Episode 4 – Roundabouts

Topics: Roundabouts


Talking Traffic Episode 4 – Roundabouts

Today is September 17, 2007, and you are listening to Talking Traffic. Wait a second. It’s not the 17th of September. It is the 17th of October! Let’s do that again. Hi! Today is October 17th, 2007, and you are listening to an episode of Talking Traffic that is a month late. You’re not supposed to apologize or allude to quote-unquote late episodes; that is advice that all the main bloggers and podcasters will tell you. I don’t plan to apologize, but I will explain.

As you might know if you read my personal blog, September and early October were a bit crazy. This was obvious from my posting about how crazy busy I was, and the fact that there weren’t any postings for about two weeks. I have many things going on in my life from professional concerns revolving around delivering projects on time (and under budget) to personal ones such as maintaining a relationship with my equally busy wife and keeping our house in reasonable order. I’m also a dedicated runner training for a half marathon, and many other things going on which are too numerous to mention.

The upshot of this is the setting of priorities, and my podcasting is not at the top of my list. Right now, I have several prime functions. Mostly they are in the list I just mentioned. The priority that achieves the top slot changes depending on what’s going on at any point in time, but they generally revolve between wife, work, house, and training. Podcasting and blogging falls inside the second tier of priorties. Thusly, you can accurately assume that for the past month, my top priorities were occupying all of my time. As varying commitments go around and around, things have to give in order to provide time for what needs to be done. But, now, hopefully, I can say that I’ll be back on top of this podcast for the foreseeable future.

Speaking of going around and around, this episode is dedicated to the Roundabout. It is by request of Chris Schierer at www.theschierers.net. That’s t-h-e, s-c-h-i-e-r-e-r dot net. Chris asked me to cover the topic of Roundabouts, as he can never remember what the difference is between roundabouts and traffic circles and rotaries. Well, Chris, this one is for you.

If you live in the northeast United States, I’m sure you’ve driven through at least one intersection that was oriented in a circular manner. You entered on one leg and either revolved around a small central island or drove like a bat out of hell around a large circular road, eventually exiting on the leg you wanted. Generally, the smaller sort of circular intersection is called a Traffic Circle, and the larger type is a Rotary. Some people will argue that I have it backwards; that a traffic circle is the big one and a rotary is the small one. Truthfully, there isn’t much distinction because most of them are too different to conform to a definition. See the show notes for examples of overhead images. Neither of these intersections are Roundabouts.

The Roundabout, or more pedantically, the Modern Roundabout, was designed by the British in the 1960′s to serve as an intersection that provides more control than an un-signed intersection, but less delay than an all-way stop or a traffic signal. The show notes have a diagram of the Modern Roundabout. If you are listening to this podcast on your computer, it will probably be handy to refer to it. I’ll do my best to describe it verbally, but a picture is worth, you know… yadda yadda.

If you will imagine yourself in the driver’s seat of a vehicle approaching a roundabout intersection, I will attempt to describe it. As you approach the intersection, you notice a dividing island that separates you from oncoming traffic. This island (called a splitter island) flares a bit wider toward the end that faces the roundabout. At the end of the island, just before you enter the roundabout, it turns you so you are facing more tangentially into the intersection, rather than directly at the center circular island. There is a yield sign facing you, requiring you to yield the right of way to traffic already inside the roundabout before you may enter the intersection. Due to the splitter island and the yield sign (assuming you didn’t have to stop) you’re probably traveling around 25 to 30 miles per hour right now. As you enter the roundabout proper, you travel around and around to the left, like a good NASCAR driver, paying attention to any vehicles that may be entering from your right. When you see the exit roadway you want to take, you enter the lane which is angled toward you by the aforementioned splitter island, and procede on your way. Other than the yield sign, there was no intersection control. No stop signs, no signals. Just each vehicle occupying a portion of the right of way until they clear the intersection.

Why Roundabouts, as opposed to “normal” intersections like stop signs or signals? Why should your hard-earned, and cleverly taxed dollars be spend on these weird European intersections? There are several reasons which I’ll touch upon: safety, operations, aesthetics, and cost.

