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[jakeroot]

And.. did we gain anything, other than a happy contractor with a great construction contract?

I'm not even sure if they're that great. One major benefit of the roundabout is the (usually) cheaper construction cost.

The ultimate goal of road design is, from my perspective, is to move traffic safely, efficiently, without undue stress. There are other factors, like emergency vehicle access, but lets not go into that for now.
So, roundabouts bring:
-mixed results in terms of safety
-throughput? My impression is that it is reduced compared to alternatives
-now we need to cut throughput down even further to improve safety
-we need to increase the level of stress for safety at a roundabout (and whatever ripple effects result from that; the stressed driver is the bad driver)
-we lose intersection scalability; sort of OK in NY as the state seems to be planning for decline

I do think roundabouts work well in areas with lots of medians. Never have been a big fan of U-turns at signals, as they limit the placement of right-turn overlap/filter arrows.

[mrsman]

https://www.tac-atc.ca/sites/default/files/conf_papers/weber_p_-_solutions_to_driver_errors_at_multi-lane_roundabouts.pdf - how to make roundabouts safer. I like their first suggestion:

A single-lane instead of a multi-lane roundabout

Two suggestions I also like:

1) higher entry angles, so entry into roundabouts is more of a right turn. I can understand failing to yield when you can just drive straight across the roundabout, but if it becomes a sharp-right, and then a gradual left turn, there might be better safety numbers.

2) circulatory lines be removed or not installed. The paper suggests that the lines make roundabouts appear as having four (or more) individual intersections, and that removing the markings make the roundabouts appear as one large intersection, increasing driver hesitation. Makes total sense to me, even if it is counter-intuitive relative to typical North American marking practices.

I think this is the proper approach.  If all of your entry roads are single lane (inbound direction), then a single lane roundabout is great.  It is reasonably safe.  It will also cause less delay than an all-way stop of signalized approach, especially if you are in a jurisdiction that will contemplate left turn signals.  And as said by others, the roundabout will take care of any left turn and u-turn movements.  But in no event should they be used in a pedestrian heavy area.

However, the multi-lane roundabouts are crash prone.  There are simply too much traffic to allow for the flow into the roundabout.  The only solution in this case would be a regular signalized intersection or a signalized traffic circle.

Signalized traffic circles are safe, but they are no picnic traffic-wise.  Usually only done in very busy areas where there are more than 4 directions coming to meet at a point.  (Think Columbus Circle in NYS or Dupont Circle in Washington DC).  The only benefit over a regular intersection, is that there is no need for separate phases for left turns.

[silverback1065]

i wish this thread would die, I regret posting in it, now i'm stuck with the notifications 

[mrsman]

The thread will only die when multi-lane roundabouts die.

Nexus 5X

[tradephoric]

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

[tradephoric]

I shared this in another thread but maybe these roundabout interchange designs would work well.  Would you still consider these roundabouts?

[kphoger]

No.

[jakeroot]

I shared this in another thread but maybe these roundabout interchange designs would work well.  Would you still consider these roundabouts?

https://www.youtube.com/watch?v=W8k1acLeK7E

https://www.youtube.com/watch?v=zVcOUY6Zs7k

Seems like you could get the same benefits by having a regular left-side merge lane. There could still be a yield sign for the off-ramp, assuming decent sight-lines.

Not sure your models fully take into account the slowdowns that would occur approaching the apex of the roundabout, as traffic along the arterial swings right, and then back left. They wouldn't have to yield, but they'd have to slow down to a reasonable speed. Seems like an annoyance along an otherwise high-speed arterial.

[kalvado]

Not sure your models fully take into account the slowdowns that would occur approaching the apex of the roundabout,

There will be no slowdowns, there will be full speed t-bones. 

[kphoger]

Not sure your models fully take into account the slowdowns that would occur approaching the apex of the roundabout,

There will be no slowdowns, there will be full speed t-bones. 

Indeed, east-west traffic doesn't even yield at the supposed roundabouts.  Basically, it's just a glorified version of what jakeroot linked to.

Not a roundabout.

[tradephoric]

Indeed, east-west traffic doesn't even yield at the supposed roundabouts.  Basically, it's just a glorified version of what jakeroot linked to.

Not a roundabout.

Teardrop roundabouts don't require traffic along the arterial to yield at the roundabout (specifically traffic along the arterial traveling away from the overpass/underpass and heading towards the roundabout... i hope that made sense!).  Here is a roundabout interchange that Calgary is proposing that is essentially the same design that i posted except that off-ramp traffic exiting the roundabout isn't merging onto the arterial street.

