Double Trumpet interchange (DDI alternative)

[tradephoric]

Would this double trumpet interchange be a better alternative to the massive DDI's found in Florida?  I juxtaposed the University Parkway and I-75 DDI over the interchange.  For roughly the same ROW as the DDI you can have a completely free flowing interchange.  Considering the closest drive is over 1500 feet away from the interchange, i believe a free flowing interchange would function well here.

https://www.youtube.com/watch?v=UhDyyx6eCSE&feature=youtu.be 

[jmacswimmer]

The SC 31/US 501 interchange near Myrtle Beach uses that exact design.

[wanderer2575]

Ironically, this was the design of the I-94/US-24 interchange in Taylor, Michigan before it was replaced with a traditional DDI SPUI.  Only difference is that the east-west carriageways were separated with a very wide median so the loop ramps were not over them.  This might work in Florida, but the loop ramp bridges would be a bugger during winter in the northern states.

(edited to say what I meant, not what I said.)

[tradephoric]

The SC 31/US 501 interchange near Myrtle Beach uses that exact design.

It's actually slightly different.  The Myrtle Beach interchange has exiting loop ramps while the SYNCHRO model has entering loop ramps.  The SYNCHRO model more closely mirrors the now defunct interchange at I-94/US-24 referenced by wanderer2575:

[tradephoric]

Here are a few advantages of the double trumpet versus the DDI:

1.  Fewer pedestrian conflicts.  Pedestrians cross 2 lanes of traffic as opposed to up to 14 at the DDI.
2.  Shorter bridge deck.  Freeway crosses 8 lanes of arterial traffic as opposed to 12 at the DDI.
3.  Less driver delay.  Free flowing interchange vs. signal controlled interchange at the DDI.

[jakeroot]

Definitely better for pedestrians, although I suppose that term is relative when we're talking about an interchange with this many lanes and all the associated woes (noise especially).

In all, I like it, although a simple Parclo B4 with signalized off-ramps might be easier? Seems like this interchange would have quite a bit of weaving with traffic attempting to reach the far lanes from the left-side entrances, although that's nitpicking.

Honestly, you could propose a regular diamond, and I'd be happier with it then the monstrosity that was actually built. We will look back on this interchange in 30 years and laugh at how much overbuilding we used to do.

[Ned Weasel]

Definitely better for pedestrians, although I suppose that term is relative when we're talking about an interchange with this many lanes and all the associated woes (noise especially).

The free-flowing interchange being better for pedestrians is debatable.  With a DDI, pedestrians have a signalized crossing.  With the free-flowing "double trumpet," pedestrians have to cross without a signal unless you add pedestrian signals to the ramps.

In all, I like it, although a simple Parclo B4 with signalized off-ramps might be easier? Seems like this interchange would have quite a bit of weaving with traffic attempting to reach the far lanes from the left-side entrances, although that's nitpicking.

A barrier to keep left-entering traffic from taking that immediate first right is definitely warranted.  But other than that, there seems to be a decent amount of space for lane changes, although that would depend on how many lane changes one needs to make and how quickly one needs to take a right turn or right exit.

Honestly, you could propose a regular diamond, and I'd be happier with it then the monstrosity that was actually built. We will look back on this interchange in 30 years and laugh at how much overbuilding we used to do.

What in the world advantage does a regular diamond ever have over a DDI except for the potentially smaller footprint if you squeeze it down to the bare minimum?  With a DDI, your signals are only two phases each, and all movements are protected (except when they don't protect the left-turning movement: https://goo.gl/maps/jeiif5LpW7Qyjarz5 .  Try watching your left mirror to be sure your trailer isn't hitting anything while looking to the right for conflicting traffic--that ain't fun!).  But with a diamond, you either have to settle for unprotected left turns crossing oncoming traffic, or suffer through a pair of three-phase signals.

Of course, if you want a really signal-efficient diamond, you could make it RIRO and then use turnarounds: https://goo.gl/maps/msgcHN6SaXsUxGUn6 .  But then, you have to have enough real estate for the turnarounds, whether you use jughandles or Michigan Lefts.

[tradephoric]

The free-flowing interchange being better for pedestrians is debatable.  With a DDI, pedestrians have a signalized crossing.  With the free-flowing "double trumpet," pedestrians have to cross without a signal unless you add pedestrian signals to the ramps.

