Crash prone 'modern roundabouts'

[jakeroot]

Or maybe they would've killed innocent people in that other car.

Still didn't get to find out. Wouldn't you rather take a chance than just kill anyone who loses control?

I'm not trying to defend the placement of the concrete, but in order for it to kill someone, it isn't just losing control. Excessive speed plays a significant role. Look at the photo... there's plenty of warning. For crying out loud, there's two sets of overhead flashing lights. Even if the concrete wasn't there, a basic mounded central island might not have changed the ultimate outcome. Depending on speed (which we still don't know), they could have been sent flying.

Given the age of the occupants, there's no guarantee they would have survived hitting a breakaway street light. With age comes added frailty. Until the full report comes out, it could have been a medical condition before they ever hit anything. It could have been a heart attack when they crashed. Until we know the medical report and the speed they were traveling at saying the concrete killed them is only one hypothesis. Is placing vertical concrete in that location advisable? No. But it's not like the concrete randomly decided to spring up and yell "boo!"

Fair enough. I shouldn't assume that the concrete killed them. But the part I bolded is basically my argument here: concrete central islands are a poor choice of decoration. If their plan is to build up a wall of bark or dirt around the concrete wall, that would be great (although not as good as flat with bushes...we don't need to be sending cars flying). Maybe they're making a hill here. In any case, even with everything else being up in the air, I'm sure you'd agree that the placement of that concrete island did not do the occupants of that car any favors. That is my only point here. Drivers f' up, a lot. I don't see why Caltrans (or any agency) needs to be throwing drivers additional curveballs. Roundabouts are hard enough for most people...

What if they kept driving straight from the base approach of a T-intersection and ran into a building or the concrete stairs leading up to a building. Would you blame the building?

Buildings serve a purpose...fucking concrete central islands do what exactly? Kill people who lose control?

Ok, don't like that scenario, how about a power pole on the outside of a curve. Or, a big 80-year old oak tree. Either of those items don't come with as much warning. When does it become the driver's fault?

Usually if they're placed in the middle of a road, there'd be some warning. Most freeways are lined with concrete barriers much like the central island of this roundabout, but importantly, they aren't directly in the path of drivers. Every now and then, someone spins out and whacks into one, but no one hits one head-on because they're on their phone or because they have a medical emergency. Usually a side-swipe. The central island of a roundabout is directly in the path of oncoming traffic. Yes, many roundabouts feature chicanes, but the lanes are so wide you can usually navigate it in a straight path anyways.

I can't recall for certain, but there' usually a clear path next to most roadways nowadays too, so that trees and power poles aren't directly adjacent to major highways (in case drivers run off the road). Then again, there'd be more of an argument for power poles and trees being adjacent to roads, since they at least have a purpose. I still don't know what that concrete island has done for anyone, besides look "pretty".

[tradephoric]

Here is a news report regarding the fatal crash Wednesday night of the elderly couple at the Highway 12 and Highway 113 roundabout.  In it, Caltrans spokeswoman Janis Mara defends the safety of roundabouts by citing 2000 IIHS safety statistics.  Lots has changed with roundabout design over the past 20 years in this country, but apparently citing a report with 20 year old data is good enough for Caltrans. This is what Janis had to say midway through the news report...

"The shape of the roundabout results in lower speeds, generally 15 to 25 miles per hour.  And as we all know, slower is safer." 

On the surface Janis' statement sounds reasonable, but just imagine if the safety of traffic signals was touted by saying "the red lights at a traffic signal results in drivers coming to a complete stop.  And as we all know, a stationary vehicle is safer than a moving vehicle." 

https://www.youtube.com/watch?v=MHiD8T_u99U

[jakeroot]

What truly bothers me is that a driver approaching that intersection at the speed limit has no chance of being able to navigate it properly. Slower is safer, sure, but what if someone isn't going slow? That's catastrophic, and it's a problem that roundabouts don't seem well-equipped to handle. Not least without a giant magnet in the central island to that can grab onto an out-of-control car.

[kphoger]

Features inside the central island have been advocated in the past as a way of making the roundabout more visible to drivers and therefore less likely to be hit.  Without some sort of retaining wall, wouldn't any built-up feature inside the island run the risk of having fill dirt and rainwater flow onto the circulating roadway and create hazardous conditions?

