Crash prone 'modern roundabouts'

[kalvado]

But anyway, a bit of harsh reminders:
Unlike college, real life doesn't give credit for the effort. No partial credit for good intentions. No extra point for beauty until core function is there.
Real life performance is a figure of merit; and if public (who is paying engineer's paycheck via taxes) is not good enough too achieve that - there is no way for engineer to get different public; but public can easily hire a different engineer.

The flaw in your argument is how you define the "core function". By that metric, there are a lot of signalized intersections and for that matter, freeways that fail to meet your standards. It is impossible to engineer the risk out of everything. Thinking/expecting perfection at every turn is insanity.

If you look at my writeup carefully, I never mentioned any safety parameter; I just massaged some butt for "stupid public" type comment.

[jakeroot]

It is impossible to engineer the risk out of everything. Thinking/expecting perfection at every turn is insanity.

No way anyone expects perfection. But I would expect engineers to do at least a modicum of research on the crash rates at complex roundabouts. tradephoric's data is informal at best, but data is data. Places like Kitchener-Waterloo or Carmel should be well aware of the constant issues with their original complex roundabouts, but they continue to build more identical intersections. Expecting them to suddenly perform better than the old roundabouts is insane.

[kalvado]

You can engineer it to be 100% but the public doesn't know any better on how it works. Signs and painted arrows can only do so much. We're seeing a lot of user error with these so maybe we need to start teaching people how to properly use these intersections.

Short of mandating additional driver training every 5 or 10 years, there's not much that can be done to educate people beyond what's already out there.  You can't force people to look at websites and pamphlets.

When you have 40,000 people killed on the road each year, and roundabouts make up about a dozen of those deaths, there's a huge room for improvements on all types of roadways and intersections.

Even with extra training, excessive risk  is an unavoidable part of a  multilane roundabout concept. This is pretty obvious once you consider possible conflicts at roundabout entry; and those risks are actually well known and anticipated, as different localities ban RTOR from the second lane, not to mention third lane; or ban RTOR altogether. Conceptually those movements  are very similar to roundabout, if you think about it.

[tradephoric]

It is impossible to engineer the risk out of everything. Thinking/expecting perfection at every turn is insanity.

The Region of Waterloo converted its most crash prone signalized intersection at Homer Watson and Ottawa Street to a roundabout.  The result has been a five-fold increase in crashes with 181 total crashes and 18 injury crashes.  It has been a complete failure at reducing accidents which was the main objective of building the roundabout to begin with.  I'd say that's the opposite of perfection.   

[jeffandnicole]

It is impossible to engineer the risk out of everything. Thinking/expecting perfection at every turn is insanity.

The Region of Waterloo converted its most crash prone signalized intersection at Homer Watson and Ottawa Street to a roundabout.  The result has been a five-fold increase in crashes with 181 total crashes and 18 injury crashes.  It has been a complete failure at reducing accidents which was the main objective of building the roundabout to begin with.  I'd say that's the opposite of perfection.   

I'd say that's taking a generalized statement and labeling it with something it wasn't intended to be labeled with.

You know, like every meme out there where people make up their own saying.

[DaBigE]

It is impossible to engineer the risk out of everything. Thinking/expecting perfection at every turn is insanity.

The Region of Waterloo converted its most crash prone signalized intersection at Homer Watson and Ottawa Street to a roundabout.  The result has been a five-fold increase in crashes with 181 total crashes and 18 injury crashes.  It has been a complete failure at reducing accidents which was the main objective of building the roundabout to begin with.  I'd say that's the opposite of perfection.   

I'd say that's taking a generalized statement and labeling it with something it wasn't intended to be labeled with.

Without doing that, there'd be no point to this thread.

[tradephoric]

I agree.  If engineers didn't use generalized roundabout safety statistics as a rational for constructing complex roundabouts, there would be a lot less crash prone roundabouts to cite on this thread.  It's great that roundabouts reduce total crashes by 38%, but that general statistic ignores the fact that there is a certain subset of roundabouts (complex roundabouts with 2x2 and 2x3 circulating lanes) that have unacceptably high crash rates.  Taking the most crash prone signalized intersection in a region of over 500k people and somehow, almost incredibly, making the crash problem 5 times worse is a failure.  Nowhere close to perfection.

