Topic: Parclo B4 interchanges (Read 9249 times)

tradephoric · « **on:** October 28, 2012, 05:52:39 PM »

tradephoric · « **Reply #1 on:** October 28, 2012, 07:46:35 PM »

A major advantage of the Parclo B4 interchange is the improved traffic signal progression along the arterial it provides. Traffic signals along a Parclo B4 only stop one direction of travel along the arterial making it extremely easy to provide good signal progression. Here is an example of a Parclo B4 interchange near Miami, Florida and the corresponding time-distance diagram:

green circular icons = traffic signals that stop only one direction of travel
red circular dots = traffic signals that stop both directions of travel

Had this interchange been designed as a Parclo A4, the time distance would look similar to this:

The problem with a Parclo A4 interchange is that the on/off ramp traffic signals stop both directions of travel along the arterial. This leads to closely spaced traffic signals that becomes nearly impossible to provide good two-way progression (good traffic signal progression in one-direction leads to poor progression in the opposite direction).

roadfro · « **Reply #2 on:** October 29, 2012, 05:14:46 AM »

^ Ah, but you wouldn't necessarily be running a 50/50 split at these intersections...might lead to some better timing possibilities if traffic volumes aren't assumed to require equal time on street and off ramp.

NE2 · « **Reply #3 on:** October 29, 2012, 05:22:19 AM »

Thanks for not giving a list of B4s but only unlabeled links. That way this thread will (hopefully) be less of a "hey look I know another one" and more of a meaningful discussion.

I suppose problems with the B4 are (in rough decreasing order of importance):
*Left turns come off the surface road, so a busy left turn needs more median width and more length, perhaps extending to the bridge
*Slowing down from freeway speeds to the loop may cause loss of control (but in an A4 it's harder to merge from the loop) - this was previously discussed in another thread
*Right turn from the freeway has no light, leading to possible weaving issues with adjacent intersections - but a light can be added without damaging progression, since it doesn't need to be synced with the nearby left turn
*Two exits rather than one, meaning more sign clutter on the freeway

tradephoric · « **Reply #4 on:** October 29, 2012, 08:11:03 AM »

Quote from: roadfro on October 29, 2012, 05:14:46 AM

^ Ah, but you wouldn't necessarily be running a 50/50 split at these intersections...might lead to some better timing possibilities if traffic volumes aren't assumed to require equal time on street and off ramp.

I chose a 50/50 split to highlight progression theory and agree that a more practical split for the off-ramp signals may have a much larger green band for the arterial. Also, in practice it’s much more common to have simultaneous greens between the closely spaced off-ramp signals to provide better progression for left-turning vehicles coming off the freeway (with green-to-green progression between the off-ramp signals left-turners off the freeway are coming straight to a red at the other off-ramp signal leading to backups on the bridge deck). Even with a more practical setup selected you can see on the time-distance diagram that progression isn’t much improved.

ARMOURERERIC · « **Reply #5 on:** October 29, 2012, 08:28:35 AM »

Plus, California seems to prefer the A4 so that the ramps can be configured as to provide a freeway bypass if there is an accident on the mainline between the on and off ramp, an aspect not available with the B4

NE2 · « **Reply #6 on:** October 29, 2012, 08:36:08 AM »

Quote from: ARMOURERERIC on October 29, 2012, 08:28:35 AM

Plus, California seems to prefer the A4 so that the ramps can be configured as to provide a freeway bypass if there is an accident on the mainline between the on and off ramp, an aspect not available with the B4

Sure it is, if you merge the right turn ramp in before the left turn ramp. Anyway, how often does this happen? I doubt it's frequent enough for Caltrans to change an interchange design.

tradephoric · « **Reply #7 on:** October 29, 2012, 08:39:08 AM »

The "practical" time-distance diagram highlights another problem with the Parclo A4 in that drivers wishing to make a left exiting the freeway can experience lengthy delays, especially when the arterial is soaking up most of the green time. In the above time-distance diagram not only are drivers experiencing poor signal progression along the arterial, but drivers exiting the freeway who need to turn left have nearly 2-minutes to wait if they just missed their green.

With a Parclo B4 all vehicles exiting the freeway are free flowing with no stop light control (like the Miami B4 interchange which can lead to weaving issues that NE2 described) or need only make a simple right turn when the off/ramp is signalized (when weaving is an issue).

haljackey · « **Reply #8 on:** October 30, 2012, 12:40:35 AM »

I like the A4 better.

tradephoric · « **Reply #9 on:** October 30, 2012, 08:41:52 AM »

The Parclo B4 interchange has similar design characteristics to the superstreet. Here’s a comparison of a superstreet in Troy, MI to a Parclo B4 interchange in Dallas, TX:

Superstreet in Troy, Michigan:

Parclo B4 in Dallas, Texas:

A major benefit of the superstreet is that it improves traffic flow along the major road (the design essentially splits up a two-way road into two one-way streets allowing for good traffic signal progression regardless of signal spacing). Researchers from North Carolina State University have found that superstreets reduce travel times by 20%, reduce total accidents by 46%, and reduce personal injury accidents by 63% when compared to conventional intersection designs.

