Crash prone 'modern roundabouts'

[jakeroot]

Consider the following scenario.  CAR A enters the roundabout mistakenly believing CAR B is about to exit.  Would u want CAR B to take 1.7 seconds or 3.4 seconds to travel the blue path?  More reaction time is a good thing!

This is probably the best reasoning I've heard in a long time. That actually makes quite a lot of sense.

[iBallasticwolf2]

Consider the following scenario.  CAR A enters the roundabout mistakenly believing CAR B is about to exit.  Would u want CAR B to take 1.7 seconds or 3.4 seconds to travel the blue path?  More reaction time is a good thing!

This is probably the best reasoning I've heard in a long time. That actually makes quite a lot of sense.

This determines that a larger diameter in a roundabout might be a good thing.

[DaBigE]

Larger roundabouts may be safer assuming the entry angles of the roundabout are designed properly and circulating speeds remain low.  Consider the following scenario.  CAR A enters the roundabout mistakenly believing CAR B is about to exit.  Would u want CAR B to take 1.7 seconds or 3.4 seconds to travel the blue path?  More reaction time is a good thing!

95 FT central island diameter:


160 FT central island diameter:

You make the assumption that vehicle B will be traveling the same speed in both scenarios. The larger the circle, the faster vehicle B can comfortably travel. The faster vehicle B is traveling, the harder it is for vehicle A to judge the gap.

[DaBigE]

...a fundamental problem with these crash prone roundabouts that can’t be fixed...

In many cases, that problem is the self-centered driver. The ones in a hurry, distracted, and like to make their own traffic rules. You can't just look at the sheer number of crashes alone. You have to look at the type/cause of crash, and the majority of the time it's been due to failure to yield or improper lane usage. If there was a way to accurately track near-misses, I bet the larger roundabouts aren't any better.

The majority of crashes at signalized intersections are due to driver error.
The majority of crashes at modern roundabouts are due to driver error. 

The very first post in this thread has a link to SEMCOG crash data for the roundabout at State & Ellsworth in Ann Arbor.  Most of the crashes were due to failure to yield (angle) and improper lane usage (sideswipe same).  The 3 years before the roundabout, the intersection averaged 18 crashes a year.  The first year after the roundabout, there were 170 crashes.  Can the roundabout at State & Ellsworth be considered a success because the majority of the 170 crashes was due to driver error?  I don't think so.

You missed the point entirely. Where is there claim that either was a success? In reducing/eliminating fatalities, maybe. Back to the point I was trying to make...Increases in education and enforcement may be the better solution in many modern roundabout cases. You seem to hang your hat solely on the size hypothesis. Size of the roundabout will not solve the overly aggressive driver nor the ill-informed. All it does is give them more room to screw up and potentially at a higher speed.

Secondly, design philosophies are continually evolving with modern roundabouts. As you have even noted, spiral designs have changed in the past 5-10 years, as have theories on radial vs. offset approach design and limiting sight on the approach. Subtle geometric modifications can have a big impact on how a roundabout operates, without necessarily changing the ICD. Am I saying all roundabouts have an appropriate ICD? No. Some are undersized, others are oversized. Some are victim to poor initial design (fast-path speeds, entry radii, etc.), others are victim to not having the design knowledge we have today. Many are victim to the site's constraints. There is no one-size-fits-all solution. Bigger/smaller isn't always better.

[iBallasticwolf2]

Larger roundabouts may be safer assuming the entry angles of the roundabout are designed properly and circulating speeds remain low.  Consider the following scenario.  CAR A enters the roundabout mistakenly believing CAR B is about to exit.  Would u want CAR B to take 1.7 seconds or 3.4 seconds to travel the blue path?  More reaction time is a good thing!

95 FT central island diameter:


160 FT central island diameter:

You make the assumption that vehicle B will be traveling the same speed in both scenarios. The larger the circle, the faster vehicle B can comfortably travel. The faster vehicle B is traveling, the harder it is for vehicle A to judge the gap.

