Crash prone 'modern roundabouts'

[tradephoric]

another big factor in roundabout performance is number of cars doing left turns. Hard to say without knowing local traffic patterns

Yeah, it reaches the point where it becomes a big guessing game.  Based on the crash statistics of that Ottawa roundabout, people may convince themselves that a central island needs to have a big mound of dirt and shrubs to obstruct drivers view from seeing across the roundabout.  But then you look at the most crash prone roundabout in Michigan in 2017, and it was a roundabout with a big mound of dirt and shrubs to obstruct the drivers view from seeing across the roundabout.  The 18 1/2 Mile and Van Dyke Ave roundabout had 165 crashes in 2017 and was constructed way back in 2005.  Here's the central island view of the Van Dyke roundabout...

[jakeroot]

I would be extremely hesitant to install stop signs, but its capacity could still be higher than a 4-way stop because multiple vehicles could occupy the circle at once.

Still, it should be a last resort. I would love to see speed tables used. They are used frequently in the Netherlands, and I don't see why they wouldn't work here, especially at intersections where vehicles must yield to pedestrians, such as at roundabouts or slip lanes. The latter situation is a frequent use-case in Boulder, CO: https://goo.gl/Q9SV8V & https://goo.gl/B9ybvs.

Multiple vehicles occupying circle for capacity boost is a... khm ....  interesting argument.
Pretty much as saying that you can enjoy your beverage faster if you use a longer straw which would holds more liquid. In both situations, however, it is the bottleneck - in case of roundabout being the entry rate - which defines total throughput.
Slowing down to 20 mph means average car takes about 0.5 second to travel its length, and that directly cuts into 2 second interval. So same story - decreasing throughput and killing suspension.
You see, this is why  traffic lights are so great - they move 2D space conflict into a time domain separation. Resolving the conflict in third dimension is still more efficient, but much more expensive.

I'm not trying to compare a roundabout with stop signs to a signal. A roundabout with stop signs will have similar throughput to a 4-way stop. I would much prefer a signal than a roundabout with stop signs.

The "more can occupy the intersection at once" argument is based on drivers who approach on opposite angles being able to enter at the same time, regardless of which direction they are going. This is possible at a four-way stop, as long as no one is turning left.

[webny99]

The "more can occupy the intersection at once" argument is based on drivers who approach on opposite angles being able to enter at the same time, regardless of which direction they are going. This is possible at a four-way stop, as long as no one is turning left.

"No one turning left" seems like a pretty big caveat. Not only that, though: traffic at a four-way stop can't proceed straight at the same time as perpendicular traffic. The only possible way four cars can all go through a four way stop at once is if they're all turning right, which renders your last point moot.

[jakeroot]

The "more can occupy the intersection at once" argument is based on drivers who approach on opposite angles being able to enter at the same time, regardless of which direction they are going. This is possible at a four-way stop, as long as no one is turning left.

"No one turning left" seems like a pretty big caveat. Not only that, though: traffic at a four-way stop can't proceed straight at the same time as perpendicular traffic. The only possible way four cars can all go through a four way stop at once is if they're all turning right, which renders your last point moot.

Allow me to clarify. If two cars arrive at a four way stop at the same time, their paths will not conflict unless one of them turns left (going straight or right obviously doesn't involve crossing paths). So, four way stops can be efficient if each car that arrives can immediately proceed without having to wait for someone else. The problem is when two cars arrive at the same time, and one of them has to turn left (not both). The car going straight (or the car waiting to turn) must wait for the other to proceed before going. At a four way stop roundabout (lol), four cars can all arrive at the same time and (in theory) perform simultaneous maneuvers (in any direction), because the area between each entry and exit leg is long enough for a vehicle. A standard roundabout with four legs is therefore capable of handling at least four cars at a time. This is difficult to describe without a diagram.

[kalvado]

The "more can occupy the intersection at once" argument is based on drivers who approach on opposite angles being able to enter at the same time, regardless of which direction they are going. This is possible at a four-way stop, as long as no one is turning left.

