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Transportation engineer here: Multi-lane roundabouts don't typically have any safety benefits. Single lane roundabouts do. Also, on the small side where you

Transportation engineer here: Multi-lane roundabouts don't typically have any safety benefits. Single lane roundabouts do. Also, on the small side where you have traffic circles rather than roundabouts, those also suck.

kmeisthax 3y ago
wait I thought "traffic circle" and "roundabout" were synonyms
- RandallBrown 3y ago
  They are in most contexts. There are "traffic circles" that aren't roundabouts but just a round block in the middle of an intersection that requires the traffic to move around in a circle so cross traffic is slowed. Also called a "traffic calming circle." https://www.seattle.gov/transportation/projects-and-programs/safety-first/traffic-operations/traffic-circles https://www.seattle.gov/transportation/projects-and-programs... That could also be a location specific term too. These circles are very common in Seattle.
  - lotsofpulp 3y ago
    I thought that was a roundabout. At least my UK family would call that a roundabout.
    - patja 3y ago
      Roundabouts have to have splitters and other features that traffic calming circles lack. Also, technically you can go whichever way you want around a traffic calming circle: counterclockwise is recommended, but many trucks physically cannot navigate them that way. And there are zero traffic laws distinguishing a traffic calming circle from an uncontrolled intersection.
- j16sdiz 3y ago
  Depends on which country/state you are from, they may or may not be the same. Look up on wikiprdia, there are even different types of roundabouts
- pests 3y ago
  This is the first time I've even heard the term "traffic circle"
  - kmeisthax 3y ago
    It's an Americanism. "Roundabout" is the God Given Queens[0] English equivalent. [0] King Charles is so uncharismatic I do not acknowledge his existence
    - pests 3y ago
      I am American! Metro Detroit area. I guess we also don't have many roundabouts. A lot of our road designs are based on Michigan Lefts[0] though which Wikipedia says are also called "median U‑turn crossover or median U‑turn, or restricted crossing U-turn (RCUT)" which I don't see much elsewhere in the country. [0] https://en.wikipedia.org/wiki/Michigan_left https://en.wikipedia.org/wiki/Michigan_left
kgermino 3y ago
> Also, on the small side where you have traffic circles rather than roundabouts, those also suck. Interesting. Can you elaborate? We have a few of them (and growing) by me and they’re fantastic as far as I can tell. No stopping every block for stop signs, I’ve seen landscaping truck with trailers sneak through them fine, and it’s so much easier on a bike or while walking. What’s the downside?
olyjohn 3y ago
Correct me if I'm wrong... but compared to a traffic light, you have to go significantly slower, so most accidents are not as severe as at a traffic light. You also aren't crashing into someone in a t-bone situation, usually they are sideswiped at more of an angle for less impact force. Of course someone could still fly into a roundabout at full speed. Anything is possible, but feel like think overall there still have to be benefits. People generally don't slow down for lights, and lots of times accelerate to make it through before it turns red.
- Clent 3y ago
  Correct. Anyone that says roundabouts are less safe doesn't have science backing their claim. While, they do not reduce the number of accidents and in some cases can increase accidents, but they consistently reduce the severity of the accidents. A three lane roundabout may be annoying when encounter as a tourist but the majority of locals going about their daily commutes quickly master them. For a three lane roundabout the inner lane is left turn only. The inner lane is left turn or straight and the right lane is right turns only. The ones in my area have ample signage indicating which lane allows for each type of turn.
  - mikepurvis 3y ago
    I would argue that the safety stats for them are still excessively car-centric. That is to say, roundabout collisions have better outcomes for car occupants specifically because they turn crashes into the kinds of crashes that cars handle best, which is low speed rear-endings, with the crumple zones doing all the heavy lifting to protect occupants. I’m much more skeptical about how roundabout users fare when they are people walking or riding bicycles. At least in my city, we've had roundabouts for years, and the yield rate for pedestrians is ghastly— like, sub-50% of drivers stopping for someone waiting to cross and remaining fully stopped until the person is completely off the roadway. In theory this is a fine of hundreds of dollars ("failure to yield to crosswalk"), but it appears to receive basically zero enforcement in practice. As a result, the fancy multi use paths that accompanied our roundabout installations get little use, since it's obvious to anyone who tries them how dangerous and unprotected they are.
mitthrowaway2 3y ago
While you're here, can I ask you a question I've always wanted to ask about traffic circles? Perhaps this is also related to why they suck for safety. At least where I live, there are two rules for traffic circles: (1) yield to traffic that is already in the traffic circle, and (2) when two cars arrive at the same time, yield to traffic on your right. Rule 1 is the same as for a roundabout, but rule (2) is a bit strange when the two are put together. It creates a scenario where if the leftmost car doesn't yield for whatever reason (perhaps they thought they arrived a microsecond earlier), then now they move forward and become "traffic in the traffic circle", which means they steal priority and the right-most car now has to yield to them. If the rule were flipped the other way, such that we default to always yielding to the car on the left, then this wouldn't happen; it would work just like a roundabout, and the order of yielding priority would be the same regardless of agreement on the exact timing of when each car arrived at the traffic circle. So, why is it that at a traffic circle we yield to traffic on the right, rather than the left? Is it just to be consistent with 4-way stops?
- eszed 3y ago
  Just clarifying: you drive on the right-hand side of the road, so "right" for you means on the outside of the circle? Yeah, that seems mad.
  - mitthrowaway2 3y ago
    Correct, right-hand side of the road.
- fshbbdssbbgdd 3y ago
