The "Father of the DDI" Defends Left Hand Exit and Entrance Ramps

[kphoger]

And imagine how much less right of way would be needed to expand freeways at crossroads.

Now imagine the headache of expanding the ramps, though.

But since it would be a SPUI, the ramps would have more capacity and would be less likely to need expansion.

In which case, the ramps would be initially constructed at lower capacity, and therefore would still need to be expanded with enough increase in traffic.  It's not like departments would simply construct ramps that are way over capacity for no good reason.

If the road changes from freeway to expressway, the slow lane would need to switch from the left to the right.

I guess it would be instinctive, drivers would quickly figure out that the slow lane is the one with all the exits and entrances.

1.  It would not be instinctive.

2.  All that weaving would be utter chaos.

[kernals12]

If the road changes from freeway to expressway, the slow lane would need to switch from the left to the right.

I guess it would be instinctive, drivers would quickly figure out that the slow lane is the one with all the exits and entrances.

MA 2 in Fitchburg? The cemetery on MA 128 in Peabody?

Sorry, I don't know what you're talking about

[hotdogPi]

If the road changes from freeway to expressway, the slow lane would need to switch from the left to the right.

I guess it would be instinctive, drivers would quickly figure out that the slow lane is the one with all the exits and entrances.

MA 2 in Fitchburg? The cemetery on MA 128 in Peabody?

Sorry, I don't know what you're talking about

Both cases I'm talking about have one or two at-grades in the middle of a freeway. If all the entrances and exits were on the left, and the right was the fast lane, you would suddenly have people getting on and off in the fast lane.

[thspfc]

This guy does make some good points. Though when he mentions the cost difference and space difference, does he factor in that the through carrigeways need to be a lot farther apart? Elimination of weaving within the interchange is kind of a moot point because unfamiliar drivers aren't expecting the left merge.
There are certain pros to left exits over huge flyovers, for sure. But overall, all right exits is better IMO. Plus, SPUIs are better than DDIs anyways. 

[kernals12]

If the road changes from freeway to expressway, the slow lane would need to switch from the left to the right.

I guess it would be instinctive, drivers would quickly figure out that the slow lane is the one with all the exits and entrances.

MA 2 in Fitchburg? The cemetery on MA 128 in Peabody?

Sorry, I don't know what you're talking about

Both cases I'm talking about have one or two at-grades in the middle of a freeway. If all the entrances and exits were on the left, and the right was the fast lane, you would suddenly have people getting on and off in the fast lane.

In a world where left hand ramps were the norm, then I'm sure they would've designed those differently.

[thspfc]

In a world where left hand ramps were the norm

So, Australia?

[Tom958]

4. Safety — The new Highway Safety Manual suggests that there is likely to be 49% more crashes with left ramps compared to other ramps. Avoiding left side ramps would seem to be a safe choice.

Might as well stop reading right there. 

I don't know about you, but I feel unsafe merging from the left because my view of the traffic stream I'm merging into is much more restricted looking to the right than it is looking to the left, and it's utterly unsurprising to me that accident statistics confirm that my discomfort is warranted. The same is true to a lesser degree with changing lanes in a rightward direction, though that's usually mitigated by having more time and distance to make the maneuver and a speed advantage over the vehicles to the right.

[1995hoo]

In a world where left hand ramps were the norm

So, Australia?

The way the OP has taken this thread strongly suggests maybe it belongs in "Fictional Highways," because that's what "a world where left hand ramps were the norm" is (also note his use of the subjunctive "were" underscoring the condition contrary to fact). It didn't necessarily start out that way because he cited an article whose author suggests that having a mixture of left- and right-side ramps wouldn't be the bad thing most of us believe it is, which is an interesting theory, but it's clear that the OP is, as usual, desperate to push a different theory from that on us. There is zero chance, none whatsoever, that left-side ramps will ever be "the norm" in the USA, and there's even less chance of all our roads being torn up and rebuilt (at huge expense and disruption) because some kid in Massachusetts thinks they're designed incorrectly.

If you then take the OP's theory (which I highly doubt he actually believes) that left-side ramps should be the standard, then extrapolating that theory to Australia or the UK, or really any other country where you drive on the left (well, maybe not Samoa as I doubt they have freeways), would mean in those countries right-side ramps should be the standard.