"No one turning left" seems like a pretty big caveat. Not only that, though: traffic at a four-way stop can't proceed straight at the same time as perpendicular traffic. The only possible way four cars can all go through a four way stop at once is if they're all turning right, which renders your last point moot.

Allow me to clarify. If two cars arrive at a four way stop at the same time, their paths will not conflict unless one of them turns left (going straight or right obviously doesn't involve crossing paths). So, four way stops can be efficient if each car that arrives can immediately proceed without having to wait for someone else. The problem is when two cars arrive at the same time, and one of them has to turn left (not both). The car going straight (or the car waiting to turn) must wait for the other to proceed before going. At a four way stop roundabout (lol), four cars can all arrive at the same time and (in theory) perform simultaneous maneuvers (in any direction), because the area between each entry and exit leg is long enough for a vehicle. A standard roundabout with four legs is therefore capable of handling at least four cars at a time. This is difficult to describe without a diagram.

Nice try, but nope, cars do NOT arrive simultaneously, there is an asynchronous flow.
A better way to look at it is for how long entering car blocks other entrances. For roundabout, it is probably 2 seconds for any entrance it passes plus 1 second for first downstream entrance. For a  4-way stop things are a bit more confusing since all other legs are affected to some extent - but effect may not be 100%. E.g car going east-west fully blocks north approach for maybe 3 seconds (wait + accelerate and pass), blocks 1 out of 3 moves(left turn) for west approach and 2 out of 3 (straight+left turn) for southern approach.
But in general, if move is affected for roundabout, it will be affected in a 4-way stop as well - EXCEPT for two oncoming lefts, which can proceed simultaneously on 4-way stop, but interfere TWICE in roundabout.
Advantage of roundabout is that duration of each conflict is less, so throughput for location with few left turns can benefit compared to a 4-way stop.
That was all for a single lane circle, 2 lanes are a bit more tricky.

[webny99]

Allow me to clarify.

Likewise.

If two cars arrive at a four way stop at the same time, their paths will not conflict unless one of them turns left (going straight or right obviously doesn't involve crossing paths).

My whole point is that going straight does involve crossing paths; if someone else approaches on the opposite road (perpendicular to you, as I said), then your paths most definitely cross. You'd T-bone each other if you both attempted to go through the four-way stop at once.
This actually adds to your argument that roundabouts have higher capacity, because at a roundabout, both cars can theoretically go at once, due to the increased distance (around the circle) and no requirement to stop.

[kalvado]

Allow me to clarify.

Likewise.

If two cars arrive at a four way stop at the same time, their paths will not conflict unless one of them turns left (going straight or right obviously doesn't involve crossing paths).

My whole point is that going straight does involve crossing paths; if someone else approaches on the opposite road (perpendicular to you, as I said), then your paths most definitely cross. You'd T-bone each other if you both attempted to go through the four-way stop at once.
This actually adds to your argument that roundabouts have higher capacity, because at a roundabout, both cars can theoretically go at once, due to the increased distance (around the circle) and no requirement to stop.

If paths would cross in the intersection, paths would also cross in roundabout (reverse is not true, by the way). It may be a conflict between cars arriving at slightly different points in time, but it is conflict nonetheless.

[jakeroot]

If two cars arrive at a four way stop at the same time, their paths will not conflict unless one of them turns left (going straight or right obviously doesn't involve crossing paths).

My whole point is that going straight does involve crossing paths; if someone else approaches on the opposite road (perpendicular to you, as I said), then your paths most definitely cross. You'd T-bone each other if you both attempted to go through the four-way stop at once.

I should have been more specific. I was only talking about two cars approaching from opposite angles, not right angles. I see now that I failed to mention that in either of my posts. My bad.

This actually adds to your argument that roundabouts have higher capacity, because at a roundabout, both cars can theoretically go at once, due to the increased distance (around the circle) and no requirement to stop.