[jakeroot]

I don't really get the point of trying to shit on his data, or even his data collection methods.

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

In terms of the "generalized statement", I've not yet seen a roundabout built where crash-reduction wasn't touted as a primary benefit.

I will give credit to anyone who can find a press-release about a new roundabout where improved safety wasn't mentioned.

[DaBigE]

I don't really get the point of trying to shit on his data, or even his data collection methods.

No one is trying to "shit on his data" anymore. Many of us are just sick of the same old points being posted over and over and over again, mixed with selective quote wordplay to continue a point/agenda that was made tens of pages ago. We don't need a repeat news feed to announce every new crash; it's beating the same dead horse over and over again (kinda similar to someone's point about installing new multilane roundabouts and expecting crashes to vanish...pot meet kettle ).

I'd love to simply ignore this thread entirely, but I keep making the same mistake of coming back when there's a new post, in the [insane?] hopes there is going to be some new information that I could maybe use. But it looks like I am better-off sticking with my Google News feed and industry-supported sources. If I want crash reports, I'll just go straight to the source and cut out the snarky middleman.

[US 89]

I will give credit to anyone who can find a press-release about a new roundabout any road project where improved safety wasn't mentioned.

FTFY

[DaBigE]

I don't really get the point of trying to shit on his data, or even his data collection methods.

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

So now we're going to conspiracy theories? What actual (not hypothetical) evidence do you have to suggest agencies are "going to great lengths" to hide data? Many agencies (a few who have been reposted within this thread), publish annual crash reports. I have yet to see one of these reports with any roundabout-related crash information being redacted. The statement could also be reversed...if traffic signals are so damn good...

Why aren't they doing this? Because these same agencies don't have the manpower to waste on such an endeavor, and if they did, the public would start an outcry as to why their tax dollars are wasted on such things. Should the PD start putting out press releases as to why their Chevy squad cars are better than Dodge or Ford? Or is there a conspiracy with maintenance records and how much time the vehicles actually spend in the shop vs on the road?

[jakeroot]

I will give credit to anyone who can find a press-release about a new roundabout any road project where improved safety wasn't mentioned.

FTFY

For sure. Safety really is the paramount issue facing transport agencies in the 21st century. But more to my point, there's no point in joking about trade's generalization about safety: every agency touts safety improvements as being a key part of road works.

I'd love to simply ignore this thread entirely, but I keep making the same mistake of coming back when there's a new post, in the [insane?] hopes there is going to be some new information that I could maybe use. But it looks like I am better-off sticking with my Google News feed and industry-supported sources. If I want crash reports, I'll just go straight to the source and cut out the snarky middleman.

Okay. So go to your sources and get some damn data. Instead, users keep coming back to this thread, telling trade to stop cherry-picking data, and then disappear again. Present some alternative data, have an actual discussion. You know, things engineers might do (although clearly don't do since they keep building these giant roundabouts, apparently thinking theirs will be different).

I don't really get the point of trying to shit on his data, or even his data collection methods.

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

So now we're going to conspiracy theories? What actual (not hypothetical) evidence do you have to suggest agencies are "going to great lengths" to hide data? Many agencies (a few who have been reposted within this thread), publish annual crash reports. I have yet to see one of these reports with any roundabout-related crash information being redacted. The statement could also be reversed...if traffic signals are so damn good...

I don't have any; note my use of "almost certain" (i.e. not actually certain). I've based my assertion on how hard it has been for trade to find and organize data on these roundabouts.

Why aren't they doing this? Because these same agencies don't have the manpower to waste on such an endeavor, and if they did, the public would start an outcry as to why their tax dollars are wasted on such things. Should the PD start putting out press releases as to why their Chevy squad cars are better than Dodge or Ford? Or is there a conspiracy with maintenance records and how much time the vehicles actually spend in the shop vs on the road?

The "public would start an outcry" if their local municipalities took the time to analyze, organize, and publish crash data? That seems like a reach, unless your citizens were seriously hard-up. Personally, I think more people would start an outcry when they find out that their tax-funded ~$4M roundabouts have done nothing, except exchange slightly-worse traffic for an increase in collisions.

