchange like either of those you've got two high-speed (or relatively high-speed) arterials that are actively hazardous to pedestrians.

The better design for pedestrians walking along the street is a proper signalized intersection.

Ron Newman
02-04-2013, 10:38 PM
I'm assuming that the underpass road is a freeway (so, no pedestrians there) but the other road is an ordinary city street.

Commuting Boston Student
02-04-2013, 11:30 PM
I'm assuming that the underpass road is a freeway (so, no pedestrians there) but the other road is an ordinary city street.

In that case, a simple pair of signalized on/off ramps will suffice.

Both of those designs are way too overbuilt for your average city street. I wouldn't consider one for any road smaller than, say, Route 1 in Saugus.

vanshnookenraggen
02-04-2013, 11:53 PM
There is a video somewhere of a guy who walks across a Diverging Diamond interchange as a pedestrian and it's completely ridiculous (in a bad way).

kz1000ps
02-05-2013, 02:05 AM
Both of those designs are way too overbuilt for your average city street. I wouldn't consider one for any road smaller than, say, Route 1 in Saugus.

Right; these are almost exclusively for areas with heavy suburban traffic. For instance my home city of Albany just got a SPUI on its busiest highway in its largest suburb, Colonie, and is right by the airport and big box central:

http://imageshack.us/a/img9/9062/screenshot20130205at232.jpg

Ron, here's the pedestrian accommodations:

http://imageshack.us/a/img443/2005/screenshot20130205at240.jpg

And on a side note, I'm taken by this bridge...they went for the deco look and pretty much nailed it!

http://img547.imageshack.us/img547/6020/screenshot20130205at243.jpg

BostonUrbEx
02-05-2013, 06:19 AM
It really isn't a very good consideration, because by the time you even need to consider breaking out an interchange like either of those you've got two high-speed (or relatively high-speed) arterials that are actively hazardous to pedestrians.

The better design for pedestrians walking along the street is a proper signalized intersection.

The funny thing about pedestrians is that you find them on every road, regardless of volume or speed. Even Rt 1 has pedestrians. So, yes, good consideration.

datadyne007
02-05-2013, 07:16 AM
That SPUI's quite beautiful, but there's legit no way for pedestrians to cross that street. I personally don't see this as a problem because these are made for major interchanges in auto-centric areas. You would never ever walk along either of those roads.

Commuting Boston Student
02-05-2013, 09:01 AM
That SPUI's quite beautiful, but there's legit no way for pedestrians to cross that street. I personally don't see this as a problem because these are made for major interchanges in auto-centric areas. You would never ever walk along either of those roads.

Right, and that's not necessarily a bad thing, either.

I certainly wouldn't want to encourage pedestrians anywhere in the vicinity of (to pick an example where I think a DDI would be appropriate) the 24/44 interchange in Raynham. And where there's a significant number of pedestrians actively endangering themselves as BostonUrbEx suggests is happening on Route 1 in Saugus, the appropriate course of action to take is to either downgrade the road or provide completely separate pedestrian facilities.

(And for the record, there's more than enough room to provide a separated pedestrian path with pedestrian overpasses on Route 1. I think I'd prefer to see it put on a road diet, but if that's not an option...)

vanshnookenraggen
02-05-2013, 10:24 AM
http://imageshack.us/a/img9/9062/screenshot20130205at232.jpg


Latham Circle Mall.... wow, memories.

Corey
02-05-2013, 11:32 AM
There is a video somewhere of a guy who walks across a Diverging Diamond interchange as a pedestrian and it's completely ridiculous (in a bad way).

You may be referencing this video (http://www.theatlanticcities.com/design/2011/11/diverging-diamond-interchange-abomination/453/) (I happened to see it posted on Reddit the other day), which provides a tour of the pedestrian and bicycle accommodations on a diverging diamond interchange in suburban Missouri. As most have pointed out, these sorts of designs are primarily intended for auto-centric areas wouldn't be as feasible in a city environment. The one posted above in Albany actually looks pretty accommodating for pedestrians, not much worse than a cloverleaf intersection (although that isn't saying much).