Are diverging diamonds a fad?

[cl94]

DDIs are bad when there are adjacent intersections within a short distance. We know that. In that case, a SPUI is better. That's one of the few cases where a SPUI will outperform a DDI, as signals can be coordinated. A DDI in the location you posted is a poor choice for that reason, but there may have been other factors.

[JKRhodes]

DDI in St. George, Utah:

UDOT DIVERGING DIAMOND INTERCHANGE (DDI) OBSERVATIONS AND EXPERIENCE:
Coordination of the DDI with adjacent signals is not easily done. Most DDIs need a lower cycle length than the adjacent signals. This may result in a vehicle having to stop at both the off ramp terminal and the next adjacent signal.

The closest adjacent signal to the St. George DDI is 550 feet away (roughly an 8 second travel time at 45 mph). Who wants to get a green light only to come to a red light 8 second later? That's the epitome of inefficiency.   In the picture you see roughly 20 cars needlessly waiting at a red light... no opposing traffic, just lots of dead air.

The heaviest movement of Eastbound St George Blvd - Northbound I-15 is favored by that intersection, and from what I saw when I drove it, it seemed to work rather well, as long as you can get through the first crossover signal. It also seemed to work well five years ago when it was a normal diamond. I never drove it during rush hour. Maybe someone from the area can chime in...

[tradephoric]

DDIs are bad when there are adjacent intersections within a short distance. We know that. In that case, a SPUI is better. That's one of the few cases where a SPUI will outperform a DDI, as signals can be coordinated. A DDI in the location you posted is a poor choice for that reason, but there may have been other factors.

Coordinating traffic signals doesn't necessarily lead to improvements in progression.  Here is an example of a Parclo A4 interchange at HWY 410 & Steeles Avenue in Toronto (the red icons represent traffic signals that stop both directions of travel):




The signals are coordinated yet progression still sucks.  Providing perfect progression in one-direction leads to green-to-red progression in the other.  Whenever you have closely spaced traffic signals that stop both directions of travel, progression will suffer.  A coordinated SPUI might be as ineffective as an uncoordinated DDI in that Utah example i posted.

[tradephoric]

That's a red herring argument, as signals all over the place are ill timed or have needless phases. It's not unique to DDIs at all.

Timing a corridor for good signal progression is a skill lost in history, like Egyptian pyramid-building techniques.  Detroit is clinging on to the lost skill, but as progression killing designs like DDIs and SPUIs emerge their efforts may prove futile.  Enjoy the good signal progression while it lasts...

https://www.youtube.com/watch?v=oea5WLUmGjs

[UCFKnights]

Timing a corridor for good signal progression is a skill lost in history, like Egyptian pyramid-building techniques.  Detroit is clinging on to the lost skill, but as progression killing designs like DDIs and SPUIs emerge their efforts may prove futile.  Enjoy the good signal progression while it lasts...

Eh, most interchanges are difficult to time, especially with high turning movement. Its not like diamond interchanges are any better. Only solutions that are better is using all ramps. Even in that video, when they were near an interchange towards the end, they had to stop. SPUI should be way better than a standard diamond as far as progression, no?

[NE2]

If you have free-flow ramps for entering onto the surface road, you need to deal with weaving (i.e. people who want to turn left immediately).

[tradephoric]

Timing a corridor for good signal progression is a skill lost in history, like Egyptian pyramid-building techniques.  Detroit is clinging on to the lost skill, but as progression killing designs like DDIs and SPUIs emerge their efforts may prove futile.  Enjoy the good signal progression while it lasts...

Eh, most interchanges are difficult to time, especially with high turning movement. Its not like diamond interchanges are any better. Only solutions that are better is using all ramps. Even in that video, when they were near an interchange towards the end, they had to stop. SPUI should be way better than a standard diamond as far as progression, no?

How does the signal progression look at this SPUI...


That SPUI has EWWY signal progression.   This is not an atypical scenario.  Many SPUIs will have corridors running 45 mph at 120 second cycle lengths and adjacent signals spaced roughly 1500 feet apart.   The SPUI stops both directions of travel along the corridor killing progression.

I don't disagree that most interchanges are difficult to time.  That's because most interchange signals stop BOTH DIRECTIONS OF TRAFFIC!  The solution?  Choose interchange designs where the signals only stop ONE DIRECTION OF TRAFFIC!!  That's it!

[kphoger]

If you have free-flow ramps for entering onto the surface road, you need to deal with weaving (i.e. people who want to turn left immediately).

Accidents caused by weaving cause less bodily and financial harm. I should hope, too, that some immediate left turns could be prevented with raised medians.