In that case the speed limit on the roundabout would have to be strictly enforced to make sure the drivers don't go faster. So post speed limit signs all over the roundabout.
Also inform drivers better. Alot of drivers are aggressive or ill-informed. There should be very good guidance for what drivers are supposed to do in a roundabout.

[tradephoric]

You make the assumption that vehicle B will be traveling the same speed in both scenarios. The larger the circle, the faster vehicle B can comfortably travel. The faster vehicle B is traveling, the harder it is for vehicle A to judge the gap.

The circulating speed of the 160 ft diameter roundabout would have to be twice as fast as the circulating speed of the 95 ft diameter roundabout before the gap time would be equal.  The reality is drivers aren't traveling twice as fast in the 160 ft diameter roundabout.  Don't ignore the benefits larger diameters may have just because the circulating speeds might be 2-3 mph faster.

Back to the point I was trying to make...Increases in education and enforcement may be the better solution in many modern roundabout cases. You seem to hang your hat solely on the size hypothesis. Size of the roundabout will not solve the overly aggressive driver nor the ill-informed. All it does is give them more room to screw up and potentially at a higher speed.

Aggressive, ill-informed drivers are present at both large and small diameter roundabouts.  Improved driver education is great, but it doesn't change the fact that smaller diameter roundabouts, based on the crash data, are more accident prone than larger ones.

Secondly, design philosophies are continually evolving with modern roundabouts. As you have even noted, spiral designs have changed in the past 5-10 years, as have theories on radial vs. offset approach design and limiting sight on the approach. Subtle geometric modifications can have a big impact on how a roundabout operates, without necessarily changing the ICD. Am I saying all roundabouts have an appropriate ICD? No. Some are undersized, others are oversized. Some are victim to poor initial design (fast-path speeds, entry radii, etc.), others are victim to not having the design knowledge we have today. Many are victim to the site's constraints. There is no one-size-fits-all solution. Bigger/smaller isn't always better.

Crash data is a good indicator to what design elements are working and which ones could be improved.  I'm sure there are ways to improve the crash rate of roundabouts with small ICD's and designers have had two decades to tweak them.  Ultimately, the lessons learned at these crash prone roundabouts with small ICDs can be applied to the larger roundabouts to make them as safe as possible.  Size isn't the only factor to look at, but it does appear to be a key factor.

The fact is State & Ellsworth is a roundabout with a 100 ft central island diameter and had 170 crashes the first year of operation.  What's the fix?  Other larger diameter roundabouts in SE Michigan didn't see anywhere close to that number of crashes the first year of operation.  Can you really just put the bulk of the blame on ill-informed drivers?  Maybe, just maybe, it's too small.

[DaBigE]

You make the assumption that vehicle B will be traveling the same speed in both scenarios. The larger the circle, the faster vehicle B can comfortably travel. The faster vehicle B is traveling, the harder it is for vehicle A to judge the gap.

The circulating speed of the 160 ft diameter roundabout would have to be twice as fast as the circulating speed of the 95 ft diameter roundabout before the gap time would be equal.  The reality is drivers aren’t traveling twice as fast in the 160 ft diameter roundabout.  Don’t ignore the benefits larger diameters may have just because the circulating speeds might be 2-3 mph faster.

I'm not ignoring the possible benefits; I am trying to look at this issue objectively from all sides.

Size isn't the only factor to look at, but it does appear to be a key factor.

Something I can agree with, although being a key factor is a matter of semantics. Until the sight to the left on entry, circulating sight, entry radii, and fastest-paths (among other design considerations) are also compared and analyzed for the problematic roundabouts, the ICD is still just a plausible theory. There are many small ICD multilane roundabouts that have not had the PDO-crash growth that some of the ones mentioned in this thread have.

[tradephoric]

There are many small ICD multilane roundabouts that have not had the PDO-crash growth that some of the ones mentioned in this thread have.

There really aren't that many 2x2 roundabouts that haven't been mentioned in this thread.  I attempted to query out the major 2x2 roundabouts in America and looked up crash data for as many as I could (found crash data for 14 of 40).  The best performing roundabout with a central island diameter of 100 feet or less still had a 300% increase in crashes.