That University Parkway DDI doesn't eliminate non-signalized crossings.  On top of the 13 lanes of signalized crossings, pedestrians must cross an unsignalized lane as well.

https://www.google.com/maps/@27.388402,-82.4511889,3a,75y,71.4h,89.31t/data=!3m7!1e1!3m5!1sM_MhvA5UUfXV7qegu8rQmg!2e0!6s%2F%2Fgeo1.ggpht.com%2Fcbk%3Fpanoid%3DM_MhvA5UUfXV7qegu8rQmg%26output%3Dthumbnail%26cb_client%3Dmaps_sv.tactile.gps%26thumb%3D2%26w%3D203%26h%3D100%26yaw%3D58.707672%26pitch%3D0%26thumbfov%3D100!7i16384!8i8192

[jakeroot]

Definitely better for pedestrians, although I suppose that term is relative when we're talking about an interchange with this many lanes and all the associated woes (noise especially).

The free-flowing interchange being better for pedestrians is debatable.  With a DDI, pedestrians have a signalized crossing.  With the free-flowing "double trumpet," pedestrians have to cross without a signal unless you add pedestrian signals to the ramps.

As someone who lives in the city and walks damn near as much as I drive: signalized crossings are a pain in the ass (I will go, regardless of the signal, as soon as there is no conflict with vehicles). A better-designed 'yield to pedestrians' crossing is much preferred, since you can go almost immediately 99.999% of the time. These crossings would all be one lane, as shown, and should be able to be crossed fairly quickly. Cars might not stop immediately, but you'll still be waiting less time than if you hit a button and wait for a walk sign, since that signal will almost certainly be synchronized to nearby signals (as opposed to a HAWK or something else).

Honestly, you could propose a regular diamond, and I'd be happier with it then the monstrosity that was actually built. We will look back on this interchange in 30 years and laugh at how much overbuilding we used to do.

What in the world advantage does a regular diamond ever have over a DDI except for the potentially smaller footprint if you squeeze it down to the bare minimum?  With a DDI, your signals are only two phases each, and all movements are protected (except when they don't protect the left-turning movement: https://goo.gl/maps/jeiif5LpW7Qyjarz5 .  Try watching your left mirror to be sure your trailer isn't hitting anything while looking to the right for conflicting traffic--that ain't fun!).  But with a diamond, you either have to settle for unprotected left turns crossing oncoming traffic, or suffer through a pair of three-phase signals.

Of course, if you want a really signal-efficient diamond, you could make it RIRO and then use turnarounds: https://goo.gl/maps/msgcHN6SaXsUxGUn6 .  But then, you have to have enough real estate for the turnarounds, whether you use jughandles or Michigan Lefts.

Well, the massive advantage is the ability to run both directions of the arterial simultaneously. DDIs are basically a giant split-phased intersection, where only some lanes can ever go at the same time. The main issue is that the arterial conflicts with itself and must run separate phases. This is not efficient at all, unless most traffic enters or exits the freeway (having only to clear one of the signals to do so). Off-ramp left turns at diamond interchanges conflict with themselves, granted, but this can be superior, depending on the flow of traffic in the area. Plus, as a pedestrian, I don't have to go and hang out in the middle of the roadway, surrounded by cars.

As a personal anecdote: I have driven through DDIs quite a bit lately, thanks to a new one near me (WA-510/I-5) and another in an area I frequent (Fishers, IN); I've never driven through either without hitting at least one red light. This is obviously by design. I have driven through numerous diamond interchanges without stopping (plenty don't have any signals at all), and I don't recall any of them being a sea of collisions compared to a DDI. The notion that fewer conflict points = fewer collisions flies in the face of basic common sense, which tells me that collisions could be caused by dozens of other things, like road curvature, driver unfamiliarity, heavy traffic, poor signalization, etc.

DDIs are like roundabouts: they take regular concepts and completely throw them out the window. This is fine in some circumstances, but it's not always superior. In many cases, I'd say both (the former more than the latter, frankly) are used inappropriately.

[vdeane]

Plus, as a pedestrian, I don't have to go and hang out in the middle of the roadway, surrounded by cars.

That is not an inherent feature of DDIs.

