SPUI vs DDI

[froggie]

Huh, so you prefer the less common one of the two. I don't think I've ever seen a B4 parclo, almost always A4.

Have you not been to Xenia?  US 35/US 42 interchange is a B4.

[kphoger]

How is a Parclo A4/B4 better than a DDI and SPUI?

Because |tradephoric| says so.

[tolbs17]

How is a Parclo A4/B4 better than a DDI and SPUI?

Because |tradephoric| says so.

But this interchange isn't one of them.

[kphoger]

How is a Parclo A4/B4 better than a DDI and SPUI?

Because |tradephoric| says so.

But this interchange isn't one of them.

What does that have to do with |tradephoric|?

[tolbs17]

How is a Parclo A4/B4 better than a DDI and SPUI?

Because |tradephoric| says so.

But this interchange isn't one of them.

What does that have to do with |tradephoric|?

Because he's against these type of interchanges.

[jakeroot]

Tradephoric says so, but I would tend to agree as well. Parclos are literally a cheap cloverleaf that has been modified to remove the weaving, replacing it with two (or possibly zero) signals that are easily implemented along a corridor. It really is a genius design that definitely is not efficient on space but is otherwise pretty neat. DDIs and SPUIs are certainly popular because they are easily crammed in to tight ROW, but I don't think that means they are appropriate everywhere. If there is room for free-flow, there is no reason to not go that route.

My main beef with SPUIs is that they rarely support permissive lefts. I seem to be the only roadgeek who appreciates the ability to turn through gaps.

Because he's against these type of interchanges.

That may be, but your random link to a North Carolina SPUI is non-sequitur: it had nothing to do with kphoger's point that Parclos are better because "tradephoric says so".

[SkyPesos]

Huh, so you prefer the less common one of the two. I don't think I've ever seen a B4 parclo, almost always A4.

Have you not been to Xenia?  US 35/US 42 interchange is a B4.

Been there. My mind was blank, as A4 Parclos seem much more familiar to me. Drove past another B4 Parclo today at I-69 and IN 46. Also, the I-376 and US 22/PA 60 interchange (which I've been through before too) is a B4 parclo with a missing movement.

[tradephoric]

I like the Parclo B4 as it performs better in nearly every measure of performance compared to the Parclo A4.


COMPARATIVE ANALYSIS BETWEEN THE DIVERGING DIAMOND INTERCHANGE AND PARTIAL CLOVERLEAF INTERCHANGE USING MICROSIMULATION MODELING
http://fau.digital.flvc.org/islandora/object/fau%3A3779

[SkyPesos]

I could see how a B4 would work better than an A4 thinking about it. The through traffic on the surface road only have to deal with one traffic signal in the interchange, instead of two signals in an A4.

^ On the chart above, what's the difference between a DDI-4 and a DDI-6?

[froggie]

^ It's in the literature tradephoric linked to.  DDI-X refers to X number of through lanes on the cross road.

[tradephoric]

I could see how a B4 would work better than an A4 thinking about it. The through traffic on the surface road only have to deal with one traffic signal in the interchange, instead of two signals in an A4.

^ On the chart above, what's the difference between a DDI-4 and a DDI-6?

The 4 and 6 represent the number of through lanes at the DDI. The Parclo models had 4 though lanes with either an acceleration lane (Parclo B4) or deceleration lane (Parclo A4) on the bridge deck.  The DDI models didn't include a deceleration lane on the bridge deck so the researchers decided to include a DDI-6 model as the number of bridge deck lanes would be the same as the Parclo models.  However, there are plenty of Parclo B4s that don't have an acceleration lane over the bridge while there are also plenty of DDIs that do have a deceleration lane over the bridge.  While i question the fairness of the methodology, the Parclo B4 model still performed exceptionally well. 

[Henry]

I'm afraid to go through a DDI, fearing that I'll accidentally go onto the right side of the road (which in this case would be wrong). Give me the SPUI any day.