Yes, exactly. I've illustrated as much below.

Advantage of roundabout is that duration of each conflict is less, so throughput for location with few left turns can benefit compared to a 4-way stop.
That was all for a single lane circle, 2 lanes are a bit more tricky.

This is all that I'm trying to say. I'm not trying to suggest that there's zero conflict points. Roundabouts just have less period of conflict because the intersection is just bigger.

To better illustrate what I'm trying to say about the simultaneous left turns, here's a diagram using an old roundabout that I designed. All of these cars can turn left at the same time, as long as they all enter at the same time, because there is significant distance between each entry point. In practice, this wouldn't work quite as well, because four cars all arriving at once is pretty unusual, and that kind of throws things off. But the point is still there:

[tradephoric]

Jake, i was about to post this picture last night because i thought it was exactly what you were describing.  It looks oddly familiar to your picture lol.  I don't think the red car entered the roundabout from the northern leg... but 3 out of 4 legs with vehicles entering the roundabout simultaneously ain't bad

[jakeroot]

Jake, i was about to post this picture last night because i thought it was exactly what you were describing.  It looks oddly familiar to your picture lol.  I don't think the red car entered the roundabout from the northern leg... but 3 out of 4 legs with vehicles entering the roundabout simultaneously ain't bad

https://i.imgur.com/P4I3NaZ.jpg

No kidding! Very similar. My diagram comes from a redesign of this intersection, which I had originally designed as a signal.

[kalvado]

This is all that I'm trying to say. I'm not trying to suggest that there's zero conflict points. Roundabouts just have less period of conflict because the intersection is just bigger.

To better illustrate what I'm trying to say about the simultaneous left turns, here's a diagram using an old roundabout that I designed. All of these cars can turn left at the same time, as long as they all enter at the same time, because there is significant distance between each entry point. In practice, this wouldn't work quite as well, because four cars all arriving at once is pretty unusual, and that kind of throws things off. But the point is still there:

* kalvado starts thinking this guy is not joking....

[jakeroot]

This is all that I'm trying to say. I'm not trying to suggest that there's zero conflict points. Roundabouts just have less period of conflict because the intersection is just bigger.

To better illustrate what I'm trying to say about the simultaneous left turns, here's a diagram using an old roundabout that I designed. All of these cars can turn left at the same time, as long as they all enter at the same time, because there is significant distance between each entry point. In practice, this wouldn't work quite as well, because four cars all arriving at once is pretty unusual, and that kind of throws things off. But the point is still there:

* kalvado starts thinking this guy is not joking....

I am describing a theory, not something that necessarily happens in practice. You don't seem to be understanding that. Obviously what happens in practice is more important, but that doesn't negate the point behind my theory. Which is, four cars can enter a roundabout simultaneously and leave without conflicting, no matter which direction they are going. Even if that's difficult in practice.

[kalvado]

This is all that I'm trying to say. I'm not trying to suggest that there's zero conflict points. Roundabouts just have less period of conflict because the intersection is just bigger.

To better illustrate what I'm trying to say about the simultaneous left turns, here's a diagram using an old roundabout that I designed. All of these cars can turn left at the same time, as long as they all enter at the same time, because there is significant distance between each entry point. In practice, this wouldn't work quite as well, because four cars all arriving at once is pretty unusual, and that kind of throws things off. But the point is still there:

* kalvado starts thinking this guy is not joking....

I am describing a theory, not something that necessarily happens in practice. You don't seem to be understanding that. Obviously what happens in practice is more important, but that doesn't negate the point behind my theory. Which is that, for cars can enter a roundabout simultaneously and leave without conflicting. Even if that's difficult in practice.

You are not describing theory. You are considering a very special relative timing which turns out to be beneficial in one case and not in the other. There are other special timings, which will play it out opposite.

As a result, you are narrowing down your scope to some very narrow detail instead of looking at a bigger picture. Which includes, at the very least, considering all sorts of relative timings.