  They added some circular medians in the middle of 4-way stops where I live, are those traffic circles? Are you suggesting that when I get to a 4 way stop, whether I yield to traffic on the left or right should depend on whether the intersection has one of those medians in the middle? Personally I find it confusing that yielding works the opposite way on roundabouts and 4-way stops, and nobody else in my neighborhood seems to know what to do either. Whether people yield to me seems to depend mostly on the size of the vehicle I am driving.
smeej 3y ago
I think you're right about single vs. 2+ lanes. It doesn't matter how many diagram signs they put up before the roundabout entry, people will NOT understand a roundabout with more than one lane, and under no circumstance will people going straight through realize they have to yield to someone coming from the opposite side who's going 270° around the circle and exiting from the inner lane. Roundabouts are fine if the rule is, "Just get in there without hitting anybody and go around until you're ready to leave," but expecting anything more than that from most drivers is a lost cause.
- yreg 3y ago
  > people will NOT understand a roundabout with more than one lane We have tons of 2-lane ones and everyone understands them…
  - dublinben 3y ago
    The number of drivers who think you can freely exit from the inner lane suggests otherwise.
    - peeters 3y ago
      Not sure exactly what you mean by freely, but in most popular roundabout designs you can exit from the inner lane. A "standard" two lane roundabout will have two lanes entering AND exiting. You never change lanes in roundabouts, at least where I'm from. Only in a massive multi-lane roundabout (that's at that point more like a ring road) would you ever change lanes to exit. e.g. for a standard two lane roundabout with four exits, if you enter in the left lane, you can exit directly out of the 2nd, 3rd, and 4th (i.e u-turn) exits, but always in the left lane of the exit. If you enter in the right lane, you can exit out of the 1st or 2nd exits, but always in the right lane of the exits. With this design cars travelling alongside you can never cross your path, only cars entering the roundabout ahead of you can cross your path and it's their responsibility to yield. The thing that most often causes problems is people in the outer lane NOT exiting (e.g. thinking they can take the third exit from the right lane) and swiping the car in the inner lane correctly taking the second exit.
gruez 3y ago
>Transportation engineer here: Multi-lane roundabouts don't typically have any safety benefits. Single lane roundabouts do. Source? Not doubting you, just want something to reference in the future.
peeters 3y ago
> Multi-lane roundabouts don't typically have any safety benefits. There seems to be quite a bit of literature that flatly disagrees with this statement. Perhaps you could add a source? The conventional wisdom where I'm from is that multilane roundabouts don't reduce the absolute number of crashes (and might even increase crash rates), but they still dramatically reduce the severity of crashes. Canada: > However recent data in the United States and Canada are showing that collision frequency at multi-lane roundabouts is often higher than expected, and sometimes higher than the prior signalized intersection. This trend is worrisome given that roundabouts are often constructed as a means of increasing safety. The good news is that fatal and injury crashes are almost always reduced with roundabouts, even if property-damage-only crashes are not. https://www.tac-atc.ca/sites/default/files/conf_papers/weber_p_-_solutions_to_driver_errors_at_multi-lane_roundabouts.pdf https://www.tac-atc.ca/sites/default/files/conf_papers/weber... Netherlands: > Multi-lane roundabouts. These are less safe than single-lane roundabouts, but still safer than other types of intersection [22]. Multi-lane roundabouts are less safe because they have a larger number of conflict points, they are more complex, driving speed is higher, and they have a less sharp entry angle [23]. https://swov.nl/en/fact/roundabouts-how-safe-are-different-kinds-roundabout https://swov.nl/en/fact/roundabouts-how-safe-are-different-k... US: > Roundabouts force drivers to slow down and all but eliminate the most severe types of intersection crashes — right-angle, left-turn and head-on collisions. Single-lane roundabouts see fewer total crashes of any type. https://www.iihs.org/news/detail/safety-at-two-lane-roundabouts-improves-over-time-new-study-shows https://www.iihs.org/news/detail/safety-at-two-lane-roundabo...
- Moru 3y ago
  The accident rate also goes down with time, people needs to get used to roundabouts.
kazinator 3y ago
All roundabouts prevent vehicles from just blowing straight through the intersection in the desired direction without so much as slowing down. They remove the middle part of the intersection where conflicting paths intersect. Two-lane roundabouts are larger; cars go around them faster. There is ambiguity. Some cars short-cut using the middle lane, and veer right out of it to their desired exit. Cars in the outer lane might be wrongly assumed to be turning off at the next exit. North American drivers get really confused by two-lane roundabouts. I don't think these situations will typically lead to anything worse than fender benders. The same intersection without a roundabout could be host to fatal collisions, when two vehicles enter at speed and hit each other in the middle.
- jameshart 3y ago
  > All roundabouts prevent vehicles from just blowing straight through the intersection in the desired direction without so much as slowing down Not at all. A large enough roundabout with good visibility from the approach can allow you to traverse it without slowing down at all - if the speed limits coming up to the yield line are appropriate, and traffic is flowing. There's a sense in which a roundabout is just a short circular section of highway with a series of on and off-ramps around the edge. Well designed onramps allow you to merge without slowing, and off-ramps let you leave without disrupting traffic. Roundabouts can permit the same.
  - kazinator 3y ago
    There is a recursive fix for that: put a small roundabout into each T shaped intersection of the huge roundabout.
- autoexec 3y ago
  > I don't think these situations will typically lead to anything worse than fender benders. Since even a fender bender can cost many thousands of dollars in repairs that's still pretty terrible. Speed bumps slow drivers down too and don't risk people turning into you because they were trying to exit from the inside lane of a roundabout.