[jeffandnicole]

This guy does make some good points. Though when he mentions the cost difference and space difference, does he factor in that the through carrigeways need to be a lot wider? Elimination of weaving within the interchange is kind of a moot point because unfamiliar drivers aren't expecting the left merge.
There are certain pros to left exits over huge flyovers, for sure. But overall, all right exits is better IMO. Plus, SPUIs are better than DDIs anyways. 

The carriageways only needs to be wider at interchanges; that'll be no different than the right of way needed for right side ramps. Between interchanges, the roadways can bump up against each other with only a guardrail or Jersey Barrier separating them.

[kernals12]

In a world where left hand ramps were the norm

So, Australia?

The way the OP has taken this thread strongly suggests maybe it belongs in "Fictional Highways," because that's what "a world where left hand ramps were the norm" is (also note his use of the subjunctive "were" underscoring the condition contrary to fact). It didn't necessarily start out that way because he cited an article whose author suggests that having a mixture of left- and right-side ramps wouldn't be the bad thing most of us believe it is, which is an interesting theory, but it's clear that the OP is, as usual, desperate to push a different theory from that on us. There is zero chance, none whatsoever, that left-side ramps will ever be "the norm" in the USA, and there's even less chance of all our roads being torn up and rebuilt (at huge expense and disruption) because some kid in Massachusetts thinks they're designed incorrectly.

If you then take the OP's theory (which I highly doubt he actually believes) that left-side ramps should be the standard, then extrapolating that theory to Australia or the UK, or really any other country where you drive on the left (well, maybe not Samoa as I doubt they have freeways), would mean in those countries right-side ramps should be the standard.

Do you not understand the idea of hypothetical? Here, I'll help you

Definition of hypothetical
: involving or being based on a suggested idea or theory : being or involving a hypothesis :

[kernals12]

And imagine how much less right of way would be needed to expand freeways at crossroads.

Now imagine the headache of expanding the ramps, though.

But since it would be a SPUI, the ramps would have more capacity and would be less likely to need expansion.

In which case, the ramps would be initially constructed at lower capacity, and therefore would still need to be expanded with enough increase in traffic.  It's not like departments would simply construct ramps that are way over capacity for no good reason.

If the road changes from freeway to expressway, the slow lane would need to switch from the left to the right.

I guess it would be instinctive, drivers would quickly figure out that the slow lane is the one with all the exits and entrances.

1.  It would not be instinctive.

2.  All that weaving would be utter chaos.

Most freeway ramps are one lane. It's not possible to have them be less than one lane, so the capacity would be higher.

[1995hoo]

In a world where left hand ramps were the norm

So, Australia?

The way the OP has taken this thread strongly suggests maybe it belongs in "Fictional Highways," because that's what "a world where left hand ramps were the norm" is (also note his use of the subjunctive "were" underscoring the condition contrary to fact). It didn't necessarily start out that way because he cited an article whose author suggests that having a mixture of left- and right-side ramps wouldn't be the bad thing most of us believe it is, which is an interesting theory, but it's clear that the OP is, as usual, desperate to push a different theory from that on us. There is zero chance, none whatsoever, that left-side ramps will ever be "the norm" in the USA, and there's even less chance of all our roads being torn up and rebuilt (at huge expense and disruption) because some kid in Massachusetts thinks they're designed incorrectly.

If you then take the OP's theory (which I highly doubt he actually believes) that left-side ramps should be the standard, then extrapolating that theory to Australia or the UK, or really any other country where you drive on the left (well, maybe not Samoa as I doubt they have freeways), would mean in those countries right-side ramps should be the standard.

Do you not understand the idea of hypothetical? Here, I'll help you

Definition of hypothetical
: involving or being based on a suggested idea or theory : being or involving a hypothesis :

Ergo, "Fictional Highways." Keep digging that hole, sonny. Maybe someday you'll reach China.

[thspfc]

In a world where left hand ramps were the norm

So, Australia?

