What are the pros and cons of a traffic circle versus a traffic light intersection?

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The debate of traffic circles (also called roundabouts or rotaries) versus traffic light intersections has been in progress for a while. Those in favor of traffic circles say that, among other things, that they are safer than traffic light intersections. This claim has been scientifically proven. On the other hand, traffic light intersections are more space-inefficient.

Even Mythbusters has joined the fun, testing the efficiency (which is one of the main arguments both sides seem to concern themselves with) of each method.

For comparison, here's a quick picture of a traffic circle:

Traffic circle

And of a four-way traffic light intersection:

Intersection

So, what are the pros and cons of a traffic circle versus a traffic light intersection?

APerson

Posted 2015-01-21T02:42:04.263

Reputation: 291

1@Pops, I believe terminology can change, and all three of those terms are often synonymous. I proposed an edit to add a disambiguation parenthetical in the first sentence.Adam Miller 2015-01-21T18:29:15.430

@AdamMiller I'm not an expert on this myself, but I was on the receiving end of a long lecture on the topic from a traffic engineer once which covered, among other things, the point that those terms have distinct meanings and anyone who uses them interchangeably is Doing It Wrong.Pops 2015-01-21T19:39:01.517

Suspect there may be international variation in the meanings of those terms. There certainly is in lay usage.Flyto 2015-02-02T22:07:08.030

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video about an unusual roundabout: The Magic Roundabout

Nick Alexeev 2015-01-21T03:23:32.313

Your link and photo refer to roundabouts, but the term "traffic circle" most typically refers to rotaries. Can you clarify which traffic device you're asking about?

Pops 2015-01-21T03:49:45.647

Answers

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I think you are talking about roundabouts, not traffic circles.

It is baffling to those of us in the UK that Americans think roundabouts are a new idea. In the UK we have so many variants, from mini-roundabouts all the way up to full motorway junctions (a giant roundabout above or below the motorway).

Motorway Roundabout

So do roundabouts take up more space? Not necessarily, this is a mini roundabout:

Mini Roundabout

It's nothing more than a slightly domed area of paint on the road, no lights are necessary, you can actually drive straight over the top of it rather than around it, its main purpose is simply to dictate who has right of way so that everyone knows who should yield and who should go.

In a busy town or city environment, roundabouts do not work well because excessive traffic from one direction with right of way can completely stop all other traffic causing congestion in other directions. Some roundabouts have lights or peak-time signals to prevent this. One great thing is that they're easy to modify (adding lights, making it mini (drive-overable), adding another entry-point, etc. Everywhere other than in busy grid-based towns/cities they are ideal.

So from a highway engineering perspective...

The main pros are:

  • Cheap to build
  • Agile (Flexible / extensible)
  • Scalable to suit any junction size
  • Mutable (add peak signals, bypass lanes, extra incoming roads)
  • Modular (google image search for "double roundabout", "magic roundabout")
  • Easy and safe for drivers to use (rules don't change in any configuration)
  • Aids navigation of complex junctions from simple road-signs (just count the exits)

Cons are:

  • Annoys drivers on country roads when you'd like to just bypass
  • Not suitable for busy city grids

jhabbott

Posted 2015-01-21T02:42:04.263

Reputation: 3 875

1Even flow round a roundabout relies on their being sufficient space/time for a car to enter a roundabout when there is nothing coming from the right. In busy city grids it's likely that traffic flow could be continuous and very dense from the right thus preventing cars from other entrances from going onto itPhizzy 2015-02-02T16:36:55.620

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There's a hierarchy of junctions, that you'll find in most standard highway design guides, such as the UK's immense Design Manual for Roads and Bridges.

Different junction designs have different motor-vehicle capacities, and operate best at different speeds. Within towns and cities, junction capacity is critical: it is that that determines the network's capacity. It's different for motorways, where it's link capacity that determines network capacity.

Junction design can take a whole bunch of different factors into effect: available space; movement of non-motor vehicles, pedestrian connectivity, and so on. Nevertheless, for a few unfortunate decades in the late 20th Century, and even today in less enlightened places, junction capacity was/is used as the determining factor to select which form of junction to use, within spatial constraints. In other places, it is just one consideration amongst many.

The highest-capacity junctions are grade-separated. The next highest are signalised roundabouts, then unsignalised roundabouts, then signalised cross-roads, then unsignalised cross-roads have the lowest capacity.

That's the biggest criterion that separates roundabouts from cross-roads. All other factors are a question of design, regulation, and patterns of behaviour: safety of all, comfort for pedestrians, ease of use for cyclists, landscape impact, cost - any of these can be better for roundabouts, or worse for roundabouts, depending on the particular design.

EnergyNumbers

Posted 2015-01-21T02:42:04.263

Reputation: 2 932

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Roundabout:

Pros

  1. As you've mentioned, the Mythbusters' test results showed that roundabouts were about 20% more efficient for cars, good for high traffic
  2. Allows more cars to cross at a time.
  3. Doesn't need traffic lights.
  4. Safer

Cons

  1. As you've also mentioned, it takes up more space.
  2. Uses more material to make.

4-way intersection

Pros

  1. Easier for pedestrians to cross street
  2. Space efficient

Cons

  1. More crashes
  2. requires traffic lights
  3. Slower than Roundabout

Overall, the roundabout has more Pros, so it is a better choice. But when you don't have space, use the classic 4 way intersection.

Pyraminx

Posted 2015-01-21T02:42:04.263

Reputation: 216

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Like the British, Australia has had roundabouts for at least five decades in some shape or form and there prevalence has increased over the last thirty years or so.

In the city were I live there is a five-way intersection and it is controlled by a single roundabout, nothing else. It's been operating for at least forty years without any issues.