kmeisthax 3y ago

wait I thought "traffic circle" and "roundabout" were synonyms

RandallBrown 3y ago
They are in most contexts. There are "traffic circles" that aren't roundabouts but just a round block in the middle of an intersection that requires the traffic to move around in a circle so cross traffic is slowed. Also called a "traffic calming circle." https://www.seattle.gov/transportation/projects-and-programs/safety-first/traffic-operations/traffic-circles https://www.seattle.gov/transportation/projects-and-programs... That could also be a location specific term too. These circles are very common in Seattle.
- lotsofpulp 3y ago
  I thought that was a roundabout. At least my UK family would call that a roundabout.
  - patja 3y ago
    Roundabouts have to have splitters and other features that traffic calming circles lack. Also, technically you can go whichever way you want around a traffic calming circle: counterclockwise is recommended, but many trucks physically cannot navigate them that way. And there are zero traffic laws distinguishing a traffic calming circle from an uncontrolled intersection.
j16sdiz 3y ago
Depends on which country/state you are from, they may or may not be the same. Look up on wikiprdia, there are even different types of roundabouts
pests 3y ago
This is the first time I've even heard the term "traffic circle"
- kmeisthax 3y ago
  It's an Americanism. "Roundabout" is the God Given Queens[0] English equivalent. [0] King Charles is so uncharismatic I do not acknowledge his existence
  - pests 3y ago
    I am American! Metro Detroit area. I guess we also don't have many roundabouts. A lot of our road designs are based on Michigan Lefts[0] though which Wikipedia says are also called "median U‑turn crossover or median U‑turn, or restricted crossing U-turn (RCUT)" which I don't see much elsewhere in the country. [0] https://en.wikipedia.org/wiki/Michigan_left https://en.wikipedia.org/wiki/Michigan_left

kgermino 3y ago

> Also, on the small side where you have traffic circles rather than roundabouts, those also suck. Interesting. Can you elaborate? We have a few of them (and growing) by me and they’re fantastic as far as I can tell. No stopping every block for stop signs, I’ve seen landscaping truck with trailers sneak through them fine, and it’s so much easier on a bike or while walking. What’s the downside?

olyjohn 3y ago

Correct me if I'm wrong... but compared to a traffic light, you have to go significantly slower, so most accidents are not as severe as at a traffic light. You also aren't crashing into someone in a t-bone situation, usually they are sideswiped at more of an angle for less impact force. Of course someone could still fly into a roundabout at full speed. Anything is possible, but feel like think overall there still have to be benefits. People generally don't slow down for lights, and lots of times accelerate to make it through before it turns red.