[webny99]

When I hear "double trumpet", I think of this.

[johndoe]

Interesting, thanks for sharing.  I fear the "off-ramp lefts" (northbound to westbound and southbound to eastbound) may require a larger radii- which would mean wider bridges.  Those movements also go through pretty serious reverse curves - something undesirable for traffic that just left the freeway at 70 mph.  The "lefts onto the freeway" parclo movements also add extra bridge area (in fairness just like every other parclo).

For pedestrian crossings, it's sort of an interesting debate between "fewer lanes and uncontrolled" vs "more lanes and signalized".  Many of the uncontrolled DDI crossings I've seen are at YIELDING rights - where cars aren't expected to blast through at higher speeds.  The FL example is really a free-flow movement so I guess you could argue peds have to be a bit braver to step out there.  I assume that's where the "peds in middle" came from - don't make them cross higher speed free lefts onto the freeway.

Cool concept, maybe it would be feasible on higher speed non-signalized arterials or system interchanges.  Something tells me DOT would see it as "excessive" to have all free-flow movements in the vicinity of other signals...not that a 12 lane DDI isn't "excessive"

[tradephoric]

I fear the "off-ramp lefts" (northbound to westbound and southbound to eastbound) may require a larger radii- which would mean wider bridges.  Those movements also go through pretty serious reverse curves - something undesirable for traffic that just left the freeway at 70 mph.

That is my biggest concern of the design too.  The Myrtle Beach design (with exiting loop ramps as opposed to entering loop ramps) could alleviate this concern but the problem is you end up dumping freeway traffic much farther down the arterial; giving drivers less time to merge to whatever lane they need to be in before making their turn-off.  Of course this isn't a problem for the Myrtle Beach interchange as the nearest exit along Carolina Bays Parkway is over 4 miles away!

[ErmineNotyours]

When I hear "double trumpet", I think of this.

I can see the Google driver's hand

[froggie]

Plus, as a pedestrian, I don't have to go and hang out in the middle of the roadway, surrounded by cars.

That is not an inherent feature of DDIs.

No, but they're designed far more often than not with the walkway in the middle.  Most literature I've seen shows it as the default configuration.

[rte66man]

When I hear "double trumpet", I think of this.

There is still one left on I-30 in DFW. It is the last relic of that road being the old Dallas-Fort Worth Turnpike.
https://www.google.com/maps/@32.7654935,-96.9194217,3760m/data=!3m1!1e3

[tradephoric]

Here are models of both the double trumpet and the University Parkway DDI.  Both models have the same traffic volumes inputted.  The double trumpet model has been modified based on some comments on this thread (there is now a barrier to keep left-entering traffic from taking that immediate right back onto the freeway and the radius of the loop ramps have been increased).

https://www.youtube.com/watch?v=SV8Zl1pArws
https://www.youtube.com/watch?v=kqfC3wjE6oc

[jakeroot]

Plus, as a pedestrian, I don't have to go and hang out in the middle of the roadway, surrounded by cars.

That is not an inherent feature of DDIs.

No, but they're designed far more often than not with the walkway in the middle.  Most literature I've seen shows it as the default configuration.

Exactly, yeah. IIRC, Utah (who has built quite a few) built some of their early DDIs in American Fork with outer pedestrian paths, but isn't as keen on them in retrospect because most of the crossings are unsignalized.

For pedestrian crossings, it's sort of an interesting debate between "fewer lanes and uncontrolled" vs "more lanes and signalized".  Many of the uncontrolled DDI crossings I've seen are at YIELDING rights - where cars aren't expected to blast through at higher speeds.  The FL example is really a free-flow movement so I guess you could argue peds have to be a bit braver to step out there.  I assume that's where the "peds in middle" came from - don't make them cross higher speed free lefts onto the freeway.

If I had to guess, I would have assumed that either the off-ramp right turns, or the on-ramp right turns, would be the highest speed movements, since both follow what was presumably a straightaway, whereas the on-ramp left turn immediately follows a "sharp" curve onto the left side of the road. In any case, I would think that the on-ramp left turn (from the left side of the road) would actually be the slowest of the free-flow movements, since drivers have just moved through a curve, and must be sure to be in the correct lane (two things combined that might be slowing drivers).