[jeffandnicole]

Here is a news report regarding the fatal crash Wednesday night of the elderly couple at the Highway 12 and Highway 113 roundabout.  In it, Caltrans spokeswoman Janis Mara defends the safety of roundabouts by citing 2000 IIHS safety statistics.  Lots has changed with roundabout design over the past 20 years in this country, but apparently citing a report with 20 year old data is good enough for Caltrans.

Well, at least that's better than what Bazoo used when trying to defend something.  His defense was utilizing data from 1979!

https://www.aaroads.com/forum/index.php?topic=17051.msg2443849#msg2443849

What truly bothers me is that a driver approaching that intersection at the speed limit has no chance of being able to navigate it properly. Slower is safer, sure, but what if someone isn't going slow? That's catastrophic, and it's a problem that roundabouts don't seem well-equipped to handle. Not least without a giant magnet in the central island to that can grab onto an out-of-control car.

No different than approaching a stop sign, a red light, a curve in the road, congestion, a person in a pedestrian crossing, etc.  The speed limit isn't a mandatory speed to drive, and a driver has a duty to exercise caution and slow down when necessary.

[kalvado]

Features inside the central island have been advocated in the past as a way of making the roundabout more visible to drivers and therefore less likely to be hit.  Without some sort of retaining wall, wouldn't any built-up feature inside the island run the risk of having fill dirt and rainwater flow onto the circulating roadway and create hazardous conditions?

Lots of collapsible energy absorbing systems have been developed.
Although, I keep enjoying all those attempts of resolving multiple issues caused by a single faulty design choice. 

[jakeroot]

What truly bothers me is that a driver approaching that intersection at the speed limit has no chance of being able to navigate it properly. Slower is safer, sure, but what if someone isn't going slow? That's catastrophic, and it's a problem that roundabouts don't seem well-equipped to handle. Not least without a giant magnet in the central island to that can grab onto an out-of-control car.

No different than approaching a stop sign, a red light, a curve in the road, congestion, a person in a pedestrian crossing, etc.  The speed limit isn't a mandatory speed to drive, and a driver has a duty to exercise caution and slow down when necessary.

Of course. But when drivers don't do their duty? Those other features tend to be more forgiving:

* stop signs can be run and you may not hit anything
* red lights are the same as stop signs
* curves often have guiderails to keep you on the road, especially on high-speed roads, that you generally won't T-bone.

Congestion and crossing pedestrians are generally not as forgiving, but are also features of roundabouts, so I'm not sure I understand your point.

[jakeroot]

Features inside the central island have been advocated in the past as a way of making the roundabout more visible to drivers and therefore less likely to be hit.  Without some sort of retaining wall, wouldn't any built-up feature inside the island run the risk of having fill dirt and rainwater flow onto the circulating roadway and create hazardous conditions?

You can probably still use a small curb, or even some rocks. A two or three foot-tall barrier just seems silly.

It can also be designed like a bioswale, where there is a drop that collects dirt and rainwater (rather than having the roadway surface be the lowest part of the design):


Image from Flickr -- Dianne Yee

[TEG24601]

It seems like Caltrans could have built their roundabout more like the ones WSDOT have built, with more nuanced approaches, and a mound in the middle.  If they want one at higher speeds, then maybe a rotary would more appropriate, just requires more land.  Then again, hearing what this road does, is seems asinine for Caltrans to keep the roadway 2 lane, and perhaps this intersection really should be grade separated.

[jakeroot]

It seems like Caltrans could have built their roundabout more like the ones WSDOT have built, with more nuanced approaches, and a mound in the middle.

What exactly do you mean when you say "nuanced approaches"? I would agree that a mound in the middle might have been better, although that could have sent the car flying!

[DaBigE]

What if they kept driving straight from the base approach of a T-intersection and ran into a building or the concrete stairs leading up to a building. Would you blame the building?

Buildings serve a purpose...fucking concrete central islands do what exactly? Kill people who lose control?

Ok, don't like that scenario, how about a power pole on the outside of a curve. Or, a big 80-year old oak tree. Either of those items don't come with as much warning. When does it become the driver's fault?

Usually if they're placed in the middle of a road, there'd be some warning. Most freeways are lined with concrete barriers much like the central island of this roundabout, but importantly, they aren't directly in the path of drivers. Every now and then, someone spins out and whacks into one, but no one hits one head-on because they're on their phone or because they have a medical emergency. Usually a side-swipe. The central island of a roundabout is directly in the path of oncoming traffic. Yes, many roundabouts feature chicanes, but the lanes are so wide you can usually navigate it in a straight path anyways.

