Are diverging diamonds a fad?

[jakeroot]

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

[cl94]

What are the operational benefits of a DDI compared to a PARCLO interchange?  There are examples of full cloverleaf interchanges being converted to DDI's as opposed to PARCLO's (IE. I-79 & US19).  A Parclo B4 has 12 conflict points as opposed to a DDI that has 14 conflict points.  A Parclo B4 can achieve good coordination (since traffic signals only stop one-direction of travel) while a DDI you get bad coordination (since closely spaced traffic signals stop both directions of travel).   Why crisscross traffic on US19 (and add two-additional conflict points) when PennDOT could simply take advantage of the grade separation between I-79 & US19 and turn it into a Parclo B4?

As was already mentioned, a DDI allows for I-70 to be widened without replacing the bridges. They're turning the auxiliary lane into an additional travel lane. Parclos are superior when space allows, but space isn't always available.

Your hung up on cost.  I'll ask you another question.  You say Parclo's are superior but there are different types of Parclos to choose from.  Are there any operational, safety, or cost benefits of picking a Parclo A4 over a Parclo B4?

As an engineer, cost is a huge concern. No matter how much safety will improve, it won't get built if the cost is too high.

A4s are superior to B4s because A4 loops are entrance ramps and the freeway can take a constant influx of traffic better than a constrained surface facility. A properly designed Parclo is superior from a safety perspective, but not always from a cost perspective.

I admit that I've only driven on one DDI, but NYSDOT did a damn good job of making it clear with relatively little signage.

[johndoe]

The term "Death Diamond", coined by yours truly, has caught on in the road community.

It has? What community are you referring to?  How many fatalities have been caused by this interchange type in the last 6 years?

Your hung up on cost.

That's how it goes in the real world!  Not impacting the bridge is a huge advantage.

[tradephoric]

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

When you are driving fast enough for your car to split in half...
http://kfoxtv.com/news/features/top-stories/stories/Woman-killed-in-car-accident-after-hitting-splitting-car-in-half-in-East-El-Paso-61639.shtml#.VR3b244a6RM

[cl94]

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

When you are driving fast enough for your car to split in half...
http://kfoxtv.com/news/features/top-stories/stories/Woman-killed-in-car-accident-after-hitting-splitting-car-in-half-in-East-El-Paso-61639.shtml#.VR3b244a6RM

She was also believed to be drunk. Engineers can't prevent stupid.

[tradephoric]

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

When you are driving fast enough for your car to split in half...
http://kfoxtv.com/news/features/top-stories/stories/Woman-killed-in-car-accident-after-hitting-splitting-car-in-half-in-East-El-Paso-61639.shtml#.VR3b244a6RM

She was also believed to be drunk. Engineers can't prevent stupid.

What does it matter that the woman was drunk or not?  Jake was asking a simple physics question.  Can you answer this question? 

Train A is traveling at 55 mph, and the conductor is piss drunk.  How far will Train A travel if the conductor of Train B is having sex with a passenger? 

[tradephoric]

What are the operational benefits of a DDI compared to a PARCLO interchange? There are examples of full cloverleaf interchanges being converted to DDI's as opposed to PARCLO's (IE. I-79 & US19).  A Parclo B4 has 12 conflict points as opposed to a DDI that has 14 conflict points.  A Parclo B4 can achieve good coordination (since traffic signals only stop one-direction of travel) while a DDI you get bad coordination (since closely spaced traffic signals stop both directions of travel).   Why crisscross traffic on US19 (and add two-additional conflict points) when PennDOT could simply take advantage of the grade separation between I-79 & US19 and turn it into a Parclo B4?

As was already mentioned, a DDI allows for I-70 to be widened without replacing the bridges. They're turning the auxiliary lane into an additional travel lane. Parclos are superior when space allows, but space isn't always available.

Your hung up on cost.  I'll ask you another question.  You say Parclo's are superior but there are different types of Parclos to choose from.  Are there any operational, safety, or cost benefits of picking a Parclo A4 over a Parclo B4?

As an engineer, cost is a huge concern. No matter how much safety will improve, it won't get built if the cost is too high.