[jakeroot]

This is all that I'm trying to say. I'm not trying to suggest that there's zero conflict points. Roundabouts just have less period of conflict because the intersection is just bigger.

To better illustrate what I'm trying to say about the simultaneous left turns, here's a diagram using an old roundabout that I designed. All of these cars can turn left at the same time, as long as they all enter at the same time, because there is significant distance between each entry point. In practice, this wouldn't work quite as well, because four cars all arriving at once is pretty unusual, and that kind of throws things off. But the point is still there:

* kalvado starts thinking this guy is not joking....

I am describing a theory, not something that necessarily happens in practice. You don't seem to be understanding that. Obviously what happens in practice is more important, but that doesn't negate the point behind my theory. Which is that, for cars can enter a roundabout simultaneously and leave without conflicting. Even if that's difficult in practice.

You are not describing theory. You are considering a very special relative timing which turns out to be beneficial in one case and not in the other. There are other special timings, which will play it out opposite.

As a result, you are narrowing down your scope to some very narrow detail instead of looking at a bigger picture. Which includes, at the very least, considering all sorts of relative timings.

The theory goes, "if four cars arrive at once, they can all proceed at once assuming no other traffic is in the circle". I don't care how rare it is. That wasn't my point. I guess it was more of an observation. Of course, if it's a line of cars all intending to turn left, then disaster. --> I think this is what you've been alluding to?

[kalvado]

The theory goes, "if four cars arrive at once, they can all proceed at once assuming no other traffic is in the circle". I don't care how rare it is. That wasn't my point. I guess it was more of an observation. Of course, if it's a line of cars all intending to turn left, then disaster. --> I think this is what you've been alluding to?

No, I am just trying to press you into describing a bigger picture. Something I often try to do while dealing with students - it just sometimes show up when I am  off campus...

[jakeroot]

The theory goes, "if four cars arrive at once, they can all proceed at once assuming no other traffic is in the circle". I don't care how rare it is. That wasn't my point. I guess it was more of an observation. Of course, if it's a line of cars all intending to turn left, then disaster. --> I think this is what you've been alluding to?

No, I am just trying to press you into describing a bigger picture. Something I often try to do while dealing with students - it just sometimes show up when I am  off campus...

It's not that I didn't see the bigger picture. It's just that my example didn't pertain to a bigger picture. My example was somewhat narrow: four cars arriving at an intersection to turn left at the same time could all proceed at the same time at a roundabout (this is only true for left-turning traffic at a traditional intersection if two cars arrive at the same time to turn left). No other intersection design permits this behavior. The problem, of course, is that there are more than four cars on the road. I think this is where we get into your point about the bigger picture.

[tradephoric]

New data released and the most crash prone roundabout in Michigan last year was at Van Dyke Avenue (M-53) and 18 1/2 Mile Road in Sterling Heights.  There was an incredible 80% spike in crashes from the previous year (going from 92 crashes in 2016 to 165 in 2017).  The three-lane roundabout was constructed in 2005 and i can't explain the massive increase in year-over-year crashes.  Injury crashes also tripled, from 4 in 2016 to 13 in 2017.

There were 3 other roundabouts that had over 100 crashes in 2017 (State & Ellsworth = 110; Martin Pkwy & Pontiac Trail = 142; and Orchard Lake & 14 Mile = 144).  The roundabout with the most injury crashes was at Orchard Lake & 14 mile with 24 injury crashes.  These crash-prone roundabouts aren't getting any better evidenced by the fact that a 13 year old roundabout had 165 crashes last year.

Where were the most Michigan roundabout accidents in 2017?
https://www.michiganautolaw.com/blog/2018/05/24/most-michigan-roundabout-accidents-in-2017/

[tradephoric]

Here's an aerial of the Van Dyke and 18 1/2 Mile with a proposed redesign. 