[sparker]

^^^^^^^^^^^
A main concern with all this is that roundabouts are just another example of what John Kingdon cited in his "garbage can" theory of general public policy -- a solution looking for a potential situation in which it can be applied.  Regardless of the data both before and after implementation, there's an institutional decision by the ranking jurisdiction (a DOT, city and/or county planners, etc.) concerning a decided preference for roundabouts.  The rash of deployment of such out here in CA by both Caltrans and local agencies seemingly at random points toward an adoption of a new "default" toward that configuration -- as the latest in a series of design "fads", placed in the field largely regardless of the specific situation.  A believer in conspiracies -- or deliberate coincidence -- might presume that there's a cross-agency goal to partially placate the naysayers of auto travel by placing these facilities where they create something of a disruption to continuous movement.  Personally, I do so hope that this isn't the case -- and that the agencies in question are continuing to engage in actual research and engineering rather than simple capitulation to some upper-level management whims!   

[jeffandnicole]

I think it's rare, if ever, when safety statistics are publicized long after any road project was completed.  I've seen numerous intersections get a traffic light, yet a year or two later there's no mention of crash data for those intersections either.  It's not like they're intentionally hiding data on roundabouts. 

And to that point - if there's 2 intersection improvements nearby; one getting a roundabout, one getting a traffic light, it would be hiding data if they only presented data on the traffic light intersection later on.  Unfortunately in this thread - and this is where the cherry-picking of data comes in - news only regarding the roundabout would be presented.  If there was 1 more crash at the roundabout, and 10 more crashes at the traffic light intersection, you can bet we'd only hear about the 1 at a roundabout.  Since we don't have a thread about Crash prone 'modern intersections', we are left to just get news on the roundabouts.

[kalvado]

I don't really get the point of trying to shit on his data, or even his data collection methods.

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

So now we're going to conspiracy theories? What actual (not hypothetical) evidence do you have to suggest agencies are "going to great lengths" to hide data? Many agencies (a few who have been reposted within this thread), publish annual crash reports. I have yet to see one of these reports with any roundabout-related crash information being redacted. The statement could also be reversed...if traffic signals are so damn good...

Why aren't they doing this? Because these same agencies don't have the manpower to waste on such an endeavor, and if they did, the public would start an outcry as to why their tax dollars are wasted on such things. Should the PD start putting out press releases as to why their Chevy squad cars are better than Dodge or Ford? Or is there a conspiracy with maintenance records and how much time the vehicles actually spend in the shop vs on the road?

And this is how engineering idiocy proliferate.
Post-mortem, lessons learned, project feedback - those are essential components of most professions; and a must for those where decisions are required.
Maybe no publishing - but an internal analysis is an absolute must.
Instead we have

What would be the criteria used to declare this project a success?

From an engineering perspective, no unusual circumstances proffer themselves (large cracks, misaligned joints, etc.) and everything is built to plan. From a public policy perspective, it comes in within budget and on time (and I believe it's actually a little early).

[kphoger]

we don't have a thread about Crash prone 'modern intersections'

Indeed, it is a thread we'll never see on AARoads.

[DaBigE]

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

So now we're going to conspiracy theories? What actual (not hypothetical) evidence do you have to suggest agencies are "going to great lengths" to hide data? Many agencies (a few who have been reposted within this thread), publish annual crash reports. I have yet to see one of these reports with any roundabout-related crash information being redacted. The statement could also be reversed...if traffic signals are so damn good...

I don't have any; note my use of "almost certain" (i.e. not actually certain). I've based my assertion on how hard it has been for trade to find and organize data on these roundabouts.

Almost certain, actually certain, spin it how you wish. Semantics doesn't change the base of your comment. "How hard"...how many agencies has he actually tried contacting? From what has been posted here, it's only been information readily available online. Just because you can't find it from an online search doesn't mean they're trying to hide anything.

Why aren't they doing this? Because these same agencies don't have the manpower to waste on such an endeavor, and if they did, the public would start an outcry as to why their tax dollars are wasted on such things. Should the PD start putting out press releases as to why their Chevy squad cars are better than Dodge or Ford? Or is there a conspiracy with maintenance records and how much time the vehicles actually spend in the shop vs on the road?