I can't recall for certain, but there' usually a clear path next to most roadways nowadays too, so that trees and power poles aren't directly adjacent to major highways (in case drivers run off the road). Then again, there'd be more of an argument for power poles and trees being adjacent to roads, since they at least have a purpose. I still don't know what that concrete island has done for anyone, besides look "pretty".

Reread what is bolded. I'm not talking about an object in the middle of a road. I'm referring to a rural two-lane highway with a curve, similar to something like this. Miss the one warning sign on a dark night, and you're going off-roading. Sure, the chances of hitting the pole are much less than hitting a wall, but the consequence is the same.

[jakeroot]

What if they kept driving straight from the base approach of a T-intersection and ran into a building or the concrete stairs leading up to a building. Would you blame the building?

Buildings serve a purpose...fucking concrete central islands do what exactly? Kill people who lose control?

Ok, don't like that scenario, how about a power pole on the outside of a curve. Or, a big 80-year old oak tree. Either of those items don't come with as much warning. When does it become the driver's fault?

Usually if they're placed in the middle of a road, there'd be some warning. Most freeways are lined with concrete barriers much like the central island of this roundabout, but importantly, they aren't directly in the path of drivers. Every now and then, someone spins out and whacks into one, but no one hits one head-on because they're on their phone or because they have a medical emergency. Usually a side-swipe. The central island of a roundabout is directly in the path of oncoming traffic. Yes, many roundabouts feature chicanes, but the lanes are so wide you can usually navigate it in a straight path anyways.

I can't recall for certain, but there' usually a clear path next to most roadways nowadays too, so that trees and power poles aren't directly adjacent to major highways (in case drivers run off the road). Then again, there'd be more of an argument for power poles and trees being adjacent to roads, since they at least have a purpose. I still don't know what that concrete island has done for anyone, besides look "pretty".

Reread what is bolded. I'm not talking about an object in the middle of a road. I'm referring to a rural two-lane highway with a curve, similar to something like this. Miss the one warning sign on a dark night, and you're going off-roading. Sure, the chances of hitting the pole are much less than hitting a wall, but the consequence is the same.

Oh no, no, I got what you meant. What I was attempting to say that was, if a power pole or tree, hypothetically, ended up in the middle of a road (historic tree or impossible-to-move power structure), there might be some kind of warning because the object will likely require a driver to maneuver around it (going straight may end up causing a crash).

In the same manner, roundabouts have warning signs (W2-6) because drivers must take action to avoid a collision with the central island. You don't really have to take action to avoid a tree or power pole on the outside of a curve, assuming some sort of guiderail is in place along the curve. Yes, you'll still crash, but it won't be a head-on crash.

[skluth]

What truly bothers me is that a driver approaching that intersection at the speed limit has no chance of being able to navigate it dproperly. Slower is safer, sure, but what if someone isn't going slow? That's catastrophic, and it's a problem that roundabouts don't seem well-equipped to handle. Not least without a giant magnet in the central island to that can grab onto an out-of-control car.

And here we get to the root of the disagreement. Some of us actually expect people to be good drivers when driving. Others want all drivers but them to be in self-driving cars because others can't be trusted. Most drivers don't have a problem with non-roundabout driving challenges like stop lights and no passing zones. I don't get why roundabouts are any different.

[jakeroot]

What truly bothers me is that a driver approaching that intersection at the speed limit has no chance of being able to navigate it dproperly. Slower is safer, sure, but what if someone isn't going slow? That's catastrophic, and it's a problem that roundabouts don't seem well-equipped to handle. Not least without a giant magnet in the central island to that can grab onto an out-of-control car.

And here we get to the root of the disagreement. Some of us actually expect people to be good drivers when driving. Others want all drivers but them to be in self-driving cars because others can't be trusted. Most drivers don't have a problem with non-roundabout driving challenges like stop lights and no passing zones. I don't get why roundabouts are any different.

Let's be real: humans are not good at operating cars. Despite cars having been around for over a hundred years, we still continue to kill each other with them every year, by the tens of thousands. Are deaths down from the 70s? Oh yeah, but are they continuing to drop? Not like they were, and 2018 apparently was the first drop in deaths since 2015, and it wasn't a big drop. For the record, I'm not part of the "everyone sucks but me" campaign. I regularly make small errors that really remind me to pay attention.