[kalvado]

Indeed, east-west traffic doesn't even yield at the supposed roundabouts.  Basically, it's just a glorified version of what jakeroot linked to.

Not a roundabout.

Teardrop roundabouts don't require traffic along the arterial to yield at the roundabout (specifically traffic along the arterial traveling away from the overpass/underpass and heading towards the roundabout... i hope that made sense!).  Here is a roundabout interchange that Calgary is proposing that is essentially the same design that i posted except that off-ramp traffic exiting the roundabout isn't merging onto the arterial street.

Parts of traffic pattern along the bottom segment of the tear are fully equivalent to a half of  2x2 roundabout. Given the speeds involved, I expect crash rate along the lines of a regular 2x2  roundabout (or worse).

[tradephoric]

With some minor tweaks that Calgary interchange could look something like this.  I only modeled half of the interchange but the other half would basically be a mirror copy.  One advantage is that traffic entering the freeway doesn't have to circulate through any roundabouts.  The question is would one lane of free-flowing arterial traffic have the same capacity of two-lanes of traffic that has to yield at the roundabout? 

[jamess]

I was just in Aruba, and thyve been adding a bunch of dutch-style roundabouts to their major roads and highways (makes sense, theyre part of the Netherlands). Went through 5 or 6 on their main highway, which is 80kph (50mph). The speeds slwos to 60 and then 40 as you near the roundabout.

Whats interesting is that there are concrete curbs between lanes. No confusion.

This made navigating even the 3 lane roundabout easy. You just stay in your lane, and you cant get out.

Island is loaded up with American tourists, and I didnt see any issues.

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

That looks dangerous. No deflection on entrance or exit.

[kalvado]

With some minor tweaks that Calgary interchange could look something like this.  I only modeled half of the interchange but the other half would basically be a mirror copy.  One advantage is that traffic entering the freeway doesn't have to circulate through any roundabouts.  The question is would one lane of free-flowing arterial traffic have the same capacity of two-lanes of traffic that has to yield at the roundabout? 

So what exactly is your point? That uncontrolled high speed intersection can be made safe? With 2 lanes intersecting another 2 lanes?
It will if traffic is about 2-3 vehicles a minute in each direction. Anything above that is a bad design.
Think rural highways intersection controlled by a single yield sign as the baseline.

[tradephoric]

So what exactly is your point? That uncontrolled high speed intersection can be made safe? With 2 lanes intersecting another 2 lanes?
It will if traffic is about 2-3 vehicles a minute in each direction. Anything above that is a bad design.
Think rural highways intersection controlled by a single yield sign as the baseline.

It's not that unusual to have two lanes of traffic merge back down to 1 shortly after exiting a roundabout.  Here's an example of this at a busy roundabout with an AADT of over 50,000 (definitely not just 2-3 vehicles per minute).  It handles it.  While this roundabout does have a high crash rate it isn't specific to the merging traffic.

https://www.google.com/maps/@42.5546443,-83.447316,137m/data=!3m1!1e3

[jeffandnicole]

I was just in Aruba, and thyve been adding a bunch of dutch-style roundabouts to their major roads and highways (makes sense, theyre part of the Netherlands). Went through 5 or 6 on their main highway, which is 80kph (50mph). The speeds slwos to 60 and then 40 as you near the roundabout.

Whats interesting is that there are concrete curbs between lanes. No confusion.

This made navigating even the 3 lane roundabout easy. You just stay in your lane, and you cant get out.

Island is loaded up with American tourists, and I didnt see any issues.

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

That looks dangerous. No deflection on entrance or exit.

I think you're too close to the roundabout.  GSV'ing back a little bit you can easily make out the deflection approaching the roundabout.  https://goo.gl/maps/qMdVZ4nVQCvF3wXLA  There isn't much deflection exiting the roundabout, but that's not extremely necessary either, and increases the visibility of pedestrians in the walkway.

[jamess]

I was just in Aruba, and thyve been adding a bunch of dutch-style roundabouts to their major roads and highways (makes sense, theyre part of the Netherlands). Went through 5 or 6 on their main highway, which is 80kph (50mph). The speeds slwos to 60 and then 40 as you near the roundabout.

Whats interesting is that there are concrete curbs between lanes. No confusion.

This made navigating even the 3 lane roundabout easy. You just stay in your lane, and you cant get out.

Island is loaded up with American tourists, and I didnt see any issues.

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

That looks dangerous. No deflection on entrance or exit.

I think you're too close to the roundabout.  GSV'ing back a little bit you can easily make out the deflection approaching the roundabout.  https://goo.gl/maps/qMdVZ4nVQCvF3wXLA  There isn't much deflection exiting the roundabout, but that's not extremely necessary either, and increases the visibility of pedestrians in the walkway.