The "public would start an outcry" if their local municipalities took the time to analyze, organize, and publish crash data? That seems like a reach, unless your citizens were seriously hard-up. Personally, I think more people would start an outcry when they find out that their tax-funded ~$4M roundabouts have done nothing, except exchange slightly-worse traffic for an increase in collisions.

That's not what I was referring to.

Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

You were suggesting a post-construction PR campaign on how good the intersection is performing, basically advertising "how great" they are. THAT's what would generate an outcry. Again, not every agency has the staffing availability to run the numbers following each and every project they complete. Most of the general public doesn't care that deeply. If you pay attention to locations where roundabouts have been present for a number of years, the comments have changed to focus on the bad driving habits and not the roundabout itself.

The larger agencies are looking at the numbers and trying to figure out what went wrong. But in most cases, since the serious injury crashes are down and traffic is generally flowing better than before, they're going to move on to one of their many other fires they have to put out. Unfortunately, real-world engineering becomes a balancing act - there are always going to be trade-offs and compromises.

I'd love to simply ignore this thread entirely, but I keep making the same mistake of coming back when there's a new post, in the [insane?] hopes there is going to be some new information that I could maybe use. But it looks like I am better-off sticking with my Google News feed and industry-supported sources. If I want crash reports, I'll just go straight to the source and cut out the snarky middleman.

Okay. So go to your sources and get some damn data. Instead, users keep coming back to this thread, telling trade to stop cherry-picking data, and then disappear again. Present some alternative data, have an actual discussion. You know, things engineers might do (although clearly don't do since they keep building these giant roundabouts, apparently thinking theirs will be different).

Yes, sir, right away sir.    I'm really eager to serve those who keep insulting my profession, and painting all of us with the same broad brush.
My sources are barely any different than what any of you have. If someone is so hell-bent on disproving roundabouts, they have the time to research balanced information. Do your own damn digging.

[DaBigE]

I don't really get the point of trying to shit on his data, or even his data collection methods.

It's almost certain that cities, counties, or states are going to great lengths to hide their actual roundabout crash data, because they know most of them don't perform well (or at least not to the touted levels prior to construction). Carmel seems to be a good example of this. The few roundabouts that actually have data, and are complex 2x1+ configurations, mostly seem to perform poorly. Yes, you can get an FOI request, but if these roundabouts are so damn good, why aren't municipalities going out of their way to mention as much? They build these things and then it's just radio silence (or until they announce a lane reduction or some other change to combat worsening conditions).

So now we're going to conspiracy theories? What actual (not hypothetical) evidence do you have to suggest agencies are "going to great lengths" to hide data? Many agencies (a few who have been reposted within this thread), publish annual crash reports. I have yet to see one of these reports with any roundabout-related crash information being redacted. The statement could also be reversed...if traffic signals are so damn good...

Why aren't they doing this? Because these same agencies don't have the manpower to waste on such an endeavor, and if they did, the public would start an outcry as to why their tax dollars are wasted on such things. Should the PD start putting out press releases as to why their Chevy squad cars are better than Dodge or Ford? Or is there a conspiracy with maintenance records and how much time the vehicles actually spend in the shop vs on the road?

And this is how engineering idiocy proliferate.
Post-mortem, lessons learned, project feedback - those are essential components of most professions; and a must for those where decisions are required.
Maybe no publishing - but an internal analysis is an absolute must.
Instead we have

What would be the criteria used to declare this project a success?

From an engineering perspective, no unusual circumstances proffer themselves (large cracks, misaligned joints, etc.) and everything is built to plan. From a public policy perspective, it comes in within budget and on time (and I believe it's actually a little early).

While I can't speak for every agency, I can say that this kind of postmortem analysis IS going on at a few I work with. Just because they don't make headlines, doesn't mean the work is not being done. Much of this work is then getting presented and/or published at TRB. Change doesn't happen overnight, especially when government agencies are involved.

[Rothman]

There's silence whether they improve safety or do not.  It's not some conspiracy.  Most transportation projects don't make the news much at all.