I'm not trying to use scare-tactics here, but there's very little reason to believe that, well, at least Americans, are capable of handling roundabouts. Engineers typically build things with multiple redundancies, to avoid catastrophic failure (look up all the redundancies for the Tower of Terror...really amazing stuff). Traffic lights technically have a fail-safe operation: if someone approaches an intersection without noticing the signal, they can, hypothetically, pass through it unharmed because there are no obstacles directly in front of them other than other cars (which there would be at roundabouts too, so it's still a fair comparison). Guiderailes are a very common safety/redundancy feature as well, and keep cars from flying off the road. They are mostly designed to deflect rather than absorb (except the end pieces), so vehicle occupants should survive. Another is breakaway sign posts, which give way with enough force, to avoid completely destroying whatever hits it.

Compare this to the Solano County roundabout. What is its failsafe? What if a driver doesn't notice the roundabout? There will absolutely be a crash. If not with another car, then with that stupid concrete island.

Honestly, if they removed the concrete island, I would be fine with this roundabout. If they installed a shit-ton of bushes in the middle, with the hope that they would absorb the energy of a car, that could be their redundancy feature.

[DaBigE]

Let's be real: humans are not good at operating cars. Despite cars having been around for over a hundred years, we still continue to kill each other with them every year, by the tens of thousands. Are deaths down from the 70s? Oh yeah, but are they continuing to drop? Not like they were, and 2018 apparently was the first drop in deaths since 2015, and it wasn't a big drop.

One, IMO, big contributing factor in the lack of drops in crashes is the ever increasing amount of technological distractions. No matter how many crashes occur, too many drivers still feel a text message is more important than the vehicle they're operating.

Engineers typically build things with multiple redundancies, to avoid catastrophic failure (look up all the redundancies for the Tower of Terror...really amazing stuff). Traffic lights technically have a fail-safe operation: if someone approaches an intersection without noticing the signal, they can, hypothetically, pass through it unharmed because there are no obstacles directly in front of them other than other cars (which there would be at roundabouts too, so it's still a fair comparison).

Sorry, I have a hard time agreeing with the notion that traffic lights have fail-safe operation. Russian roulette-style fail-safe, maybe. Miss the signal and you are either really lucky, or fail in a really ugly fashion. At least with a roundabout, you have the chance the splitter island curb will slow-down or redirect a errant vehicle (granted, those chances decrease on multilane approaches).

[kalvado]

At least with a roundabout, you have the chance the splitter island curb will slow-down or redirect a errant vehicle (granted, those chances decrease on multilane approaches).

Bingo! Just make sure it is actually slowed down, not launched up by a truck apron; nor compacted by chunk of solid concrete. SOmething like soft soil, maybe with some vegetation?...

[tradephoric]

Sorry, I have a hard time agreeing with the notion that traffic lights have fail-safe operation. Russian roulette-style fail-safe, maybe. Miss the signal and you are either really lucky, or fail in a really ugly fashion. At least with a roundabout, you have the chance the splitter island curb will slow-down or redirect a errant vehicle (granted, those chances decrease on multilane approaches).

Blowing through the middle of a roundabout with a concrete retaining wall in the central island is like playing Russian Roulette with 6 bullets in the chamber.  Fixed object fatalities at roundabouts is a real concern and here is what the FHWA had to say on the issue in its 2015 report "A Review of Fatal and Severe Injury Crashes at Roundabouts" .

Fixed-object crash types were the most common crash types among the Injury A and B crashes at roundabouts in Washington and Wisconsin and were involved in 85 percent of fatal crashes at roundabouts. Of the 39 fatal fixed object crashes at roundabouts, 35 involved vehicles striking the curb. In some cases, multiple fixed objects were involved in a single crash. For example, a vehicle may have struck the curb first and a sign post second.

Given that roundabouts rely on effective channelization using raised features such as splitter islands and central islands, as well as on signing to communicate legal movements and right-of-way to users, some fixed objects are inherent in the design of roundabouts and must be present in the design. Other objects that were hit, such as boulders, retaining walls, trees, and landscaping, may be optional at many roundabout locations. Based on these observations, the following sections summarize recommendations made about fixed objects...

Rigid Fixed Objects
Thoughtful placement of fixed objects is a basic principle of proper roadside design. In 2011, the American Association of State Highway and Transportation Officials (AASHTO) published an updated Roadside Design Guide (RDG), and the revisions to Chapter 10 on Roadside Safety in Urban or Restricted Environments represented a new perspective on roadside design in lower speed and urban contexts.(7) Roundabouts share many of the characteristics discussed in this chapter of the RDG, and hence the updated guidance may be of value for roundabout design as well. In particular, as described in the RDG, it could be worthwhile to consider what may be "high risk"  locations for fixed objects unique to roundabouts, given the movements of traffic approaching, circulating, and leaving the vicinity of a roundabout.