Clent 3y ago
Correct. Anyone that says roundabouts are less safe doesn't have science backing their claim. While, they do not reduce the number of accidents and in some cases can increase accidents, but they consistently reduce the severity of the accidents. A three lane roundabout may be annoying when encounter as a tourist but the majority of locals going about their daily commutes quickly master them. For a three lane roundabout the inner lane is left turn only. The inner lane is left turn or straight and the right lane is right turns only. The ones in my area have ample signage indicating which lane allows for each type of turn.
- mikepurvis 3y ago
  I would argue that the safety stats for them are still excessively car-centric. That is to say, roundabout collisions have better outcomes for car occupants specifically because they turn crashes into the kinds of crashes that cars handle best, which is low speed rear-endings, with the crumple zones doing all the heavy lifting to protect occupants. I’m much more skeptical about how roundabout users fare when they are people walking or riding bicycles. At least in my city, we've had roundabouts for years, and the yield rate for pedestrians is ghastly— like, sub-50% of drivers stopping for someone waiting to cross and remaining fully stopped until the person is completely off the roadway. In theory this is a fine of hundreds of dollars ("failure to yield to crosswalk"), but it appears to receive basically zero enforcement in practice. As a result, the fancy multi use paths that accompanied our roundabout installations get little use, since it's obvious to anyone who tries them how dangerous and unprotected they are.

mitthrowaway2 3y ago

While you're here, can I ask you a question I've always wanted to ask about traffic circles? Perhaps this is also related to why they suck for safety. At least where I live, there are two rules for traffic circles: (1) yield to traffic that is already in the traffic circle, and (2) when two cars arrive at the same time, yield to traffic on your right. Rule 1 is the same as for a roundabout, but rule (2) is a bit strange when the two are put together. It creates a scenario where if the leftmost car doesn't yield for whatever reason (perhaps they thought they arrived a microsecond earlier), then now they move forward and become "traffic in the traffic circle", which means they steal priority and the right-most car now has to yield to them. If the rule were flipped the other way, such that we default to always yielding to the car on the left, then this wouldn't happen; it would work just like a roundabout, and the order of yielding priority would be the same regardless of agreement on the exact timing of when each car arrived at the traffic circle. So, why is it that at a traffic circle we yield to traffic on the right, rather than the left? Is it just to be consistent with 4-way stops?

eszed 3y ago
Just clarifying: you drive on the right-hand side of the road, so "right" for you means on the outside of the circle? Yeah, that seems mad.
- mitthrowaway2 3y ago
  Correct, right-hand side of the road.
fshbbdssbbgdd 3y ago
They added some circular medians in the middle of 4-way stops where I live, are those traffic circles? Are you suggesting that when I get to a 4 way stop, whether I yield to traffic on the left or right should depend on whether the intersection has one of those medians in the middle? Personally I find it confusing that yielding works the opposite way on roundabouts and 4-way stops, and nobody else in my neighborhood seems to know what to do either. Whether people yield to me seems to depend mostly on the size of the vehicle I am driving.

smeej 3y ago

I think you're right about single vs. 2+ lanes. It doesn't matter how many diagram signs they put up before the roundabout entry, people will NOT understand a roundabout with more than one lane, and under no circumstance will people going straight through realize they have to yield to someone coming from the opposite side who's going 270° around the circle and exiting from the inner lane. Roundabouts are fine if the rule is, "Just get in there without hitting anybody and go around until you're ready to leave," but expecting anything more than that from most drivers is a lost cause.