A major disadvantage of the superstreet is that traffic along the minor road wishing to turn left or go straight are not permitted to do so. They must first make a right at the major road and then make a U-turn at a crossover along the median. However, a Parclo B4 interchange doesn’t experience this disadvantage as drivers coming off the freeway decide which off-ramp to take so that they are traveling in their desired direction after making a right turn (no u-turn required).

The Parclo B4 interchange may see the same safety benefits and travel time reductions as a superstreet without dealing with the same disadvantages.

hbelkins · « **Reply #10 on:** October 30, 2012, 10:09:32 AM »

Your second Kentucky example is US 127 and the Bluegrass Parkway. This was formerly one of the "toll booth" interchanges where all four ramps were loops. After the toll booth was taken out, the diamond ramps were added because this was the heaviest volume of the other two toll booth exits.

I know the first example (Watterson Expressway at Taylorsville Road) was rebuilt, but I don't remember what the old exit's footprint was like.

tradephoric · « **Reply #11 on:** November 02, 2012, 10:53:40 AM »

Left-turns are required at both the Parclo A4 and Parclo B4 interchanges. Drivers turning left from the arterial onto the freeway (Parclo B4 design) only need to cross one direction of travel along the arterial while drivers turning left exiting the freeway (Parclo A4 design) must cross both directions of travel. I've read literature that indicates the Parclo A4 interchange is more desirable because it eliminates left-turns from the arterial. While this is true, the Parclo A4 interchange doesn't eliminate left turns coming off the freeway (which is potentially a more dangerous movement due to the fact drivers must cross both directions of travel along the arterial).

Parclo A4 off-ramp where drivers cross both directions of travel along the arterial:

Parclo B4 on-ramp where drivers cross only one-direction of travel along the arterial:

tradephoric · « **Reply #12 on:** November 02, 2012, 02:42:26 PM »

Another advantage of the Parclo B4 interchange is that it helps distributes traffic more evenly along the arterial. Having on-ramps on each side of the arterial helps prevents uneven queues along the arterial as half of the entering freeway traffic merges to the left lane and half merge to the right lane along the arterial.

The Parclo A4 interchange at the top of the picture may be leading to uneven queues along the arterial as drivers wishing to enter the freeway fight to get into the right-lane.

tradephoric · « **Reply #13 on:** November 07, 2012, 01:02:26 PM »

http://maps.google.com/?ll=40.44743,-80.16609&z=17&t=h

One near Pittsburgh added to the list.

johndoe · « **Reply #14 on:** November 16, 2012, 10:44:31 AM »

Interesting research! On these time space diagrams, have you experimented with actual volumes, or is this all theoretical?

I think one thing the interchanges around here struggle with is space: loop ramps wouldn't fit properly, and outer roads are way too close. Even though you could provide progression for throughs on the arterial, they're going to run into issues with the adjacent signals that have more phases.

Maybe the biggest concern I would have would be confused drivers on the freeway taking the wrong exit. The Washington and Texas examples fix this by using a CD system, but of course that's more bridge cost.

It's like we tell people at public meetings: There is not one magic solution that can be plopped into every location...it all depends on the traffic patterns and geometric constraints.

tradephoric · « **Reply #15 on:** November 27, 2012, 12:19:08 PM »

Quote from: johndoe on November 16, 2012, 10:44:31 AM

Interesting research! On these time space diagrams, have you experimented with actual volumes, or is this all theoretical?

It is mainly theoretical to highlight how a Parclo A4 interchange can lead to poor two-way signal progression along an arterial regardless of how short the green for the off-ramp is... basically poor signal spacing is the main issue.

Quote from: johndoe on November 16, 2012, 10:44:31 AM

I think one thing the interchanges around here struggle with is space: loop ramps wouldn't fit properly, and outer roads are way too close. Even though you could provide progression for throughs on the arterial, they're going to run into issues with the adjacent signals that have more phases.

Agree with all of these comments.

Quote from: johndoe on November 16, 2012, 10:44:31 AM

Maybe the biggest concern I would have would be confused drivers on the freeway taking the wrong exit. The Washington and Texas examples fix this by using a CD system, but of course that's more bridge cost.

The C/D lanes do help alleviate this problem and is a valid point. I personally would have the 0.1% of drivers who don't know where they are going making u-turns along the arterial because they took the wrong exit than inconvenience the 99.9% of the rest of the drivers who know where they want to go. It's like driving around Detroit... occasionally you see the oblivious driver who is new to the area cutting across 3-lanes of traffic to successfully complete their first Michigan Left turn. The rest of us are amused and congratulate their accomplishment by giving them the one-finger salute.