From a safety perspective, the Modern Roundabout is designed to reduce the number of conflict points experienced by vehicles. A conflict point is a location where two traffic streams cross. For example, at a normal four way intersection, a straight-through movement will cross the opposing left-turn movement. That is one conflict point. Another is where the straight-through movement of the main street crosses the straight-through movement of the minor street. I know what you’re thinking, “those two movements aren’t supposed to go at the same time,” and you’re right, but the summation of conflict points is concerned with all potential conflicts, not just the ones that are legal.

A typical four-way stop or signal controlled intersection has thirty-two conflict points, all told. Roundabouts by comparison reduce the number of conflicts down to eight! And, again by design, they remove all of the really dangerous types of conflicts. Statistically, the most dangerous type of crash you can be involved in is an angle crash, where the two vehicles strike at right angles. These occur where one vehicle runs a red light or stop sign and is t-boned by another, or where a turning vehicle strikes, or is struck by, a through vehicle. Roundabouts remove all of the conflict points that lead to crossing or angle collisions. Every movement into or out of the roundabout is a right turn, thus causing any collisions to occur as a sideswipe, which is a much safer way to encounter a ton of steel, glass and plastic. Roundabouts are also designed as a traffic calming device, reducing the speeds of the entering roadways. Drivers need to slow in order to navigate the roundabout, which allows time to react to unexpected situations, plus reducing the total energy of any collisions that may occur.

One thing that I have ignored so far is how roundabouts affect pedestrians. At a normal four way intersection, there are breaks in the flow of one or the other traffic stream, which allows pedestrians to cross in relative safety. Roundabouts do not have this type of operation. A functioning roundabout has a continuous flow of vehicles through all lanes, requiring pedestrians to have a much higher situational awareness in order to cross the traffic stream. These pedestrian crossings are on the approach legs of the roundabout, with a refuge on the splitter island, so while the pedestrians have to navigate a traffic stream that isn’t conditioned to stop, they only need to pay attention to one approach direction at a time. From a safety standpoint, the studies seem to indicate that roundabouts are safer for pedestrians than stop-controlled or signalized intersections, but there is considerable debate on this matter, and it is a topic for a full podcast. Especially about how blind pedestrians navigate a roundabout. Again, another full-podcast topic.

That is how roundabouts effect the safety of an intersection, through reduction in conflict points, slower speeds, and lower-severity crashes. Now let’s discuss operations.

Operations of an intersection are improved by jamming more vehicles through per hour. Unfortunately, there are many considerations that make that sentence an extremely complicated one. You can infinitely improve the operations of a main street by putting the main street approach to a signal on green all the time. This has unfortunate consequences on the minor street, though, as they will never move and end up getting their shotguns and shooting out the signal lamps. Not a recommended practice. Roundabouts do something similar to this premise, but with fewer shotguns: Essentially, all approaches are treated equally, no matter what is the “main” approach and what is minor. Everyone has to yield as they enter the roundabout and thus vehicles are served as they approach the yield sign. No need to wait for a green light, merely a gap in traffic acceptable to the driver.

Another reason why roundabouts operate well is because of their lower speeds. The design forces drivers to slow on their approach, and the diameter of the internal circle causes drivers circulating inside to move at a speed much lower than would be possible in a typical stop or signal controlled intersection. How does this help operations, you ask? Wouldn’t a higher speed mean more vehicles can get through the intersection? The answer is counter intuitively, No. Imagine you are at a T-intersection, sitting at the minor-road stop sign, wanting to make a turn onto the major road. Traffic is creeping by at 10 miles per hour. How big a gap in traffic do you think you need to get into the traffic stream safely? Not very big, right? Now imagine the same intersection, but you’re trying to turn onto a road where vehicles are zipping by at 55 mph. You’ll need a much larger gap between vehicles in order to turn onto the road then accelerate up to speed before the vehicle behind you decides to test the worthiness of your bumper. This same principle applies to roundabouts. Slower absolute speeds allow drivers to accept shorter gaps in order to enter the intersection, thus improving the number of vehicles that can clear the intersection in an hour.

Roundabouts can also lend themselves to very attractive aesthetics. The center island can be planted with attractive shrubs or grasses (but NOT trees that block vision), thus lending to a friendlier environment. Roundabouts also cost less than a signalized intersection, at least for construction, equipment, and maintenance, because there is no need to operate and maintain the signal infrastructure. Now, a Roundabout will require more space, thus more Right of Way than a typical intersection, which may put the total cost edge over that of a typical intersection, especially in built-up areas where land is at a premium. Of course, the flip side of that is where a roundabout has become saturated due to a large increase in traffic, an upgrade to a signalized intersection won’t typically require any more land purchases for the necessary equipment to be installed.