yreg 3y ago
> people will NOT understand a roundabout with more than one lane We have tons of 2-lane ones and everyone understands them…
- dublinben 3y ago
  The number of drivers who think you can freely exit from the inner lane suggests otherwise.
  - peeters 3y ago
    Not sure exactly what you mean by freely, but in most popular roundabout designs you can exit from the inner lane. A "standard" two lane roundabout will have two lanes entering AND exiting. You never change lanes in roundabouts, at least where I'm from. Only in a massive multi-lane roundabout (that's at that point more like a ring road) would you ever change lanes to exit. e.g. for a standard two lane roundabout with four exits, if you enter in the left lane, you can exit directly out of the 2nd, 3rd, and 4th (i.e u-turn) exits, but always in the left lane of the exit. If you enter in the right lane, you can exit out of the 1st or 2nd exits, but always in the right lane of the exits. With this design cars travelling alongside you can never cross your path, only cars entering the roundabout ahead of you can cross your path and it's their responsibility to yield. The thing that most often causes problems is people in the outer lane NOT exiting (e.g. thinking they can take the third exit from the right lane) and swiping the car in the inner lane correctly taking the second exit.

gruez 3y ago

>Transportation engineer here: Multi-lane roundabouts don't typically have any safety benefits. Single lane roundabouts do. Source? Not doubting you, just want something to reference in the future.

peeters 3y ago

> Multi-lane roundabouts don't typically have any safety benefits. There seems to be quite a bit of literature that flatly disagrees with this statement. Perhaps you could add a source? The conventional wisdom where I'm from is that multilane roundabouts don't reduce the absolute number of crashes (and might even increase crash rates), but they still dramatically reduce the severity of crashes. Canada: > However recent data in the United States and Canada are showing that collision frequency at multi-lane roundabouts is often higher than expected, and sometimes higher than the prior signalized intersection. This trend is worrisome given that roundabouts are often constructed as a means of increasing safety. The good news is that fatal and injury crashes are almost always reduced with roundabouts, even if property-damage-only crashes are not. https://www.tac-atc.ca/sites/default/files/conf_papers/weber_p_-_solutions_to_driver_errors_at_multi-lane_roundabouts.pdf https://www.tac-atc.ca/sites/default/files/conf_papers/weber... Netherlands: > Multi-lane roundabouts. These are less safe than single-lane roundabouts, but still safer than other types of intersection [22]. Multi-lane roundabouts are less safe because they have a larger number of conflict points, they are more complex, driving speed is higher, and they have a less sharp entry angle [23]. https://swov.nl/en/fact/roundabouts-how-safe-are-different-kinds-roundabout https://swov.nl/en/fact/roundabouts-how-safe-are-different-k... US: > Roundabouts force drivers to slow down and all but eliminate the most severe types of intersection crashes — right-angle, left-turn and head-on collisions. Single-lane roundabouts see fewer total crashes of any type. https://www.iihs.org/news/detail/safety-at-two-lane-roundabouts-improves-over-time-new-study-shows https://www.iihs.org/news/detail/safety-at-two-lane-roundabo...

Moru 3y ago
The accident rate also goes down with time, people needs to get used to roundabouts.

kazinator 3y ago

All roundabouts prevent vehicles from just blowing straight through the intersection in the desired direction without so much as slowing down. They remove the middle part of the intersection where conflicting paths intersect. Two-lane roundabouts are larger; cars go around them faster. There is ambiguity. Some cars short-cut using the middle lane, and veer right out of it to their desired exit. Cars in the outer lane might be wrongly assumed to be turning off at the next exit. North American drivers get really confused by two-lane roundabouts. I don't think these situations will typically lead to anything worse than fender benders. The same intersection without a roundabout could be host to fatal collisions, when two vehicles enter at speed and hit each other in the middle.

jameshart 3y ago
> All roundabouts prevent vehicles from just blowing straight through the intersection in the desired direction without so much as slowing down Not at all. A large enough roundabout with good visibility from the approach can allow you to traverse it without slowing down at all - if the speed limits coming up to the yield line are appropriate, and traffic is flowing. There's a sense in which a roundabout is just a short circular section of highway with a series of on and off-ramps around the edge. Well designed onramps allow you to merge without slowing, and off-ramps let you leave without disrupting traffic. Roundabouts can permit the same.
- kazinator 3y ago
  There is a recursive fix for that: put a small roundabout into each T shaped intersection of the huge roundabout.
autoexec 3y ago
> I don't think these situations will typically lead to anything worse than fender benders. Since even a fender bender can cost many thousands of dollars in repairs that's still pretty terrible. Speed bumps slow drivers down too and don't risk people turning into you because they were trying to exit from the inside lane of a roundabout.