[tradephoric]

Two massive interchanges from Florida, a DDI and a Parclo B4.  The I-75/Sheridan interchange was recently constructed and i think it would have been a perfect location for a contraflow Parclo B4.  The advantageous of this design are:

*traffic turning left onto the freeway only have to navigate one traffic signal as opposed to two.
*traffic turning left onto the freeway only have to cross 3-lanes of traffic as opposed to 5-lanes.
*prevents left-turns from gridlocking when queues back up into the intersection.
*it's kick ass!


https://www.youtube.com/watch?v=0v1QPaYISFA&t=15s

[jakeroot]

One other design feature that I particularly like about B4 Parclos is two off-ramps rather than just one. This provides two separate areas for vehicle storage, massively reducing the chance of vehicles backing up onto the freeway. Parclo B4's can be free-flowing, but assuming a pedestrian-friendly, NTOR multiple-turn lane design, you would want a lot of storage. And you definitely get that with two separate ramps. Free-flow ramps can overwhelm the arterial, and a single off-ramp can backup to the mainline (A4 style), but a signalized design meets in the middle.

There are three B4-style off-ramps here in Tacoma, but only one has fully-signalized off-ramps. Here at 74th, you can see the off-ramp split. Total off-ramp length from the gore point of I-5 Southbound to both stop lines is ~ 3430 feet, or just under two-thirds of a mile (the double right to westbound 74th is a rare-for-WA double-right NTOR). For a relatively tight urban design, it's remarkably efficient. Needless to say, backups to the mainline simply do not occur. It was built in the 80s (note the overpass is a ridiculous eight lanes), but certainly may never need rebuilding ever again.

[tolbs17]

I guess this interchange would be called a "Parclo B3".

While that interchange looks overbuilt, 80% of the traffic uses the loop to exit.

https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=b7a26d6d8abd419f8c27f58a607b25a1

[tradephoric]

Exiting a freeway at an unsignalized Parclo A4 versus entering a freeway at an unsignalized Parclo B4.  As a driver which would you rather encounter?  The turning movements at a Parclo B4 interchange are simple and helps explain why there are so many unsignalized Parclo B4s.

One other design feature that I particularly like about B4 Parclos is two off-ramps rather than just one. This provides two separate areas for vehicle storage, massively reducing the chance of vehicles backing up onto the freeway.

I do like the added off ramp queue space the Parclo B4 provides.  In addition, the Parclo B4 only has one on ramp with traffic merging onto the freeway as opposed to two at a Parclo A4.  Limiting the number of on ramps where traffic is merging onto the freeway sounds like a net benefit for safety too.

[kphoger]

I'm afraid to go through a DDI, fearing that I'll accidentally go onto the right side of the road (which in this case would be wrong). Give me the SPUI any day.

Having driven through both kinds plenty of times, I actually worry about conflicting with oncoming traffic at a SPUI more than I do at a DDI.  I've never once been at all unclear about where to go at any DDI I've used.

[jakeroot]

I guess this interchange would be called a "Parclo B3".

While that interchange looks overbuilt, 80% of the traffic uses the loop to exit.

https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=b7a26d6d8abd419f8c27f58a607b25a1

You need to check your link. Google Maps might be better than ArcGIS Online.

One other design feature that I particularly like about B4 Parclos is two off-ramps rather than just one. This provides two separate areas for vehicle storage, massively reducing the chance of vehicles backing up onto the freeway.

I do like the added off ramp queue space the Parclo B4 provides.  In addition, the Parclo B4 only has one on ramp with traffic merging onto the freeway as opposed to two at a Parclo A4.  Limiting the number of on ramps where traffic is merging onto the freeway sounds like a net benefit for safety too.

Certainly possible. Having a single on ramp seems like it might help with queue management as well, especially when used in tandem with ramp meters. Rather than setups like this, where one ramp could become much busier than the other, with no way for one ramp to relieve the other.

I would contend the opposite issue (queue management issues with multiple off-ramps) is less problematic as you can add turn lanes, improve signal timing, add roundabouts, etc to improve flow onto the arterial if it actually becomes a problem (rare from my experience). Freeways are rather sacred in comparison and need to be managed more carefully, due to high speeds and limited "creative" alternative design options.