I dont think that deflection point makes much sense. Its too early, and actually makes the crosswalk harder to see.

Theres no streetview in Aruba, and with the disclaimer that Ive never driven in mainland Netherlands, their roundabouts had very steep entrance and exit points - youre literally turning on and off.

https://goo.gl/maps/UQkkkURo5k5BpgXv6

Made it seem safer from my experience.

In the US ones, drivers just gun it.

[mrsman]

Agreed on those points.

The Dutch roundabout you linked to is single lane and has a wide circumference.  For drivers, it has the feel of yielding (or even stopping) to make a right turn and then taking it to where you want.  Reasonably safe. 


The problem with many of the ones  in the US, is that by altering the geometry, the circumference, and the number of lanes, you basically are dealing with a manuever of handling 4 closely spaced one-way intersections.  (I.e. a town center intersection but where everything is really close.)  You yield to the first intersection and then you have the right of way in the others.  But the problem deals with that you don't know when it is safe, because you may misjudge if traffic on your left will exit or go straight or continue around the roundabout.*  And the straight move isn't such a great deflection, so some traffic isn't even adequately slowing down.

IMO, if traffic can be narrowed to single lane on all approaches, a roundabout could be an option that will eliminate delays from traffic signals.  if it doesn't and you need multi-lane, you also likely need a traffic signal.


* To some extent, you experience similar problems on multi-lane 4-way stop corners.  When those are busy, the only way of being safe is keeping track of which direction got to the intersection first.  But it is harder to do that for multi-lane approaches.  IMO, if a road is busy enough for multiple lanes, it's busy enough to require traffic signals.  If it isn't busy enough, then narrow the road.

[kphoger]

Not sure your models fully take into account the slowdowns that would occur approaching the apex of the roundabout,

There will be no slowdowns, there will be full speed t-bones. 

Indeed, east-west traffic doesn't even yield at the supposed roundabouts.  Basically, it's just a glorified version of what jakeroot linked to.

Not a roundabout.

Teardrop roundabouts don't require traffic along the arterial to yield at the roundabout (specifically traffic along the arterial traveling away from the overpass/underpass and heading towards the roundabout... i hope that made sense!).

A teardrop interchange, in my opinion, is just a single squished roundabout interchange—not a pair of roundabouts.  Take a huge circle, squish the middle in, and you end up with two teardrops.  With that in mind, traffic on the arterial does indeed yield upon entry to the roundabout; where you describe them not having to yield is, in my opinion, still the middle of a big roundabout.

In your models, unless I was seeing them incorrectly, traffic on the arterial does not yield at all.  The way I see it, you simply modeled a ParClo with some wide spots in the medians and curvy trajectories.  I'm not saying it's a bad idea, I'm just saying it has little to do with a roundabout.

Here is a roundabout interchange that Calgary is proposing that is essentially the same design that i posted except that off-ramp traffic exiting the roundabout isn't merging onto the arterial street.

And that is a proper teardrop/dogbone interchange with some extra ramps thrown in.  All approaches appear to yield at entry.

[jakeroot]

A teardrop interchange, in my opinion, is just a single squished roundabout interchange—not a pair of roundabouts.  Take a huge circle, squish the middle in, and you end up with two teardrops.  With that in mind, traffic on the arterial does indeed yield upon entry to the roundabout; where you describe them not having to yield is, in my opinion, still the middle of a big roundabout.

I'm very tempted to agree with you, although I think from most driver's perspective, it will appear as though they are travelling through two roundabouts. Drivers aren't looking at these things overhead, and I'm half-certain that most drivers, particularly out west, are not familiar with "roundabout interchanges" like those in the UK and parts of the northeast...they just see two roundabouts, although with the second not requiring a yield.

Just for the record, WA signs their teardrop interchanges with two separate roundabout warning signs: one for the first entry, and another for the second entry. There are some obvious issues with this, but, apparently, states seem to see two separate roundabouts as well (or at least that's common practice).

[jeffandnicole]

I was just in Aruba, and thyve been adding a bunch of dutch-style roundabouts to their major roads and highways (makes sense, theyre part of the Netherlands). Went through 5 or 6 on their main highway, which is 80kph (50mph). The speeds slwos to 60 and then 40 as you near the roundabout.

Whats interesting is that there are concrete curbs between lanes. No confusion.

This made navigating even the 3 lane roundabout easy. You just stay in your lane, and you cant get out.

Island is loaded up with American tourists, and I didnt see any issues.

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

That looks dangerous. No deflection on entrance or exit.

