SPUI vs DDI

[tolbs17]

Wonder why they didn't use a SPUI at western Blvd and I-440 like they are using at I-840/Lawndale Dr.

The SPUI takes up less space and you don't have to drive on the wrong side.

I say keep DDIs for existing interchange improvements and SPUIs for a new freeway.

[Ned Weasel]

SPUIs take up more space in the sense that you have to have an extra-wide overpass or underpass to accommodate the left turns meeting at a singular point in the middle.  DDIs are a lot easier to convert from a standard diamond, and they take up roughly the same footprint.

[SkyPesos]

I see DDIs more when there are more traffic getting on the freeway than continuing on the arterial, as left turns onto the freeway are free-flow. Traffic signals are 2 phases instead of 3, though there are 2 sets of signals instead of 1. Left turns ignore the second one though as mentioned already. Also, it's easier to convert from a standard diamond or parclo.

[hbelkins]

I see DDIs more when there are more traffic getting on the freeway than continuing on the arterial, as left turns onto the freeway are free-flow. Traffic signals are 2 phases instead of 3, though there are 2 sets of signals instead of 1. Left turns ignore the second one though as mentioned already. Also, it's easier to convert from a standard diamond or parclo.

I'm not sure what the selling points of a SPUI are, other than a smaller footprint (since the U stands for "Urban") but the DDI, as noted above, is generally implemented when you have a large amount of traffic entering the freeway from the intersecting route.

[SeriesE]

I see DDIs more when there are more traffic getting on the freeway than continuing on the arterial, as left turns onto the freeway are free-flow. Traffic signals are 2 phases instead of 3, though there are 2 sets of signals instead of 1. Left turns ignore the second one though as mentioned already. Also, it's easier to convert from a standard diamond or parclo.

I'm not sure what the selling points of a SPUI are, other than a smaller footprint (since the U stands for "Urban") but the DDI, as noted above, is generally implemented when you have a large amount of traffic entering the freeway from the intersecting route.

It's a bit easier to understand for drivers, and for me the 3 phases doesn't matter since suburban arterials in California has 4 phases anyway and there's 1 fewer light compared to DDI, parclo, or plain diamond so less delays since here the 2 signals are more often not synchronized than do..

[tolbs17]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

[SkyPesos]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I've seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

[tolbs17]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I've seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

BS. Why?

[SkyPesos]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I’ve seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

BS. Why?

The WB 70 ramp onto Mid Rivers backed up onto the freeway frequently because of the left turn, as it's where the mall and other businesses are. Also, there's a left turn right south of the I-70 overpass from SB Mid Rivers to EB Veterans Memorial at the same location as the EB 70 exit ramp, and there's actually a good amount of cars coming off WB 70 using that turn, so that's one benefit of a B4 parclo gone.  I moved out of the St Louis area when the Parclo was getting converted to a DDI, so I don't know how well the DDI works compared to the parclo, but the WB exit ramp changed from 2 exit only lanes to 1 exit only and 1 option lane, so I guess it worked well enough that an option lane could be used on the left instead of an exit only?

[thspfc]

SPUIs work better for high-volume left turn movements.

[KEK Inc.]

SPUIs work better for high-volume left turn movements.

How is it better than a DDI for left-turn movements?  You literally hug the curb when you turn left on a DDI and follow the same signal phase as thru-traffic. 

[kphoger]

The SPUI takes up less space and you don't have to drive on the wrong side.

1.  You don't drive on the wrong side at a DDI either.  You drive on the left side.  And the left side is the correct side.

2.  I'm pretty sure those aren't the only two factors under consideration when choosing between a list of alternatives.

[tolbs17]

The SPUI takes up less space and you don't have to drive on the wrong side.

1.  You don't drive on the wrong side at a DDI either.  You drive on the left side.  And the left side is the correct side.

2.  I'm pretty sure those aren't the only two factors under consideration when choosing between a list of alternatives.

How about an improved diamond interchange?