[kphoger]

I guess this interchange would be called a "Parclo B3".

While that interchange looks overbuilt, 80% of the traffic uses the loop to exit.

https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=b7a26d6d8abd419f8c27f58a607b25a1

I'm seeing a four-quadrant interchange with a loop ramp beyond the crossroad.  Using the typical Ontario terminology, that would make it a B4, not a B3.

[kphoger]

I guess this interchange would be called a "Parclo B3".

While that interchange looks overbuilt, 80% of the traffic uses the loop to exit.

https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=b7a26d6d8abd419f8c27f58a607b25a1

You need to check your link. Google Maps might be better than ArcGIS Online.

I went ahead and zoomed in.  Specifically, 81% of the total exiting traffic on eastbound US-264 uses the loop ramp (to US-301 North), with the remaining 19% using the direct ramp (to US-301 South).

But |tolbs17| is missing the bigger picture:  the on-ramp there has an AADT that's 40% higher than both of them put together.  If you moved the loop ramp to the other side of US-301, changing it from B4 to A4 configuration, then the loop ramp would be carrying 88% more traffic than the adjacent off-ramp–which you might notice is more than the existing 81% difference.

For what it's worth, exiting traffic is only 8% of the total eastbound US-264 traffic., and the AADT of the existing loop ramp is only 1500.

[jakeroot]

I guess this interchange would be called a "Parclo B3".

While that interchange looks overbuilt, 80% of the traffic uses the loop to exit.

https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=b7a26d6d8abd419f8c27f58a607b25a1

You need to check your link. Google Maps might be better than ArcGIS Online.

I went ahead and zoomed in.  Specifically, 81% of the total exiting traffic on eastbound US-264 uses the loop ramp (to US-301 North), with the remaining 19% using the direct ramp (to US-301 South).

But |tolbs17| is missing the bigger picture:  the on-ramp there has an AADT that's 40% higher than both of them put together.  If you moved the loop ramp to the other side of US-301, changing it from B4 to A4 configuration, then the loop ramp would be carrying 88% more traffic than the adjacent off-ramp–which you might notice is more than the existing 81% difference.

For what it's worth, exiting traffic is only 8% of the total eastbound US-264 traffic., and the AADT of the existing loop ramp is only 1500.

Interesting. When I open the link, the ArcGIS map's extent is set to the entire state of North Carolina. I only was able to find the interchange in question thanks to your comments (US-264 @ US-301).

As to your observation: I completely agree that the overall picture is key to correctly assessing the success of an interchange. To be fair, such an assessment may actually show A4 parclos to be better under certain circumstances: perhaps there is so much entering traffic that having only one on-ramp with traffic taking turns is going to be worse for flow than an on-ramp loop and adjacent direct right turn ramp. But, such a situation seems much less common, and backups from a single off-ramp (A4 style) seem like a bigger issue that is more easily addressed with off-ramp loops and direct ramps.

[kphoger]

When I open the link, the ArcGIS map's extent is set to the entire state of North Carolina. I only was able to find the interchange in question thanks to your comments (US-264 @ US-301).

Same here.  I got the location from his hyperlink, then found that location on the NCDOT map and zoomed in.

[jakeroot]

When I open the link, the ArcGIS map's extent is set to the entire state of North Carolina. I only was able to find the interchange in question thanks to your comments (US-264 @ US-301).

Same here.  I got the location from his hyperlink, then found that location on the NCDOT map and zoomed in.

I'm an idiot. I didn't notice that "this" was a hyperlink in his original post. I was trying to find the Parclo using the ArcGIS map.

THIS is why I'm glad you and I underline our links by default. So much easier to spot.

[kphoger]

I didn't notice that "this" was a hyperlink in his original post.

THIS is why I'm glad you and I underline our links by default. So much easier to spot.

  Glad to know someone noticed!

[tolbs17]

And there's another interchange close by. So some traffic uses that one as well. While AADT might not look high enough to warrant signals, you are crossing a wide 6-lane boulevard therefore that's why it's signalized.