[jakeroot]

I don't disagree that most interchanges are difficult to time.  That's because most interchange signals stop BOTH DIRECTIONS OF TRAFFIC!  The solution?  Choose interchange designs where the signals only stop ONE DIRECTION OF TRAFFIC!!  That's it!

I'd love to know the numbers on the I-5/S 38th Street interchange in Tacoma, Washington. It's a parclo B4. Traffic flow is always steady ... very little waiting at any of the signals (though it does help that, in Washington, I can turn left onto the on-ramp after a stop).

[tradephoric]

The SPUI at I-94 & Telegraph has a larger footprint than the Parclo B4 at I-75 & 14 Mile.  MDOT, in their infinite wisdom, decided a progression killing SPUI would work better than a progression saving Parclo B4 at I-94 & Telegraph.

[cl94]

No, it's that a parclo with modern curve radii wouldn't have fit in the area occupied by the effed-up interchange it replaced while providing little improvement. And what's with the obsession over B4s? They're the worst type of parclo to have because there's unrestricted flow onto the surface road.

[Revive 755]

If you have free-flow ramps for entering onto the surface road, you need to deal with weaving (i.e. people who want to turn left immediately).

Worst case you could always modify the loop ramps to be signal controlled instead of free-flow.

[froggie]

They're the worst type of parclo to have because there's unrestricted flow onto the surface road.

As a budding engineer, surely you realize that how the unrestricted flow affects the surface road will depend on the volume of that flow and the location/design of downstream impediments on the surface road.

Furthermore, one point about B4's that hasn't been made yet is that they only require ONE signal per direction for through movement on the surface road, vice two for an A4.

In rural situations, a safety case can also be made for a B4 over an A4.  In a B4, non-loop turning traffic only needs to worry about traffic from one direction on the surface-road (namely the oncoming traffic).  In an A4, the non-loop turning traffic has to watch for BOTH directions on the surface-road.

[vtk]

The SPUI at I-94 & Telegraph has a larger footprint than the Parclo B4 at I-75 & 14 Mile.  MDOT, in their infinite wisdom, decided a progression killing SPUI would work better than a progression saving Parclo B4 at I-94 & Telegraph.

Your measurements included portions of the freeway itself, as well as the surface road, which would be there even without an interchange. Importantly, you included greater length of freeway for the SPUI, possibly tipping the area comparison. To make an effective real estate comparison, do not include areas that would be within highway ROW without any interchange.

[tradephoric]

 

No, it's that a parclo with modern curve radii wouldn't have fit in the area occupied by the effed-up interchange it replaced while providing little improvement. And what's with the obsession over B4s? They're the worst type of parclo to have because there's unrestricted flow onto the surface road.

Signalize the off-ramps so there's not unrestricted flow then.  Revive 755 already touched on this simple solution.  Here's an example of a Parclo B4 in Florida with signalized off-ramps:

https://www.google.com/maps/@26.007247,-80.3406062,709m/data=!3m1!1e3

Note: I-75 & Pines Blvd use to have unrestricted flow onto the surface road but the interchange was reconfigured around 2009 to signalize the off-ramps.

[kphoger]

Question: Is signal progression actually a consideration that agencies make when deciding between different interchange configurations?

I know we like to think of a solution as "good" of we can personally drive through it with zero or one red light, and likewise "bad" if we can personally drive through it with what we feel are unnecessary stops. But traffic needs in the bigger picture are much more complex, and the best solution doesn't always mean what we think it might.

[TEG24601]

The SPUI at I-94 & Telegraph has a larger footprint than the Parclo B4 at I-75 & 14 Mile.  MDOT, in their infinite wisdom, decided a progression killing SPUI would work better than a progression saving Parclo B4 at I-94 & Telegraph.

Well, M-DOT wasted a lot of money.  They moved from a free-flowing, left exiting system that worked and handled the traffic, to a non-free flowing system that can't.  Even the parclo would have been worse than leaving 94/24 alone.

[jakeroot]

Signalize the off-ramps so there's not unrestricted flow then.  Revive 755 already touched on this simple solution.  Here's an example of a Parclo B4 in Florida with signalized off-ramps:

WSDOT does that a lot. It bugs me, since I've always thought that Parclos should look like the kind that Ontario builds (with proper curves, etc). But, seeing the reason, I'm less put-off.

[jeffandnicole]

DDI in St. George, Utah:

UDOT DIVERGING DIAMOND INTERCHANGE (DDI) OBSERVATIONS AND EXPERIENCE:
Coordination of the DDI with adjacent signals is not easily done. Most DDIs need a lower cycle length than the adjacent signals. This may result in a vehicle having to stop at both the off ramp terminal and the next adjacent signal.

The closest adjacent signal to the St. George DDI is 550 feet away (roughly an 8 second travel time at 45 mph). Who wants to get a green light only to come to a red light 8 second later? That's the epitome of inefficiency.   In the picture you see roughly 20 cars needlessly waiting at a red light... no opposing traffic, just lots of dead air.