There was a 300%-1400% increase in crashes at roundabouts with central island diameters of 100 feet or less.  There was a 36%-318% increase in crashes at roundabouts with central island diameters between 120 and 170 feet.  The worst performing large diameter roundabout was almost as good as the best performing small diameter roundabout.

Sure, you may be able to find an aesthetic 2x2 roundabout in a subdivision that hasn't seen an increase in crashes, but that really isn't the focus.  If you have crash data on major 2x2 roundabouts that I don't, by all means share it with the rest of us. 

Here is a list of 40 multi-lane roundabouts that I think could be meaningful to this conversation.  This was queried from a database of over 5,100 modern roundabouts.  This was the criteria used to query out the list:

-roundabouts constructed within the last 10 years
-all approaches have 2 entry lanes
-roundabouts have 4-legs
-main & side streets are major state or county routes (ie. likely high AADT roundabouts)
-interchange roundabouts (and frontage road roundabouts) not included



Now the hard part is finding accurate before/after crash data to make any type of analysis.  Here is a google KMZ file that includes the 40 roundabouts in the chart above:

http://www.mediafire.com/download/cd570rarros2c3g/Multi-Lane+Roundabouts+%28with+high+AADT%29.kmz

In a previous post, I queried out 40 multi-lane roundabouts that have 2-entry lanes for all 4-legs (ie. the roundabouts in America that likely have the highest traffic volumes).  I've tracked down before/after crash data for 14 of them.  The excel file includes data links to articles/publications where the data was obtained.


http://www.mediafire.com/view/p0u5ovrupr0175m/Multi-lane_roundabout_crashes.xlsx

The takeaway is that smaller roundabouts seem to be more accident prone than larger roundabouts.  Would there have been 171 crashes at State & Ellsworth if the roundabout was constructed with a central island diameter of 140' as opposed to 100'?  This thread wasn't meant to bash roundabouts but to discuss ways to reduce the crash frequency at the highest AADT roundabouts.  My thought is make them just a little bit bigger.

[Rothman]

Are the 3-2-1, 1-2-3 columns years before and after?

[tradephoric]

Are the 3-2-1, 1-2-3 columns years before and after?

Yes.  3-2-1 represents 3 years before, 2 years before, 1 year before.  1-2-3 represents 1 year after, 2 years after, 3 years after.  The year of construction is not included.  The cells highlighted in grey are average crashes over multiple years.  If you open the excel file it provides links where the crash data was obtained.

http://www.mediafire.com/view/p0u5ovrupr0175m/Multi-lane_roundabout_crashes.xlsx

[Rothman]

Interesting that the crashes for the New Scotland/NY 85/NY 140 roundabout are only one year before and after.  As I've said before, the most mindboggling thing for me going through there are the people that think they can continue to go around the roundabout in the right lane and don't look when they essentially turn left to do so.  I'd bet that kind of thing accounts for a lot of the crashes in the year after the roundabout was built.

[tradephoric]

Interesting that the crashes for the New Scotland/NY 85/NY 140 roundabout are only one year before and after.  As I've said before, the most mindboggling thing more me going through there are the people that think they can continue to go around the roundabout in the right lane and don't look when they essentially turn left to do so.  I'd bet that kind of thing accounts for a lot of the crashes in the year after the roundabout was built.

The roundabout at New Scotland/NY 85/NY 140 is one of the worst performing roundabouts that has a central island diameter between 120-170 feet.  The faded spiral markings could definitely be a contributing factor to the high crash rate seen at this roundabout and has been discussed previously in this thread (the smaller diameter roundabout in Malta, NY also has painted spiral markings).  By the way, I believe the crash data is average crashes over multiple years but the article (linked in the excel file) misrepresented the data to be from only one year before and after. 

Bethlehem, NY (New Scotland Road and Route 140)


Malta, NY (Route 9, Route 67 and Dunning Street)

[DaBigE]

There are many small ICD multilane roundabouts that have not had the PDO-crash growth that some of the ones mentioned in this thread have.