I think you're too close to the roundabout.  GSV'ing back a little bit you can easily make out the deflection approaching the roundabout.  https://goo.gl/maps/qMdVZ4nVQCvF3wXLA  There isn't much deflection exiting the roundabout, but that's not extremely necessary either, and increases the visibility of pedestrians in the walkway.

I dont think that deflection point makes much sense. Its too early, and actually makes the crosswalk harder to see.

Theres no streetview in Aruba, and with the disclaimer that Ive never driven in mainland Netherlands, their roundabouts had very steep entrance and exit points - youre literally turning on and off.

https://goo.gl/maps/UQkkkURo5k5BpgXv6

Made it seem safer from my experience.

In the US ones, drivers just gun it.

What?  There's no deflection in that GSV example whatsoever.  It's literally a straight line to the roundabout, and then a right turn.

[tradephoric]

Here is one more model of the Calgary interchange design but with exiting roundabout traffic merging down to one lane before joining the arterial street.  I do find it ironic that I'm defending whether or not my design is a roundabout considering how many times i have been accused of hating roundabouts or something to that effect.  I just believe that the simpler a roundabout design is the less chances there are for accidents and with this design there is just not that many ways for cars to crash into each other.
   

[jamess]

I was just in Aruba, and thyve been adding a bunch of dutch-style roundabouts to their major roads and highways (makes sense, theyre part of the Netherlands). Went through 5 or 6 on their main highway, which is 80kph (50mph). The speeds slwos to 60 and then 40 as you near the roundabout.

Whats interesting is that there are concrete curbs between lanes. No confusion.

This made navigating even the 3 lane roundabout easy. You just stay in your lane, and you cant get out.

Island is loaded up with American tourists, and I didnt see any issues.

The Veterans Memorial Dr and Greenwich Ave roundabout in Warwick may be one of the most dangerous roundabouts in the country.  It's a full 2x2 roundabout with a large retaining wall in the central island and over the weekend a police officer crashed into the retaining wall and suffered several injuries.  The officer's name hasn't been released and no word yet if it's one of the LivePD officers out of Warwick.

Warwick police officer injured when SUV crashes at roundabout
https://www.providencejournal.com/news/20190714/warwick-police-officer-injured-when-suv-crashes-at-roundabout

That looks dangerous. No deflection on entrance or exit.

I think you're too close to the roundabout.  GSV'ing back a little bit you can easily make out the deflection approaching the roundabout.  https://goo.gl/maps/qMdVZ4nVQCvF3wXLA  There isn't much deflection exiting the roundabout, but that's not extremely necessary either, and increases the visibility of pedestrians in the walkway.

I dont think that deflection point makes much sense. Its too early, and actually makes the crosswalk harder to see.

Theres no streetview in Aruba, and with the disclaimer that Ive never driven in mainland Netherlands, their roundabouts had very steep entrance and exit points - youre literally turning on and off.

https://goo.gl/maps/UQkkkURo5k5BpgXv6

Made it seem safer from my experience.

In the US ones, drivers just gun it.

What?  There's no deflection in that GSV example whatsoever.  It's literally a straight line to the roundabout, and then a right turn.

The right turn into and out of the roundabout is the deflection.

[jakeroot]

I dont think that deflection point makes much sense. Its too early, and actually makes the crosswalk harder to see.

Theres no streetview in Aruba, and with the disclaimer that Ive never driven in mainland Netherlands, their roundabouts had very steep entrance and exit points - youre literally turning on and off.

https://goo.gl/maps/UQkkkURo5k5BpgXv6

Made it seem safer from my experience.

In the US ones, drivers just gun it.

What?  There's no deflection in that GSV example whatsoever.  It's literally a straight line to the roundabout, and then a right turn.

The right turn into and out of the roundabout is the deflection.

Exactly. American roundabouts are designed for speed: long gentle curves which eventually point you directly into the circulating lanes, with the exit often being in a straight line as well.

Dutch [turbo] roundabouts (and by extension, Aruban roundabouts), in comparison, are designed more around turns on-and-off a large circle, rather than around several different intersections/crossover points. Turbo roundabouts still (usually) use circulating markings, like the American roundabouts. This is why American roundabouts are often considered a hybrid between the original English roundabout (where our original roundabout designs came from), and Dutch turbo roundabouts, as unlike English roundabouts, we do include circulating lane lines.

Below, I've mocked up a comparison. The red line represents the American roundabout, and the curves that one might shall use through the roundabout. The blue lines are closer to how a Dutch roundabout might be designed: There is no deflection approaching the roundabout. Drivers simply come up to the circle, and turn right into it, with the sharp right turn being the deflection.