[jakeroot]

You were suggesting a post-construction PR campaign on how good the intersection is performing, basically advertising "how great" they are. THAT's what would generate an outcry. Again, not every agency has the staffing availability to run the numbers following each and every project they complete. Most of the general public doesn't care that deeply.

"Going out of their way" was a poor choice of words on my part. I was thinking more along the lines of a press release talking about safety improvements. Along the lines of "20% reduction in injuries in the two years after construction as compared to before", or "crashes halved at busy county intersection after new roundabout". WSDOT already did this when they removed two traffic lights along an expressway: "Highway 500 reconfiguration passes 6-month crash test." They could do this for roundabouts too, no? If they want to build more, they could at least confirm that they work. That's not unreasonable.

The reason I consider things like this important, is because agencies do go on PR campaigns about the benefits of roundabouts. What major DOT doesn't have some webpage dedicated entirely to roundabouts and their benefits? Here is WashDOT's page on the matter, with titles such as "Improve safety", "Reduction in collisions", and "Reduce delay, improve traffic flow". As with other agencies, they continue to cite that nearly-20-year-old study from the IIHS, and literally nothing else. Then, there's the numerous videos online about how to drive roundabouts, why they are built, where they came from; again, after they are built, it's just complete silence.

Even more interesting, on WashDOT's page for "Safety", they have sub-pages about cable barriers, rest areas, and...yep, roundabouts. Click on "roundabout", and there are six separate pages on what they are, their benefits, navigation, etc. You're gonna tell me that taxpayers are perfectly fine with agencies creating dedicated webpages, graphics, and PR campaigns specifically for these intersections, but they'll lose it if they announce post-construction safety improvements? Come on. I think both are perfectly reasonable.

If you pay attention to locations where roundabouts have been present for a number of years, the comments have changed to focus on the bad driving habits and not the roundabout itself.

That's not a good thing, though. How many signals or stop signs have similar levels of comments? Sure, people bitch and moan about red-light running, or failing to yield, but people don't go on and on about how "no one but them knows how to use one".

[DaBigE]

You were suggesting a post-construction PR campaign on how good the intersection is performing, basically advertising "how great" they are. THAT's what would generate an outcry. Again, not every agency has the staffing availability to run the numbers following each and every project they complete. Most of the general public doesn't care that deeply.

"Going out of their way" was a poor choice of words on my part. I was thinking more along the lines of a press release talking about safety improvements. Along the lines of "20% reduction in injuries in the two years after construction as compared to before", or "crashes halved at busy county intersection after new roundabout". WSDOT already did this when they removed two traffic lights along an expressway: "Highway 500 reconfiguration passes 6-month crash test." They could do this for roundabouts too, no? If they want to build more, they could at least confirm that they work. That's not unreasonable.

No, it's not unreasonable, but our PR people get crucified with any instructional mailing or other campaign that comes out, whether it be for roundabouts, diverging diamonds, the FYA, or a reduction in road salt use. Generally, if you stop hearing complaints, you accept the "victory"; you don't go poking the bear with an "I told you so" campaign. No news is good news.

How many signals or stop signs have similar levels of comments? Sure, people bitch and moan about red-light running, or failing to yield

Isn't that a little contradictory?

That's not a good thing, though. How many signals or stop signs have similar levels of comments? Sure, people bitch and moan about red-light running, or failing to yield, but people don't go on and on about how "no one but them knows how to use one".

It happens every time someone gets t-boned at a red light or stop sign, DUI or otherwise. The other week there was an editorial in our local paper asking the DOT to install FYAs at an intersection because no one seemingly knows what a green ball means. It happens more than you may think.

[Scott5114]

Questions for discussion: what makes multi-lane roundabouts broadly acceptable in the United Kingdom but not in the United States? Is the crash data substantially the same between the two countries? If not, why not?

[kalvado]

Questions for discussion: what makes multi-lane roundabouts broadly acceptable in the United Kingdom but not in the United States? Is the crash data substantially the same between the two countries? If not, why not?