Roundabouts are on the lips of a lot of politicians and city planners in the United States in recent years. They’ve begun to acquire widespread acceptance as an effective intersection alternative. In fact, New York State has issued a policy statement directing engineers to make roundabouts the preferred alternative when considering intersection types. A website dedicated to the roundabout exists on the New York State DOT website. See the show notes for the url. Even as I speak, there is work going on here in Cobb County, Georgia for the design and installation of at least two new roundabouts. Unfortunately, I’m not on those projects, although I know the people who are, and I’m jealous.

Roundabouts are not the solution to all traffic problems. Beyond a certain volume of traffic, they become less effective than traffic signals at controlling vehicles safely. They have higher right of way costs than stop or signal controlled intersections, which escalates the initial installation cost. Some drivers, especially older ones, may have difficulty navigating the intersection. Pedestrians and bicycles may not be well treated by roundabouts, and large vehicles can have difficulty navigating around the intersection. Careful design is necessary to make sure that trucks are accommodated correctly.

All of the same information as what I’ve told you, and much more is available in the Roundabouts, An informational Guide published by the Federal highway Administration. It has lots of good information in there if you’re interested in some of the more technical aspects. It also talks about multi-lane roundabouts, which I did not touch. Everything I’ve said today deals with single-lane roundabouts. Multi-lane roundabouts are the same as single-lane roundabouts, but more so. Their designs are more complex in order to maintain correct traffic flow and safety considerations. Perhaps we will revisit multi-lane roundabouts in a later episode.

For now, I’ll leave you with these parting thoughts. Roundabouts are good things, but they are not a panacea. They will not work in all situations and must be correctly designed for a particular intersection. That being said, they are an excellent and cost-effective approach to middle-volume intersections. If you end up at a public meeting and someone stands up saying, “I’ll not have any of them new-fangled intersections ’round here” just ask them if they prefer their taxes to go up in order to maintain the signal that will be installed instead.

This episode of Talking Traffic has been brought to you through the kind offices of my not being crazy busy any more. Show notes can be found at www.talkingtraffic.org, under Episode 4. All comments and suggestions are appreciated and can be sent to me at the email address Bill @ talkingtraffic.org. This podcast is released under a Creative Commons 3.0 license for Attribution, Non-Commercial, No-Derivatives. This means you can copy and re-release the podcast, but please don’t change it, or sell it, and make sure you link back to talkingtraffic.org.

For the next episode, I haven’t decided yet what to do. I’ve been kicking around in my mind doing a safety-themed podcast, but this one might be a monster, and will require more forethought and planning than others. I will think on it, and see what comes about.

Episode Notes

Here is a link to the FHWA guidebook, Roundabouts, An informational Guide
Here is a link to the New York state website on Roundabouts.

The music on this podcast is Mercurial Girl by Five Star Fall, off of Automatic Ordinary. Find them at Magnatune.com, which allows podcasters such as me access to high quality music licenses for free.

Diagram of a Modern Roundabout

Diagram of a Modern Roundabout.  Source: Roundabouts, an Informational Guide, published by the Federal Highway Administration.  http://www.tfhrc.gov/safety/00068.htm

Examples of Traffic Circles

The Traffic Circle from Hell: Dupont Circle, Washington DC


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Another Traffic Circle from Hell: Charles de Gaul Plaza, Paris, France

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Examples of Rotaries

Notice the high-speed nature of these intersections. NOT a roundabout

Portsmouth, NH

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Bourne, MA. Although this looks like a roundabout (see below), it’s not. It is too big to operate in the intended fashion.

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Examples of Roundabouts

There are two roundabouts in this image, from Vail, Colorado. These are the ramp terminals for an exit off of Interstate 70.

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Asheville, NC

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Clearwater, Florida. This is an example of a two lane roundabout.

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5 Responses to Episode 4 – Roundabouts

  1. Chris says:

    Woohoo! Thanks Bill!

  2. Pingback: Roundabout Time by Schierer Space

  3. Pingback: Traffic Tidbits: 23 October 2007 : Talking Traffic

  4. Pingback: Episode 11 - Traffic Calming : Talking Traffic

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