If you put that Solano County roundabout in context it's just a really bad spot to have an unnecessary rigid fixed object.  Drivers traveling east on Highway 12 leaving Fairfield have been traveling down the rural highway for over 10 miles without so much as traveling through a traffic signal.  The posted speed limit is 55 mph and they are traveling at a downhill grade as they approach the Highway 12/Highway 113 roundabout.  For some drivers that roundabout could very easily sneak up on them which is a recipe for disaster once they encounter a concrete wall in the middle of the intersection. 

[sparker]

^^^^^^^^^^^
Unfortunately, Caltrans planning seems to be going through a prolonged phase of concept over reality; putting a roundabout on CA 12 anywhere between I-5 and I-80 -- a "shortcut" from the North Bay/"wine country" to Valley N-S arterials that avoids much of Bay traffic -- was and is a recipe for disaster, as evidenced by the fatality.  It's as if the prevailing "wisdom" (right!) is to attempt to micromanage rather than expedite open-road travel -- in this case, forcing high-speed through traffic into sporadic slowdowns to achieve -- well, what? -- a drop in the aggregate speed; a punitive measure for actually being out on the road to begin with!? -- the rationale, or lack thereof, behind putting obstacles in the way of rural through traffic is astonishing!  These are connecting corridors, folks, not parts of a miniature golf course!  I've been through 12/113 (pre-roundabout) numerous times; a channelized and signalized intersection -- with plenty of advance notice, particularly on 12 in both directions, would have been appropriate, not a center-of road structure.  I understand the desire of some planners to attempt to get folks out of their cars as much as possible -- but not through their windshields!

[DaBigE]

^^^^^^^^^^^
Unfortunately, Caltrans planning seems to be going through a prolonged phase of concept over reality; putting a roundabout on CA 12 anywhere between I-5 and I-80 -- a "shortcut" from the North Bay/"wine country" to Valley N-S arterials that avoids much of Bay traffic -- was and is a recipe for disaster, as evidenced by the fatality.  It's as if the prevailing "wisdom" (right!) is to attempt to micromanage rather than expedite open-road travel -- in this case, forcing high-speed through traffic into sporadic slowdowns to achieve -- well, what? -- a drop in the aggregate speed; a punitive measure for actually being out on the road to begin with!? -- the rationale, or lack thereof, behind putting obstacles in the way of rural through traffic is astonishing!  These are connecting corridors, folks, not parts of a miniature golf course!  I've been through 12/113 (pre-roundabout) numerous times; a channelized and signalized intersection -- with plenty of advance notice, particularly on 12 in both directions, would have been appropriate, not a center-of road structure.  I understand the desire of some planners to attempt to get folks out of their cars as much as possible -- but not through their windshields!

Based on your description, a signal is no better of a solution. Having people come to a complete stop in the middle of a rural 55-mph highway is no better than a roundabout. By your description, a grade separated interchange is the best solution, as any other form of intersection control has no choice but to negatively impact throughput on the mainline at some point.

I don't recall seeing one person defend the vertical concrete surface in the middle of the roundabout; can we finally move on?

[tradephoric]

I don't recall seeing one person defend the vertical concrete surface in the middle of the roundabout; can we finally move on?

Your previous comment didn't sound like a ringing endorsement to get rid of retaining walls in the central islands of roundabouts.  Rather it sounded like you were questioning what would be a better way to go out...

Even if the concrete wasn't there, a basic mounded central island might not have changed the ultimate outcome. Depending on speed (which we still don't know), they could have been sent flying.

Of course if you catapult your vehicle high enough into the air, you may clear the dangerous fixed objects in the central island (boulders, retaining walls, trees, etc.).  Here's a few examples of this strategy (these are both non-fatal crashes):

https://www.youtube.com/watch?v=xTk_kPeA63c

https://www.youtube.com/watch?v=2Wgw4Fg8Vuw

[kphoger]

It's a new kind of runaway truck ramp.

[DaBigE]

I don't recall seeing one person defend the vertical concrete surface in the middle of the roundabout; can we finally move on?

Your previous comment didn't sound like a ringing endorsement to get rid of retaining walls in the central islands of roundabouts.  Rather it sounded like you were questioning what would be a better way to go out...