Is the cost of each interchange design relevant to the question i asked?  Even if a DDI interchange is less costly than a PARCLO, how do i know that the savings are worth it if the operational benefits aren't known?  Maybe the DDI is a guaranteed piece of SHIT! 

https://www.youtube.com/watch?v=a5dpBpaFiMo

[cl94]

Modified DDIs have existed for a while, not including 40 year old DDIs in France. Take I-95 Exit 18 in Rhode Island. It's really no different than a one-way pair (which is exactly how it functions). So far, in the 6 years they've existed in the US, there haven't been many major issues. Speed isn't as much of an issue because they're not meant to be used on high-speed roadways. They're a simple solution with little construction and, if major problems become apparent, they can be reverted to standard diamonds with little effort.

We understand that you don't like the idea. Engineering studies have shown that the design can be effective and increase efficiency when replacing other designs.

[kkt]

Train A is traveling at 55 mph, and the conductor is piss drunk.  How far will Train A travel if the conductor of Train B is having sex with a passenger? 

A long way.  Drink provokes the desire, but it takes away the performance.

[tradephoric]

Engineering studies have shown that the design [DDI's] can be effective and increase efficiency when replacing other designs.

The flip side is DDI's can decrease efficiency when replacing "other designs".  Your sentence is fluff when you don't define what the other designs are.  Feel free to cite these phantom engineering studies.

[jakeroot]

^Whether or not the woman was drunk is irrelevant to Jake's question.  It's a simple physics question he's after.  Physics questions on test

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

When you are driving fast enough for your car to split in half...
http://kfoxtv.com/news/features/top-stories/stories/Woman-killed-in-car-accident-after-hitting-splitting-car-in-half-in-East-El-Paso-61639.shtml#.VR3b244a6RM

She was also believed to be drunk. Engineers can't prevent stupid.

What does it matter that the woman was drunk or not?  Jake was asking a simple physics question.

Allow me to rephrase: "At what point while driving on a DDI are everyday citizens, who are paying attention to the road and not driving distracted or too quickly, travelling fast enough to die?"

A normal, everyday citizen (i.e. those not drinking/driving distracted/driving too fast for conditions) should not have any problems navigating a DDI. The death was because of her apparent intoxication, not the engineering of the junction. As cl94 said, engineers can't prevent stupid.

[dfwmapper]

...and will eventually prove to be deadly.

At what point while driving on a DDI are you travelling fast enough to die?

When you are driving fast enough for your car to split in half...
http://kfoxtv.com/news/features/top-stories/stories/Woman-killed-in-car-accident-after-hitting-splitting-car-in-half-in-East-El-Paso-61639.shtml#.VR3b244a6RM

Not exactly an argument against the general concept of a DDI though, considering a full on freeway terminates at this one. Some drunk old lady doing 60 through the original diamond probably would have had a much worse result T-boning someone else. Maybe a DDI isn't the greatest choice for a freeway to freeway interchange, but there aren't a lot of alternatives that don't involve expensive flyovers or tearing down the existing freeway.

[bugo]

Driving on the wrong side of the road doesn't require a learning curve? I know if I hadn't read about Death Diamonds on the internet I would have turned right at the first traffic light into oncoming traffic. They are confusing, cheap (in the bad way) and will eventually prove to be deadly.

Why would you have done that?  Because the signage and pavement markings were unclear?  Because the center median didn't establish a clear division of directions of traffic?  Because you didn't understand the concept of a two-way street becoming a pair of one-way streets?  Or because your future self spoke to you from this present reality and told you to make an incorrect maneuver, just to prove a point?

Because of the tendency to stay on the right (*correct*) side of the road which happens to be the right side. Crossing over to the left is inherently more dangerous than staying on the right is.

The Death Diamond is nothing more than a cheap "solution" to a problem that usually doesn't exist. I've driven through the MO 13/I-44 interchange before and after it was converted and I didn't see any improvement after it was converted to a Death Diamond. Build a proper interchange or leave the diamond the fuck alone. An even better solution would have been to put frontage roads along I-44 and use slip ramps to go to and from the frontage road to the mainline. Sure it would be more expensive than a Death Diamond but hey, you get what you pay for.

[NE2]

Yo Hibby, 'death diamond' is bugo's term for roundabout. Go make out in the hay.