They say a picture is worth a thousand words.  Sometimes, those words are wrong.

A picture is simply a single point in time.  I could sit atop a building for hours, wait for the perfect moment, snap a picture, and develop an entire story around that one moment in time.  Nevermind pictures that I could've snapped every second for several hours would've revealed something else.

While it appears the traffic is waiting there for air, there's also traffic at the bottom of the picture.  That traffic may had just made it thru the intersection, and the signal simply didn't cycle thru for the green light yet.  Looking at the red pickup with the trailer - he certainly seems to be someone moving.  He may have just went thru that intersection a second earlier, justifying the red light and the waiting traffic.

And in a construction zone, signals are rarely optimized for the most efficient flow of traffic.  Traffic detectors may still need to be installed and programmed.  Traffic shifts and lane closures frequently happen.  It's fairly clear there's little if any signage in place.

If you want to criticize a DDI because of this picture, I'll just pull up photos of traffic sitting at red lights at all sorts of intersections.  Even the most optimized traffic corridors won't have free-flowing traffic 100% of the time.

[tradephoric]

They say a picture is worth a thousand words.  Sometimes, those words are wrong.

A picture is simply a single point in time.  I could sit atop a building for hours, wait for the perfect moment, snap a picture, and develop an entire story around that one moment in time.  Nevermind pictures that I could've snapped every second for several hours would've revealed something else.

While it appears the traffic is waiting there for air, there's also traffic at the bottom of the picture.  That traffic may had just made it thru the intersection, and the signal simply didn't cycle thru for the green light yet.  Looking at the red pickup with the trailer - he certainly seems to be someone moving.  He may have just went thru that intersection a second earlier, justifying the red light and the waiting traffic.

And in a construction zone, signals are rarely optimized for the most efficient flow of traffic.  Traffic detectors may still need to be installed and programmed.  Traffic shifts and lane closures frequently happen.  It's fairly clear there's little if any signage in place.

If you want to criticize a DDI because of this picture, I'll just pull up photos of traffic sitting at red lights at all sorts of intersections.  Even the most optimized traffic corridors won't have free-flowing traffic 100% of the time.

A picture is worth a thousand words and a time-distance diagram is worth ten thousand.  You're fooling yourself if you can't acknowledge that DDIs, SPUIs and Parclo A4s lead to poor signal progression along a corridor.  Closely spaced traffic signals that stop both directions of travel along a corridor will lead to poor signal progression.  That's exactly what you get with a DDI.

I for one believe drivers get stopped at way too many red lights and DDIs compound the problem.

[NE2]

Yawn. DDIs are not intended for interchanges where the majority of surface traffic does not enter or exit the freeway. Therefore signal progression has no meaning when considering a DDI.

[tradephoric]

Yawn. DDIs are not intended for interchanges where the majority of surface traffic does not enter or exit the freeway. Therefore signal progression has no meaning when considering a DDI.

There are hundreds of SPUIs and Parclo A4s where good progression along the corridor should be a key concern.  However, these designs also stop both directions of travel and suffer from poor signal progression.  I know this thread was originally about DDIs but it has morphed into interchange designs that lead to poor signal progression (SPUIs and Parclo A4s included).  The point you are making in regards to DDIs can't be applied to the hundreds of SPUIs and Parclo A4s out there.   

Don't you want to drive 10, 15, or 20 miles along a major corridor and never get stopped at a red light?  Good progression is lost when progression killing interchange designs are chosen (ie. DDIs, SPUIs, & PARCLO A4s).

[NE2]

Don't you want to drive 10, 15, or 20 miles along a major corridor and never get stopped at a red light?

That would be nice, but lights are rarely timed for bike speeds.

[Bickendan]

Don't you want to drive 10, 15, or 20 miles along a major corridor and never get stopped at a red light?

That would be nice, but lights are rarely timed for bike speeds.

This. You need a one-way grid for it to work, and even in downtown Portland, the signal timing leaves a bit to be desired.

[tradephoric]

Don't you want to drive 10, 15, or 20 miles along a major corridor and never get stopped at a red light?

That would be nice, but lights are rarely timed for bike speeds.

This. You need a one-way grid for it to work, and even in downtown Portland, the signal timing leaves a bit to be desired.

Getting off topic, but here is downtown Portland's grid in action.  The city has very condensed grid so the 'green wave' that achieves perfect progression is only 12 mph (at 56 second cycle lengths).  A driver is pretty much limited to driving 12 mph through downtown Portland.

https://www.youtube.com/watch?v=4mlZMPqJBLE
https://www.youtube.com/watch?v=7jGWdCknurM