The way the OP has taken this thread strongly suggests maybe it belongs in "Fictional Highways," because that's what "a world where left hand ramps were the norm" is (also note his use of the subjunctive "were" underscoring the condition contrary to fact). It didn't necessarily start out that way because he cited an article whose author suggests that having a mixture of left- and right-side ramps wouldn't be the bad thing most of us believe it is, which is an interesting theory, but it's clear that the OP is, as usual, desperate to push a different theory from that on us. There is zero chance, none whatsoever, that left-side ramps will ever be "the norm" in the USA, and there's even less chance of all our roads being torn up and rebuilt (at huge expense and disruption) because some kid in Massachusetts thinks they're designed incorrectly.

If you then take the OP's theory (which I highly doubt he actually believes) that left-side ramps should be the standard, then extrapolating that theory to Australia or the UK, or really any other country where you drive on the left (well, maybe not Samoa as I doubt they have freeways), would mean in those countries right-side ramps should be the standard.

Do you not understand the idea of hypothetical? Here, I'll help you

Definition of hypothetical
: involving or being based on a suggested idea or theory : being or involving a hypothesis :

Ergo, "Fictional Highways." Keep digging that hole, sonny. Maybe someday you'll reach China.

I think we're all getting a little tired of kernals12's contrarian posts. He went on a two-week tirade in the weather board about how global warming is good because it makes it warmer in Boston in the winter. All of his posts are just looking for a negative reaction.

[kphoger]

This whole line of discussion assumes that every single interchange along a stretch of freeway can be a SPUI /slash/ tight diamond interchange.

Traffic volumes warrant a cloverleaf somewhere?  Too bad!
Railroad along a crossroad necessitates a ParClo?  Too bad!
Stoplight synchronization along a crossroad destroyed by a SPUI?  Too bad!

Because, unless ALL of the interchanges can have left-side ramps, the right lane cannot simply be "the fast lane".

[CtrlAltDel]

In a fit of boredom, I created this lefthand cloverleaf. It's more or less workable, except that the footprint is enormous, about four times the size of a regular cloverleaf.

Behold:



Also, now that I think about it, the loop ramps are arguably unnecessary, in that you can just do three straight ramps in a row to end up in the right direction (much like you can do three loop ramps in a row on a regular cloverleaf), but that doesn't really save any space.

[kernals12]

In a fit of boredom, I created this lefthand cloverleaf. It's more or less workable, except that the footprint is enormous, about four times the size of a regular cloverleaf.

Behold:

It also looks like the weaving on left hand turns is even worse than on a normal cloverleaf.

Here's a better low cost interchange with left hand ramps

[GaryV]

That one has both left and right, so it fails k's theory that all ramps could be on the left.  (could be - hypothetical - happy?)

[1995hoo]

This whole line of discussion assumes that every single interchange along a stretch of freeway can be a SPUI /slash/ tight diamond interchange.

Traffic volumes warrant a cloverleaf somewhere?  Too bad!
Railroad along a crossroad necessitates a ParClo?  Too bad!
Stoplight synchronization along a crossroad destroyed by a SPUI?  Too bad!

Because, unless ALL of the interchanges can have left-side ramps, the right lane cannot simply be "the fast lane".

This sort of follows the point I was trying to make about how the original post, and the blog entry linked there, envisioned BOTH left- AND right-side exits:

Left hand ramps are frowned upon by highway engineers due to the safety problem posed by having people merge into the fast lane when they're entering the freeway, but in 2011, Gilbert Chlewicki, the man who invented the diverging diamond interchange (or rather reinvented it after France tried it and gave up on it in the 70s, decided to defend the undefendable.

....

Lane Balancing — When all ramps are on the right side of the highway, it requires a heavier load of traffic in the right lanes of the highway. When an interchange has a high volume of traffic using the ramps, this can cause delays on the entire highway due to the heavy weaving needed that slows traffic down. When left ramps are introduced, traffic can be more evenly distributed among all the lanes of the highway, which can improve overall traffic flow.

....

....

Think about it for a minute. kernals12 has been going on about having all left-side exits, or making them the predominant style. But wouldn't that just mean you'd have the same "imbalance problem" theorized in the paragraph quoted above, just on the left instead of on the right? The blog writer clearly envisions a mix of ramps on both sides so as to "balance" the traffic load.