In some cases, like the University Parkway DDI in Florida, the interchange is so large that even the turns are high speed. I don't think this has been seriously considered in terms of how those affect pedestrian movements. A pedestrian trying to cross a freeway to freeway ramp seems insane, but trying to cross to a regular porkchop island via a slip lane isn't that crazy because the latter is across a travel lane that has much slower speeds (higher reaction time for motorists, reduced chance for injury if there is a collision, etc). These DDIs seem designed for very high speed free-flow movements. Why are more not designed in an urban context, with sharper turns? Especially if these are so damn great for pedestrians.

Good example of an overbuilt DDI in terms of speed: Bruce Highway & Caloundra Road north of Brisbane, QLD. Absolutely gigantic DDI with some of the largest curves I've ever seen. The curves are appropriate for the rural area that it's in, but it would be wildly inappropriate in an urban environment. Yet I don't think it would be beyond some DOTs to build a DDI that large in an urban area. Just a waiting game at this point.

As a side-note: some of the DDIs I've seen in New York are actually quite well designed for pedestrians. The one at 590 & Winton Road in Rochester is quite tight compared to most.

[jakeroot]

Probably more appropriate for a second reply, than to tack onto my above reply.

The DDI at Winton Road and I-590 near Rochester, NY is a great example an "urban" DDI with a great pedestrian experience, with outer pedestrian paths more normal for an urban roadway, and sharp curves for the turns (like at porkchop islands or regular diamonds). The off-ramps are signalized, as is often the case at DDIs, but the turns are still quite sharp and likely better for pedestrian visibility. As well, the on-ramps are very sharp left turns with I'm sure quite slow speeds. Again, great for pedestrians.

[kphoger]

As a pedestrian, I'm not sure I'd care one way or the other.  Sidewalk, between the median barriers, doesn't matter to me either way.

[jakeroot]

As a pedestrian, I'm not sure I'd care one way or the other.  Sidewalk, between the median barriers, doesn't matter to me either way.

I think it's more about which one is easier and less weird for a pedestrian. Crossing out into the middle of the road, only to cross back again, is weird. Pedestrians are expressly denied the privilege of entering the middle of roundabouts, and are largely not permitted to stand in the median of roadways. Yet, it's status quo for a DDI.

In terms of "ease", the argument is signalized crossing = better. In my opinion, unsignalized (faster) = better. And of course, when I say "unsignalized", I don't necessarily mean "no warning lights or flashers", but it's easier to just "keep going" as a pedestrian when you don't have to cross into the middle and/or wait for several signalized crossings in a row, when you can quickly whack those ADA buttons to activate a flasher, wait a couple seconds for traffic to stop (if they haven't already by your presence at a crosswalk), and then just keep moving.

[Duke87]

I would definitely rather, as a pedestrian, cross a single lane without a traffic signal than multiple lanes with one. The shorter the crossing, the easier it is to find a gap in traffic to scamper across through.

But at the same time I can also see where an unsignalized crossing might raise the hackles of a designer who has to worry about how less spry pedestrians will navigate the interchange. "Just look for a gap in traffic and then quickly scamper across" doesn't work for everyone.

[I-55]

I would definitely rather, as a pedestrian, cross a single lane without a traffic signal than multiple lanes with one. The shorter the crossing, the easier it is to find a gap in traffic to scamper across through.

But at the same time I can also see where an unsignalized crossing might raise the hackles of a designer who has to worry about how less spry pedestrians will navigate the interchange. "Just look for a gap in traffic and then quickly scamper across" doesn't work for everyone.

I would think that if someone is crossing an interchange in (what I assume to be) a suburban area that they can handle 2 unsignalized crossings. These areas (correct me if I'm wrong) aren't in downtowns where you park in a garage and walk the streets everywhere. Out here, I don't see anyone walking anywhere except to just take a walk, and if you're doing that there then you are probably spry enough.

[kphoger]

If I were a blind pedestrian, I'd sure rather have signalized crossings than un- at such a major traffic location.

[tradephoric]

Florida is proposing another DDI at I-75 and MLK Jr. Blvd in Hillsborough County.  FDOT's aerial highlights how an existing loop ramp could easily fit into the same ROW as the proposed DDI.  These Florida DDI's are massive.