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

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:

And of a four-way traffic light intersection:

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

• video about an unusual roundabout: The Magic Roundabout – Nick Alexeev Jan 21 '15 at 3:23
• 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 Jan 21 '15 at 3:49
• @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 Jan 21 '15 at 18:29
• @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 Jan 21 '15 at 19:39
• Suspect there may be international variation in the meanings of those terms. There certainly is in lay usage. – Flyto Feb 2 '15 at 22:07

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).

So do roundabouts take up more space? Not necessarily, this is a 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
• 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
• Even 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 it – Phizzy Feb 2 '15 at 16:36

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.

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.

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

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.

• If "Doesn't need traffic lights" is a pro for roundabouts, then "Doesn't need a roundabout" is a pro for traffic lights, unless you have already decided that roundabouts are better than traffic lights. The conclusion should not be part of the evidence. – CJ Dennis Feb 1 '20 at 2:41

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.

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.

• The 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 Mar 15 '16 at 18:04

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.

• I think you mean that when one road reaches saturation point it will starve the others. There's no problem with an unequal amount of traffic from different roads in general. In fact I can think of one roundabout with almost no cross traffic that works very well for all vehicles because the main road is usually only about half of its saturation point. – CJ Dennis Feb 1 '20 at 2:45

People using the road make mistakes (like running stop signs and red lights), always have and always will. Crashes will always be with us, but they need not result in fatalities or serious injury.

Modern roundabouts are the safest form of intersection in the world - the intersection type with the lowest risk of fatal or serious injury crashes - (much more so than comparable signals). Modern roundabouts require a change in speed and alter the geometry of one of the most dangerous parts of the system - intersections. The reduction in speed and sideswipe geometry mean that, when a crash does happen at a modern roundabout, you usually need a tow truck, not an ambulance. Visit the Insurance Institute for Highway Safety for modern roundabout FAQs and safety facts. Roundabouts are one of several proven road safety features (FHWA).

Modern, slow and go, roundabout intersections have less daily delay than a stop light or stop sign, especially the other 20 hours a day people aren’t driving to or from work (it’s the #2 reason they’re built). Average daily delay at a signal is around 12 seconds per car. At a modern roundabout average daily delay is less than five seconds. Signals take an hour of demand and restrict it to a half hour, at best only half the traffic gets to go at any one time. 'At best' because traffic signals must have the yellow and all red portion (6+ seconds per cycle) for safety, and modern roundabouts do not. At a modern roundabout, drivers entering from different directions can all enter at the same time. Don’t try that with a signalized intersection.

First cost is the wrong way to compare projects. It would be like buying a car without knowing the fuel economy or safety of the thing, just its price to buy.

 Present Value Life Cycle Cost Analysis (LCCA) is the best way to compare two or more choices.  When comparing modern roundabouts to signals for a 20-year life cycle (the standard period), modern roundabouts usually cost less.  Costs to compare include: first cost (design/land/construction), operation and maintenance (electricity, re-striping, upgrades, etc.), crash reduction (what’s your/your family’s safety worth?), daily delay (what’s your time worth?), daily fuel consumption (spend much on gas?), point source pollution (generated by stopped vehicles = health cost), area insurance rates (this costs more where it is less safe to drive).  Each of these things, and others, can be estimated for any two choices and everyone near or using the project area will pay some portion of all these costs (and gain the benefits).


Many people confuse other and older styles of circular intersections with modern roundabouts. High speed, east coast rotaries, large multi-lane traffic circles (Arc D’Triomphe, Dupont Circle), and small neighborhood traffic circles are not modern roundabouts. The Brits even call a merry-go-round a kid’s roundabout. Go to http://www.k-state.edu/roundabouts/photos.htm to see pictures.

BTW, safety projects traditionally qualify for 80% Federal reimbursement.

Mini-roundabouts are less common in North America, but frequently used in the UK. Their footprint is smaller, making them suitable for retrofit situations with right of way constraints (and lower cost). They are also commonly used where truck U-turns are not needed, so achieve all the safety benefits of compact modern roundabouts at a much lower cost. They are all truck apron, and in the UK are sometimes just paint on the road.

Single-lane modern roundabouts (50-120 feet in diameter) can handle intersections that serve up to 20,000 vehicles per day with peak-hour flows between 2,000 and 2,500 vehicles per hour.  Two- and three-lane modern roundabouts (150-220 feet in diameter) can serve up to 60,000 vehicles per day and handle 2,500 to 5,500 vehicles per hour.  Right-turn slip lanes can increase those numbers if needed (just like for signal intersections).  Much depends on how balanced the entries are, but only in determining how many lanes are needed for each movement – just like at a signal controlled intersection.


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 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.

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.

• Can 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 Feb 25 '15 at 0:46

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...

In any design regarding public traffic your biggest problem is going to be managing the human factors of competence, confidence and error. The entire purpose of traffic management is to control human behaviour. With automated traffic it is simply an issue of coordination but humans are not well coordinated. Some are slow, others are nervous, other people are frustrated while even more are driving along on autopilot or putting on make-up...

These factors are what make a junction of roads work or fail.

The cons of traffic lights are electricity consumption and telecommunication with the rest of the network. A good system will work at all times.

A modular traffic circle will always work as long as nobody fearful turns up and flawless around each stage of turns. As capacity is reached they can become a gridlocked bottleneck but are excellent on one way street systems. Traffic lights work best at minor junctions to regulate traffic flow. In this respect the UK has ubiquitous installations of traffic lights on traffic circles which is an oxymoron contrary to traffic flow behaviours. The UK problem would be reduced drastically of drivers actually obeyed the 3 second rule of following distance but they steadfastly do not.

Volume of traffic is a major consideration also. Where volume is low and speed is low, a painted circle should suffice but why bother when a 4 way stop does exactly the same thing? Intersections are always an opportunity for an hazard to be amplified or reduced. You just have to work out which hazard needs reduction and the opposite hazard increased. Whatever the design, it is to manage human behaviour in that location.

Roundabouts let traffic flow more smoothly and there is also less reliance on technology that can malfunction due to a series of factors (i.e. power outage, etc).

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.

• This seems quite opinion based. – JMac Jul 12 '17 at 10:30