Having a straight shot exiting the roundabout could simplify the roundabout operation IMO.  Drivers circulating the roundabout who drive over a "straight only" arrow pavement marking would actually continue to drive "straight" as oppose to weave through the roundabout.  The green path in the picture is simply easier to drive than the blue path.   Notice that the blue path hugs the inside of the roundabout for much longer than the green path, so a driver who wishes to enter the roundabout doesn't know the circulating drivers intention till much later on.  With the blue path it's a guessing game if the vehicle is about to exit the roundabout or continue circulating through it.  Makes it much more likely a driver pulls out infront of someone because they "guessed" wrong.

[jakeroot]

I've read that straight exit paths aren't good for pedestrian crossings (higher exit speed). But I would think visibility would be better not having a curve. I like what I see. Are they considering any actual revisions?

[tradephoric]

I've read that straight exit paths aren't good for pedestrian crossings (higher exit speed). But I would think visibility would be better not having a curve. I like what I see. Are they considering any actual revisions?

I haven't heard of any official plans to upgrade the roundabout.  I'm just playing around with some ideas i have.  Hey Jake, what program did you use to create the rendering of the roundabout in reply #1782?  I wanted to mess around with the Van Dyke roundabout some more but don't have a good program to use.

[tradephoric]

Here's a redesign for the Van Dyke roundabout.  Three main changes...

1.  Straight out exits. 
2.  Arrows in circulating lanes moved farther up to the entry legs.
3.  Central island reconfigured to be more rectangular than circular. 

[jakeroot]

I've read that straight exit paths aren't good for pedestrian crossings (higher exit speed). But I would think visibility would be better not having a curve. I like what I see. Are they considering any actual revisions?

I haven't heard of any official plans to upgrade the roundabout.  I'm just playing around with some ideas i have.  Hey Jake, what program did you use to create the rendering of the roundabout in reply #1782?  I wanted to mess around with the Van Dyke roundabout some more but don't have a good program to use.

Sorry for the delay in response. My above diagram was created with Adobe Illustrator, which I've been using off-and-on for about ten years. It's one of my favorite computer programs. Most of what I design in that program is built to the scale of a background image.

[tradephoric]

I've read that straight exit paths aren't good for pedestrian crossings (higher exit speed). But I would think visibility would be better not having a curve. I like what I see. Are they considering any actual revisions?

I haven't heard of any official plans to upgrade the roundabout.  I'm just playing around with some ideas i have.  Hey Jake, what program did you use to create the rendering of the roundabout in reply #1782?  I wanted to mess around with the Van Dyke roundabout some more but don't have a good program to use.

Sorry for the delay in response. My above diagram was created with Adobe Illustrator, which I've been using off-and-on for about ten years. It's one of my favorite computer programs. Most of what I design in that program is built to the scale of a background image.

No problem Jake.  I used (fumbled) with Photoshop to make the overlay design above but I'm just not very good with that program.  I'll take a look at Adobe Illustrator.   

[MNHighwayMan]

No problem Jake.  I used (fumbled) with Photoshop to make the overlay design above but I'm just not very good with that program.  I'll take a look at Adobe Illustrator.   

Or if you're unwilling to pay Adobe the unreasonable prices they charge to use their software, Inkscape is the free and open-source alternative to Adobe Illustrator. It's what I use, and is more than good enough to create sign graphics (or things like roundabout layouts), even if it lacks a lot of the fancier features Illustrator has.

[jakeroot]

No problem Jake.  I used (fumbled) with Photoshop to make the overlay design above but I'm just not very good with that program.  I'll take a look at Adobe Illustrator.   

Or if you're unwilling to pay Adobe the unreasonable prices they charge to use their software, Inkscape is the free and open-source alternative to Adobe Illustrator. It's what I use, and is more than good enough to create sign graphics (or things like roundabout layouts), even if it lacks a lot of the fancier features Illustrator has.

Adobe's CC programs are expensive if you're a non-vocational user, but they're dirt cheap for those who use them professionally. That doesn't necessarily describe me, but $50/month for Adobe's entire suite is a bargain if you want my opinion.