[BigRedDog]

Having never driven through a DDI: what kind of an angle would the "wrong way" right turn be? It seems like a driver would be close to making a 90^o right turn in order to stay on the right. In other words, if a driver comes to the first signal, they are going to just go straight across at the intersection - and likely not even realize they're on the wrong side of the road until they are actually into the DDI.

[tradephoric]

"At what point while driving on a DDI are everyday citizens, who are paying attention to the road and not driving distracted or too quickly, travelling fast enough to die?"

A normal, everyday citizen (i.e. those not drinking/driving distracted/driving too fast for conditions) should not have any problems navigating a DDI. The death was because of her apparent intoxication, not the engineering of the junction. As cl94 said, engineers can't prevent stupid.

It's stupid to assume that a sober driver traveling safely through a DDI is immune to other people's stupidity.  The crossing angle at a DDI has a high angle of incidence which is one of the disadvantages of the design.  A drunk driver blowing through a red light at a DDI is susceptible to being involved in a near head on crash.  You can't stop stupid but you can try to limit its effects.

[Bickendan]

Driving on the wrong side of the road doesn't require a learning curve? I know if I hadn't read about Death Diamonds on the internet I would have turned right at the first traffic light into oncoming traffic. They are confusing, cheap (in the bad way) and will eventually prove to be deadly.

Why would you have done that?  Because the signage and pavement markings were unclear?  Because the center median didn't establish a clear division of directions of traffic?  Because you didn't understand the concept of a two-way street becoming a pair of one-way streets?  Or because your future self spoke to you from this present reality and told you to make an incorrect maneuver, just to prove a point?

Because of the tendency to stay on the right (*correct*) side of the road which happens to be the right side.

England and other left-hand drive countries disagree.

Crossing over to the left is inherently more dangerous than staying on the right is.

I-5 through the Grapevine disagrees. As do a few full-freeway interchanges that feature DDI type interchanges (though they do have the 'weaknesses' of left-hand exits and entrances).

You wear your bias on your sleeve calling it a 'Death Diamond' when there's no evidence the term's warranted outside of a ski resort.

[jakeroot]

The crossing angle at a DDI has a high angle of incidence which is one of the disadvantages of the design.

The FHWA's (albeit small) study on "Drivers' Evaluation of the Diverging Diamond Interchange" reported that none of the participants performed a wrong-way manoeuvre at the crossover point. There were wrong-way manoeuvers whereby an elderly driver repeatedly turned left onto the offramp after the initial crossover, but they attribute this error to poor signing.

Obviously you're quoting a more recent study...if you could please provide a link, I'd love to read it.

[tradephoric]

The crossing angle at a DDI has a high angle of incidence which is one of the disadvantages of the design.

The FHWA's (albeit small) study on "Drivers' Evaluation of the Diverging Diamond Interchange" reported that none of the participants performed a wrong-way manoeuvre at the crossover point. There were wrong-way manoeuvers whereby an elderly driver repeatedly turned left onto the offramp after the initial crossover, but they attribute this error to poor signing.

Obviously you're quoting a more recent study...if you could please provide a link, I'd love to read it.

If two drivers going in opposite directions travel through a crossing conflict point of a DDI intersection, the angle of the two vehicles striking would be nearly head on.  Put another way, the crossing angle at a DDI has a high angle of incidence   Do i really need to cite a study to make that point?

My interest is comparing the operational differences between a DDI and a PARCLO.  You cite a study that compares a DDI to a diamond.  Don't feel obligated to answer a question that you don't have the answer to.  I readily admit i don't know the operational differences between a DDI and a PARCLO.  That's why i keep asking the question (probably to the annoyance of some, but ooh well).  Cite me that study and I'll be happy!

[cl94]

Except the angle of incidence thing is relatively inapplicable to this situation because they're 2 one way streets that just happen to cross without a single turn movement and you can see traffic in the opposing direction.

We don't know the differences between a DDI and a parclo because they're intended for different situations. Will there be a study? Possibly. But as of now, they're not considered to be comparable.

[tradephoric]

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.

[Bickendan]

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.

[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.

That's like telling a 600 lbs guy to keep eating 12,000 calories a day since they are already morbidly obese.

[NE2]

If most traffic is turning (as should happen if a DDI is warranted), the problem exists no matter what type of interchange you have.

[Brandon]

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.

They can be timed very well.  However, DDIs don't help; Michigan Lefts do.