There really aren't that many 2x2 roundabouts that haven't been mentioned in this thread.  I attempted to query out the major 2x2 roundabouts in America and looked up crash data for as many as I could (found crash data for 14 of 40).

Forty? That's it? You're being to limited in your search and you indirectly admit that you don't have enough accurate crash data for all the ones you do list.

Sure, you may be able to find an aesthetic 2x2 roundabout in a subdivision that hasn't seen an increase in crashes.

There is no such thing. You don't build a multilane roundabout just because it looks pretty.

Here is a list of 40 multi-lane roundabouts that I think could be meaningful to this conversation.  This was queried from a database of over 5,100 modern roundabouts.  This was the criteria used to query out the list:

-roundabouts constructed within the last 10 years    Reasonable
-all approaches have 2 entry lanes    Why exclude roundabouts that may have one or two legs with a single lane entry?
-roundabouts have 4-legs    Exactly 4 or >=4? Why exclude 3-leg 2x2 roundabouts?
-main & side streets are major state or county routes (ie. likely high AADT roundabouts)    Not an accurate assumption...while likely, there still are a number that are solely local arterial roads, and not numbered/lettered routes. Ex: Thompson/Lein/Zeier, Madison, Thompson/Main, Sun Prairie
-interchange roundabouts (and frontage road roundabouts) not included    Why? Just because interchange roundabouts don't have 4 complete approaches?

As I said above, you're being too limiting in your search. Some might say you're cherry-picking data. The lack of complete crash data skews the whole process. I could list many multilane roundabouts that you haven't mentioned, but they would/should get thrown-out because the intersection did not exist before the roundabout, thus there is no before period to compare to.

This is going in nothing but circles (pun not intended). I tried to find common ground in your reasonable query on roundabout size, but you are blinded into focusing solely on size. Is size a factor? Yes. Does it deserve more scientifically-acceptable research? Most certainly. Is it the only factor where crashes are a problem? No. Roundabout design isn't that simplistic. You started a discussion on "crash-prone 'modern roundabouts'", when a more accurate subject would be 'why modern roundabouts are too small'.

[Rothman]

I drive through that roundabout a couple of times a week and can say with certainty that crashes are not due to faded pavement markings.  The thing is still well-marked and even signed.

[tradephoric]

I drive through that roundabout a couple of times a week and can say with certainty that crashes are not due to faded pavement markings.  The thing is still well-marked and even signed.

Even with perfectly visible pavement markings, drivers are going to cheat.  The driver in the blue SUV is asking themselves "is the silver Honda in the outside or inside lane?"  Let's hope they don't guess wrong. 

Hell, i'd encourage you to take video of the Bethlehem roundabout and post it online so we can see for ourselves if improper lane use is a problem at this roundabout.  It wouldn't be that hard to do.

[Rothman]

Heh.  Like I said, the crashes in the 1st year afterwards are probably due to people turning left from the right lane and I'd bet the frequency of lane misuse has dropped -- although I still witness it from time-to-time.

Might make that video when I get a chance.

[tradephoric]

Heh.  Like I said, the crashes in the 1st year afterwards are probably due to people turning left from the right lane and I'd bet the frequency of lane misuse has dropped -- although I still witness it from time-to-time.

Might make that video when I get a chance.

Here's the next best thing to a video.  Here are streetview images tracking a vehicle through the roundabout in Malta, NY (which has an identical spiral design to the roundabout in Bethlehem NY).  At some point the blue SUV may think it's safe to enter the roundabout (since the silver Honda is in the inside left turn only lane).







The silver Honda is cheating their way through the roundabout.

[lordsutch]

Most of the crashes were in year one... the year the pavement markings almost certainly weren't faded. Hence faded lines don't, and can't, explain jack.

As for the silver Honda... dude is like 1 foot across the line for maybe 20 feet without endangering any other traffic. People drift across lane lines all the time. Give it a rest.

Free advice from one data scientist: stop torturing the data to fit an agenda.

[tradephoric]

Most of the crashes were in year one... the year the pavement markings almost certainly weren't faded. Hence faded lines don't, and can't, explain jack.