Good question. As much as I hate to say that — engineers' level of understanding.  A few miles upthread we pressed @cl94 into confession that there is no interest (hence funding) from FHWA for roundabout-related research. If you look at the publications, most of related research happens in Irel.and and France.    While there is no very direct relation between academic research and showels in the ground, problem  understanding tends to go around.
My impression is that US engineers don't understand limitations of roundabouts in terms of traffic throughput, scalability, limitations. FHWA guildelines for road diet, for example, seem to have much better understanding of situation.
Squeezing roundabouts to the bare minimum size is another problematic trend. Being only fourth largest country in the world, US really needs smallest possible footprints

Yes, sir, right away sir.    I'm really eager to serve those who keep insulting my profession, and painting all of us with the same broad brush.

It could be worse... Some people critisize engineers for a living! For example, one of my most pressing pre-xmas deadlines was posting grades for two ENGXXX courses. I am not your dream Santa with 3 F's passed around. My only excuse — I didn't enjoy that at all.

While I can't speak for every agency, I can say that this kind of postmortem analysis IS going on at a few I work with. Just because they don't make headlines, doesn't mean the work is not being done. Much of this work is then getting presented and/or published at TRB. Change doesn't happen overnight, especially when government agencies are involved.

At least we're on the same page with seing the value in such analysis.
I am not fully with @jakeroot in expecting a lot of post-construction PR to go around (although no PR department is going to pass on opportunity to send out "a paper analysing recent projects was publishest by StateDOT engineers" , even without going into headlines?). And  from the general perspective, most of pre-construction PR quotes ancient papers — if the data about previous project existed, you think DOT would pass on the opportunity to include "a similar project 2 miles down the road resulted in 3x drop of accident rate" in public comment presentation?
Something I still need to see.

And while we're at this.... Merry Christmas, ladies and gentlemen!

[jeffandnicole]

"Going out of their way" was a poor choice of words on my part. I was thinking more along the lines of a press release talking about safety improvements. Along the lines of "20% reduction in injuries in the two years after construction as compared to before", or "crashes halved at busy county intersection after new roundabout". WSDOT already did this when they removed two traffic lights along an expressway: "Highway 500 reconfiguration passes 6-month crash test." They could do this for roundabouts too, no? If they want to build more, they could at least confirm that they work. That's not unreasonable.

This is where the nitpicking comes to play.  The news report you found was for a very specific, relatively unusual project.   But then you used this one example to claim they should be doing this for roundabouts.  Why not for traffic lights?  Or Interchanges?  Or widenings?  Or smoothing out a dangerous curve?  Or for an addition of chevron arrows?  Or for different paint for lines?  Or for reflectors?  Or for different asphalts?  Or for LED lighting?

This shows that most projects don't get full analysis afterwards. But finding one that does doesn't mean it's justified on every project, and especially towards one specific type of project.

Questions for discussion: what makes multi-lane roundabouts broadly acceptable in the United Kingdom but not in the United States? Is the crash data substantially the same between the two countries? If not, why not?

I tried doing searching for "roundabout crash analysis in France", "roundabout crash analysis in Paris France" and the same replacing "analysis" with "data" (and a few other similar searches).  Very few relevant results appear, and of the few, the analysis just talks mostly about the data collection rather than the results.  One search led me to roundabouts in Korea, but again, no real conclusion about their safety was mentioned.

So, the reasoning we have in the US may be a fallacy - we simply don't hear viewpoints of other countries, and the crash data and results in other countries.  We think of the US as not accepting of roundabouts because we are specifically looking for articles and viewpoints where people complain about them, and searches where we want to see Europe and other areas as favorable to them because they've had them.  But there's no definite conclusion I can quickly find that states they're safer in Europe, or people actually accept them and favor them in Europe (or elsewhere).

[kalvado]

Questions for discussion: what makes multi-lane roundabouts broadly acceptable in the United Kingdom but not in the United States? Is the crash data substantially the same between the two countries? If not, why not?

I tried doing searching for "roundabout crash analysis in France", "roundabout crash analysis in Paris France" and the same replacing "analysis" with "data" (and a few other similar searches).  Very few relevant results appear, and of the few, the analysis just talks mostly about the data collection rather than the results.  One search led me to roundabouts in Korea, but again, no real conclusion about their safety was mentioned.

You need to know where to look for.
https://link.springer.com/book/10.1007/978-3-642-84537-6#toc - is there anything remotely comparable for US? Published in 1991, mind it..