It's easier and simpler to install a roundabout for such an intersection than to install a system of traffic lights.

Depending to the zealotry of the relevant traffic authority, if roundabouts are placed at every intersection on a major road for a considerable length of road, it can be very frustrating for drivers on the major road. In this situation the roundabout can slow traffic. Prior to the installation of the roundabout at those intersection the minor road intersecting the major road were controlled by Stop or Give Way signs, no traffic lights, and at when the major road was busy vehicles trying to enter the major road had very long waiting times. Hence, the roundabouts made the traffic flow easier for the vehicles on the minor roads faster and fairer.

As jhabbott states in his/her answer about, roundabouts can be small and cheap. A small painted circle and on the road and signs on the roads at the intersection informing drivers that the intersection is a roundabout. Even very large roundabouts can be virtually level with the rest of the road, allowing large heavy vehicles like buses and semi-trailers to drive straight through providing it is safe to do so.

Fred

Posted 2015-01-21T02:42:04.263

Reputation: 6 658

1The effect to slow traffic may very well be by design. In Spain, I've seen roundabouts with exactly two exist at the entrance of a town, which serves no other purpose than to slow traffic (and to permit traffic participants who have changed their mind to not enter the town after all ;-)gerrit 2016-03-15T18:04:24.823

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Roundabouts are only good for equal distribution of incoming traffic. As soon as one road feeds more traffic it will starve the other roads.

Small roundabouts are less efficient than large ones. So to get an efficient roundabout you need some space to lay it. Mini's are only good to specify equal right of way to the roads in low traffic areas.

Double lane roundabouts are only 30% more efficient than single lane roundabouts. Compare that to traffic light where doubling the lanes will actually double the capacity.


Traffic lights however can be tuned to prioritize certain roads. For example a left turn at an intersection crossing opposing traffic can get a phase where they can go and the opposing direction has red and then don't have to worry about giving way to that traffic (a conflict-free left turn in countries that drive on the right). Or directions can only come up when there is traffic waiting for it.

Traffic lights can also filter traffic to spread out the incoming flow of a subsequent intersection instead of letting it jam. Or coordinate them to get a "green wave" so traffic doesn't need to wait at the second intersection.

Things like public transport can also benefit as buses and trams can get be equipped with transponders to "log in" the intersection and better green times (earlier and/or longer). This is usually at the cost of other regular traffic though.

A well designed traffic light will be so you only need to queue up at most once before you can get through, waiting at most 2 minutes.

ratchet freak

Posted 2015-01-21T02:42:04.263

Reputation: 4 083

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This answer is based on personal observation and commonsense. There is little science but some engineering intuition. I don't claim any civil or traffic engineering expertise. Like most engineering solution no one size fits all.

Traffic Circle (Round about)

Traffic Circles are good for areas with low traffic volume, warm climate and with large population of discipline. law abiding, courteous drivers who rarely use cell phone while driving. This will enable the traffic to flow smoothly and everyone will be happy drivers.

Traffic circles are not so great for areas prone to heavy snow or back ice. During heavy snow it is extreme difficult for snow plows to clean the roads properly, thus leaves snow which eventually turn to ice and cause accidents. Using extra salt is and option but then in return salt eventually pollutes water sources. Therefore low cost stop signs are a great alternative, as well as more expensive traffic lights.

Traffic Lights

Apart from the power requirements, overzealous drivers trying to beat the red light, and driver under influence who fail to recognize traffic light, I believe traffic light have good benefit. Traffic light give the ability automate the follow of traffic, allow an override option for pedestrians as well keep those unlawful, rude drivers in check. In summary a good engineer will pick the best solution for the particular application based on data, analysis and skill.

So to all those Civil and Traffic engineers, do you think you can find a solution to this situation?

This happen about a week ago, not too far away from my residence.

https://www.youtube.com/watch?v=O7ApxVyskuI

Mahendra Gunawardena

Posted 2015-01-21T02:42:04.263

Reputation: 4 093

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Something that I didn't see mentioned here is a HUGE saving in wasted fuel, burnt by cars and truck sitting at a traditional intersection, waiting for a light to change. To me, this is the best feature. Roundabouts are planet friendly period!

Another spinoff to this is the reduced frustration to drivers who are sitting, waiting and waiting and waiting...

Roger Williams

Posted 2015-01-21T02:42:04.263

Reputation: 1

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Most of the pros regarding a traffic circle have been touched on here. It's also worth mentioning that traffic circles, despite their efficiency in decreasing accidents in heavily-congested areas, are truly harrowing -- often fatal -- for the blind & impaired.

Long, R. G., Guth, D. A., Ashmead, D. H., Emerson, R. W., & Ponchillia, P. E. (2005). Modern roundabouts: Access by pedestrians who are blind. Journal of Visual Impairment & Blindness, 99(10), 611-621.

tajcook93

Posted 2015-01-21T02:42:04.263

Reputation: 56

1Can you elaborate a little on the contents of the paper? I understand exactly what you mean here, but fleshing it out a bit more could make for a better answer.HDE 226868 2015-02-25T00:46:44.753

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There is really no argument against the traffic circle. They use less space (contrary to what some have said here). They reduce accidents, injuries, and fatalities substantially. They improve traffic flow, which increases fuel efficiency. They are more free and cooperative, whereas traffic lights are totalitarian. Lighted intersections should be abolished and replaced with circles or grade separated intersections for rush hour situations that might cause a flow problem (which I think is exaggerated). Lights are an outdated panic reaction that has no place in modern society.

Bob

Posted 2015-01-21T02:42:04.263

Reputation: 1

This seems quite opinion based.JMac 2017-07-12T10:30:32.100