I think that idea is more interesting as a discussion topic than kernals12's attempt to change the focus. As a practical matter, though, to some degree I kind of like having more of the traffic concentrated to one side if I don't want to exit. For example, back in the 1990s, before the Springfield Interchange in Virginia was rebuilt, if I were headed from Fairfax to Alexandria on the Beltway, I could get all the way over to the left and I'd bypass the long queue of crawling or stopped traffic over to the right that was trying to head to southbound I-95. The "imbalance" was a good thing as far as I was concerned. And as I said earlier in the thread, if there were a lot of exits on both sides and the highway had only two lanes in each direction, there would be no real "passing lane," and that would be a problem.

[kernals12]

In a fit of boredom, I created this lefthand cloverleaf. It's more or less workable, except that the footprint is enormous, about four times the size of a regular cloverleaf.

Behold:



Also, now that I think about it, the loop ramps are arguably unnecessary, in that you can just do three straight ramps in a row to end up in the right direction (much like you can do three loop ramps in a row on a regular cloverleaf), but that doesn't really save any space.

Could you draw a stack interchange with left handed ramps?

[jeffandnicole]

Anyone who is in favor of left-sided ramps probably failed the entire concept of walking to the ride side of the hallway since Kindergarten.

[CtrlAltDel]

Could you draw a stack interchange with left handed ramps?

Here you go. You could probably shave a bit more space from the middle if you wanted to.

[GaryV]

... the ride side of the hallway since Kindergarten.

You had a ride side? Was it kind of like the "Stand/Walk" markings you see in airports?   

Must have been some kind of experimental model school.   

[kernals12]

Could you draw a stack interchange with left handed ramps?

Here you go. You could probably shave a bit more space from the middle if you wanted to.

I was assuming you'd have flyovers for the left turns, like in the Lodge-Ford Interchange in Detroit and then your right turns would branch off of those connecting to a flyover going in the opposite direction.

[SEWIGuy]

Anyone who is in favor of left-sided ramps probably failed the entire concept of walking to the ride side of the hallway since Kindergarten.

But if I wanted to turn left in Kindergarten, I wouldn't go to the right and "fly over" to head left.   

That being said, I do think that there are times where things get "over engineered" to eliminate left hand exits.  Whether or not the cost of that engineering makes sense is unknown to me.

[jeffandnicole]

Anyone who is in favor of left-sided ramps probably failed the entire concept of walking to the ride side of the hallway since Kindergarten.

But if I wanted to turn left in Kindergarten, I wouldn't go to the right and "fly over" to head left.   

That being said, I do think that there are times where things get "over engineered" to eliminate left hand exits.  Whether or not the cost of that engineering makes sense is unknown to me.

This thought process immediately shows that people only care about themselves, not the overall flow of traffic.  That's where engineering comes in.

In terms of my example, sure, if you want to take an exaggerated view of things, you're right. 

But, people should keep to the right when walking.  When people stop at their lockers and such, the flow of traffic moves towards the center of the hallway.  But what happens when a few people stop to talk in the middle of the hallway?  Congestion ensues. People have to walk around them, which entails getting close to those already stopped at their lockers.

If you don't want to use a school as an example, think of a supermarket where people stop their carts in the middle.  Or a walking trail, where people stop to talk.  In each case, the ones stopped (just a few) are causing the congestion, and the others have to figure a way around.

To continue on, when you believe things are over engineered, again, you are looking at a very small area, thru horse blinders, without considering the overall impact of traffic flow.  You are also very unaware of all the costs of highway construction.  Why even bother with interchanges? To be cheaper, all roadways should just have intersections.  Traffic lights cost money, so just use stop signs. Wider roadways cost money, so make them one lane each way.  Don't bother with shoulders.  Eliminate signage.  Drainage is a huge cost of roadway building, so just let the roadways and surrounding infrastructure flood.  The base of the roadway goes a foot or more down below the surface.  2 inches of asphalt would definitely reduce the costs.

So the cost to build something is an important part of a project, but it's by far not the only thing.  If it costs $10 million to build something that isn't expected to congest for 20 years, or $1 million to build something that will instantly congest, transportation departments would rather spend the $10 million even though it's more money.