[1995hoo]

The SPUI takes up less space and you don't have to drive on the wrong side.

1.  You don't drive on the wrong side at a DDI either.  You drive on the left side.  And the left side is the correct side.

2.  I'm pretty sure those aren't the only two factors under consideration when choosing between a list of alternatives.

How about an improved diamond interchange?

Isn't that what a DDI is?

[kphoger]

The SPUI takes up less space and you don't have to drive on the wrong side.

1.  You don't drive on the wrong side at a DDI either.  You drive on the left side.  And the left side is the correct side.

2.  I'm pretty sure those aren't the only two factors under consideration when choosing between a list of alternatives.

How about an improved diamond interchange?

Isn't that what a DDI is?

That's what two of the three letters stand for, after all!

[CtrlAltDel]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I've seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

That was a parclo A4:

[tolbs17]

The SPUI takes up less space and you don't have to drive on the wrong side.

1.  You don't drive on the wrong side at a DDI either.  You drive on the left side.  And the left side is the correct side.

2.  I'm pretty sure those aren't the only two factors under consideration when choosing between a list of alternatives.

How about an improved diamond interchange?

Isn't that what a DDI is?

https://xfer.services.ncdot.gov/PDEA/Web/NC150/r2307_rdy_phm_psh_06.pdf

Look at this. NCDOT went with a regular diamond interchange and they did not push it to be a SPUI or a DDI.

[SkyPesos]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I've seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

That was a parclo A4:

I mix those two up pretty frequently, thanks. So B4 is the variant with the loop ramp off the freeway...
Though I question if a B4 parclo is better than an A4 or DDI, as you're still making a left turn getting onto the freeway. There probably is a reason why there are much more A4s out there than B4s.

[CtrlAltDel]

Nowadays the Parclo B4 would be the alternative if they didn't want to go with a DDI.

I’ve seen a couple of B4 Parclos replaced with a DDI actually. One of them is I-70 exit 222 (Mid Rivers Mall Dr) in MO.

That was a parclo A4:

I mix those two up pretty frequently, thanks. So B4 is the variant with the loop ramp off the freeway...
Though I question if a B4 parclo is better than an A4 or DDI, as you're still making a left turn getting onto the freeway. There probably is a reason why there are much more A4s out there than B4s.

The trick is to remember that "A" means before and "B" means after. Or more seriously, I've long thought that the labels should be reversed because the way they are now is far from intuitive. Or, maybe, go with "dp" for what is currently "A," and "qb" for what is currently "B," since the loop ramps combined with the mainline vaguely look like those letters. The more I think about it, the more I like this idea, sophomoric meaning of "dp" notwithstanding.

Past that, I'm sure tradephoric will be by before too long to point out the relative advantages and disadvantages of these different types of parclo, whatever label you give them. 

[webny99]

How about an improved diamond interchange?

Isn't that what a DDI is?

That's what two of the three letters stand for, after all!

As for the third letter, though, Diverging ≠ Improved.

Or at least, they're not synonyms, even if you could argue that something could be both at once.

[hotdogPi]

If exit numbers are increasing, a parclo A4 would be where an exit would have suffix A as a cloverleaf, and the same for B.

[SkyPesos]

If exit numbers are increasing, a parclo A4 would be where an exit would have suffix A as a cloverleaf, and the same for B.

From what I got from the thread already, I thought there generally wouldn't be exit number suffixes for A4 exits, as it's a single ramp.

[tolbs17]

A disadvantage of a SPUI interchange is that you cannot make a U-turn going in the opposite direction compared to the other interchanges.

[SkyPesos]

A disadvantage of a SPUI interchange is that you cannot make a U-turn going in the opposite direction compared to the other interchanges.

I though SPUIs are one of the easier interchanges to make a U-turn.

[Revive 755]

I've heard SPUI's work better than DDI's when there are high through volumes on the road crossing the freeway and signal coordination is a concern.