I don't have year by year crash data for the NY roundabout.  How do u know that the majority of crashes happened in year one?

As for the silver Honda... dude is like 1 foot across the line for maybe 20 feet without endangering any other traffic. People drift across lane lines all the time. Give it a rest.

Dude is entirely in the inner lane in the 3rd picture.  I only had streetview (not many examples to pick from) so sorry if i couldn't find a better example of the scenario

Free advice from one data scientist: stop torturing the data to fit an agenda.

I'm just looking for trends.  The crash data shows that there are fewer crashes at larger diameter roundabouts.  Therefore, I'm in favor of larger roundabouts.  If the data had shown that there are fewer crashes at smaller diameter roundabouts, then i would be in favor of smaller diameter roundabouts.

[Rothman]

Most of the crashes were in year one... the year the pavement markings almost certainly weren't faded. Hence faded lines don't, and can't, explain jack.

I don't have year by year crash data for the NY roundabout.  How do u know that the majority of crashes happened in year one?

Eh...your percentage increases treats them as if you did have year-to-year.  Can't have it both ways.

[jakeroot]

The crash data shows that there are fewer crashes at larger diameter roundabouts.  Therefore, I'm in favor of larger roundabouts.  If the data had shown that there are fewer crashes at smaller diameter roundabouts, then i would be in favor of smaller diameter roundabouts.

But the data is only representative of the few roundabouts you can find data for. I don't think making such large generalizations is a good idea without a significant amount of data.

[tradephoric]

Most of the crashes were in year one... the year the pavement markings almost certainly weren't faded. Hence faded lines don't, and can't, explain jack.

I don't have year by year crash data for the NY roundabout.  How do u know that the majority of crashes happened in year one?

Eh...your percentage increases treats them as if you did have year-to-year.  Can't have it both ways.

Cells highlighted in grey are average crashes over multiple years.  The before and after crash data cells for the Bethlehem, NY and Malta, NY roundabouts are highlighted in grey.  I'm not trying to fool anyone.

[tradephoric]

The crash data shows that there are fewer crashes at larger diameter roundabouts.  Therefore, I'm in favor of larger roundabouts.  If the data had shown that there are fewer crashes at smaller diameter roundabouts, then i would be in favor of smaller diameter roundabouts.

But the data is only representative of the few roundabouts you can find data for. I don't think making such large generalizations is a good idea without a significant amount of data.

The Insurance Institute for Highway Safety study only evaluated 9 multi-lane roundabouts.  This 15 year old study is routinely cited in roundabout articles and is the go to source when it comes to roundabout safety statistics.  I'm evaluating the safety performance of 14 multi-lane roundabouts.  The amount of data being evaluated is not insignificant.

[Rothman]

Most of the crashes were in year one... the year the pavement markings almost certainly weren't faded. Hence faded lines don't, and can't, explain jack.

I don't have year by year crash data for the NY roundabout.  How do u know that the majority of crashes happened in year one?

Eh...your percentage increases treats them as if you did have year-to-year.  Can't have it both ways.

Cells highlighted in grey are average crashes over multiple years.

I'm going to have call baloney on this repeated assertion as well regarding the New Scotland roundabout.  You're saying that the average in the first three years was 9.6 per year and then the average in the three years afterwards was 38.3. per year.

That's total baloney.  In essence, you're saying that there was at least one incident in the roundabout every 10 days for three years after it was built. I highly suspect that you have warped the data into something for which it was not intended.

[tradephoric]

I'm going to have call baloney on this repeated assertion as well regarding the New Scotland roundabout.  You're saying that the average in the first three years was 9.6 per year and then the average in the three years afterwards was 38.3. per year.

That's total baloney.  In essence, you're saying that there was at least one incident in the roundabout every 10 days for three years after it was built. I highly suspect that you have warped the data into something for which it was not intended.

I don't understand what is so hard to believe.  Does 38 crashes a year sound too high to you?  To put it into perspective, there are over 50 intersections in SE Michigan that have averaged at least 38 crashes a year.

http://semcog.org/Data-and-Maps/High-Frequency-Crash-Locations