Quote from: johndoe on November 16, 2012, 10:44:31 AM

It's like we tell people at public meetings: There is not one magic solution that can be plopped into every location...it all depends on the traffic patterns and geometric constraints.

Someone should have told the Ontario Ministry of Transportation that they could plop something in other than a Parclo A4 interchange!
Here's a map of all the Parclo A4's in Toronto:

jakeroot · « **Reply #16 on:** August 04, 2017, 07:48:33 PM »

A Parclo B4 interchange in the UK:

A21/Sevenoaks Bypass at Westerham Road, just south of the M25/M26 junction: https://goo.gl/fFyLsF

jakeroot · « **Reply #17 on:** April 29, 2019, 03:08:54 AM »

I found this thread many years ago...never have forgotten about it, as it has a great listing of Parclo B4 interchanges (thanks tradephoric).

The two British Columbia Parclo interchanges listed are no longer in operation. The first link (Hwy 1 @ Brunette Ave) is now a half-diamond/half-parclo, and the second link (Hwy 1 @ Willingdon Ave) is now a folded diamond (though totally rebuilt, with the loops now in the opposite corners).

The only remaining Parclo B4 interchange in British Columbia, which was not listed, is Hwy 91 @ Nordel Way, at the south end of the Alex Fraser Bridge. It was built in 1985 (top right corner of photo), and was rebuilt in 1997.

The rebuild in 1997 did several interesting things:

* upgraded the SW loop to two lanes, which then required three eastbound lanes over Hwy 91.
* westbound through movement restricted down to one lane (previously two)
* limited westbound capacity required the rebuild of the NW loop to get rid of what would have been an extremely short merge (old roadbed was visible for a while)
* the left turn for southbound Hwy 91 traffic had to be relocated west, off the bridge, the only remaining place where there was room for a left turn lane

Technically it has been modified twice since: in 2005, to add an HOV bypass to the northbound on-ramp, and then again very recently to add a left turn light for northbound Hwy 91 traffic. That actually highlights the incredible capacity of the Parclo B4...it was built 34 years ago, and operated without any signals for 33 of those years. Very impressive stuff.

Buck87 · « **Reply #18 on:** April 29, 2019, 10:28:14 AM »

Quote from: tradephoric on October 28, 2012, 05:52:39 PM

Ohio
http://maps.google.com/maps?ll=40.779124,-82.470337&z=17&t=h&hl=en

This one dates back to either the 50's or 60's and has some interesting geometry, including a surface street (Beal Rd) that intersects a pair of the ramps. I once went to a funeral at the Open Bible Church immediately north of this interchange and found it pretty neat at the time that we had to use part of the US 30 entrance ramp to get there.

Quote

http://maps.google.com/maps?ll=41.404642,-82.11731&z=17&t=h&hl=en

This one used to be a full cloverleaf. It was technically a Parclo B4 for a few years when the two loop ramps from OH 57 to OH 2 were removed, but it has since been converted to a diamond (as shown on Google maps)

X99 · « **Reply #19 on:** April 29, 2019, 10:29:25 AM »

Quote from: jakeroot on April 29, 2019, 03:08:54 AM

The two British Columbia Parclo interchanges listed are no longer in operation.

Neither is the one in Alaska. Google Maps Satellite shows the parclo, but the road lines from the map show it as a DDI.

tradephoric · « **Reply #20 on:** April 29, 2019, 01:13:28 PM »

Quote from: jakeroot on April 29, 2019, 03:08:54 AM

I found this thread many years ago...never have forgotten about it, as it has a great listing of Parclo B4 interchanges (thanks tradephoric).

Thanks Jake! My borderline infactuation with the Parclo B4 has gotten me some flack in the past, but i remember i enjoyed researching the Parclo B4 and putting together that somewhat comprehensive list. Over the years i have put together a few Parclo B4 models (or variations thereof) of Michigan interchanges that i think would work well. The first model is just a standad conversion from a full cloverleaf to a Parclo B4, but the other two models are Parclo B4s with contraflow lefts. While this type of design may not be practical in many parts of the country, Metro Detroit with it's abundance of wide-boulevards makes this type of design somewhat practical (as it wouldn't require much additional ROW).

https://www.google.com/maps/@43.4147058,-83.8910697,539m/data=!3m1!1e3

https://www.google.com/maps/@42.4969279,-83.4450483,778m/data=!3m1!1e3

https://www.google.com/maps/@42.562294,-83.1584594,460m/data=!3m1!1e3

News:

Author Topic: Parclo B4 interchanges (Read 9249 times)