How I missed tradephoric's cherry-picked commentary...said no one ever. 

For your selective eyesight; the whole quote you selectively read (emphasis added):

Or maybe they would've killed innocent people in that other car.

Still didn't get to find out. Wouldn't you rather take a chance than just kill anyone who loses control?

I'm not trying to defend the placement of the concrete, but in order for it to kill someone, it isn't just losing control. Excessive speed plays a significant role. Look at the photo... there's plenty of warning. For crying out loud, there's two sets of overhead flashing lights. Even if the concrete wasn't there, a basic mounded central island might not have changed the ultimate outcome. Depending on speed (which we still don't know), they could have been sent flying.

Given the age of the occupants, there's no guarantee they would have survived hitting a breakaway street light. With age comes added frailty. Until the full report comes out, it could have been a medical condition before they ever hit anything. It could have been a heart attack when they crashed. Until we know the medical report and the speed they were traveling at saying the concrete killed them is only one hypothesis. Is placing vertical concrete in that location advisable? No. But it's not like the concrete randomly decided to spring up and yell "boo!"

The context was whether or not the concrete killed the occupants, not whether or not it should have been there in the first place.

[tradephoric]

The word 'but' is used to nullify all words preceding it and implies the sentence said before it wasn't worth saying.  Someone's true feelings always comes after the word but, not before.

I'm not trying to defend the placement of the concrete, but in order for it to kill someone, it isn't just losing control. Excessive speed plays a significant role. Look at the photo... there's plenty of warning. For crying out loud, there's two sets of overhead flashing lights. Even if the concrete wasn't there, a basic mounded central island might not have changed the ultimate outcome. Depending on speed (which we still don't know), they could have been sent flying.

Given the age of the occupants, there's no guarantee they would have survived hitting a breakaway street light. With age comes added frailty. Until the full report comes out, it could have been a medical condition before they ever hit anything. It could have been a heart attack when they crashed. Until we know the medical report and the speed they were traveling at saying the concrete killed them is only one hypothesis. Is placing vertical concrete in that location advisable? No. But it's not like the concrete randomly decided to spring up and yell "boo!"

[DaBigE]

Tradephoric, you're taking the statements out of context regardless of the presence of the word 'but' and you know it. STFU with your word games. Whatever I say doesn't matter, since you only hear what you want to hear. Do yourself a favor and ignore my posts; I am done putting up with your BS.

[sparker]

^^^^^^^^^^^
Unfortunately, Caltrans planning seems to be going through a prolonged phase of concept over reality; putting a roundabout on CA 12 anywhere between I-5 and I-80 -- a "shortcut" from the North Bay/"wine country" to Valley N-S arterials that avoids much of Bay traffic -- was and is a recipe for disaster, as evidenced by the fatality.  It's as if the prevailing "wisdom" (right!) is to attempt to micromanage rather than expedite open-road travel -- in this case, forcing high-speed through traffic into sporadic slowdowns to achieve -- well, what? -- a drop in the aggregate speed; a punitive measure for actually being out on the road to begin with!? -- the rationale, or lack thereof, behind putting obstacles in the way of rural through traffic is astonishing!  These are connecting corridors, folks, not parts of a miniature golf course!  I've been through 12/113 (pre-roundabout) numerous times; a channelized and signalized intersection -- with plenty of advance notice, particularly on 12 in both directions, would have been appropriate, not a center-of road structure.  I understand the desire of some planners to attempt to get folks out of their cars as much as possible -- but not through their windshields!

Based on your description, a signal is no better of a solution. Having people come to a complete stop in the middle of a rural 55-mph highway is no better than a roundabout. By your description, a grade separated interchange is the best solution, as any other form of intersection control has no choice but to negatively impact throughput on the mainline at some point.

I don't recall seeing one person defend the vertical concrete surface in the middle of the roundabout; can we finally move on?

When it comes to a signalized intersection at the 12/113 junction, I clearly mentioned quite a bit of advance notice; previous Caltrans installations of this type have placed warnings of a red signal phase in the form of flashing notification to that effect at least 3/4 mile back from the interchange itself.  That has proven to be quite effective at getting traffic to slow down if and when a red signal is imminent; ample time & space to drop from 65-70 to zero.  I doubt whether advance notification of this sort is placed for a roundabout; the ones I've encountered simply have stepped speed drops.  I'm just questioning the appropriateness of this format in high-speed non-urban situations; it seems like the "garbage can" method -- a solution searching for a problem!