I’ve read a few articles that mention the safety benefits of a modern roundabout over a traditional intersection—significant reductions in collisions and fatalities—but they all seem to talk in the sense of a signal lane circle with two-lane roads.
Is there any data on how a two-lane or three-lane roundabout fares, from a safety standpoint, compared to a standard intersection of a four or six lane road?
Anecdotally, roundabouts are the favored intersection construction locally. Some are two intersections of two lane roads. Some are intersections of four lanes roads.
We have more serious accidents (i.e. head-on or -bone collisions, more serious injuries) at intersections controlled by traffic lights/signs. Accidents at roundabouts tend to have lower speeds and relatively safer contact (i.e. side to side collisions, less serious injuries).
At least that is the anecdote from our 9-1-1 centre.
It gets even better when they are more complicated: http://news.bbcimg.co.uk/media/images/63126000/jpg/_63126500_msn_magic_roundabout_470x350.jpg
That intersection is known as “The Magic Roundabout”
You might try searching for “traffic circle” instead. In Ann Arbor, they use the term “traffic circle” for much larger “circular traffic thingies” than “roundabouts”. Not sure if that’s a universal practice, or a regional thing. Also not sure what a generic name a little more technical than “circular traffic thingies” would be.
“Traffic circle” will also bring up circles that aren’t roundabouts. A circle where there are traffic lights at each entrance, or where traffic in the circle has to yield to entering traffic (yes, this exists, and yes, it’s stupid) is a traffic circle, but not a roundabout.
Hmm. Driver confusion probably accounts for some of this lower speed. 
I don’t know where to find data, but multi-lane roundabouts have been common in Britain for many decades. I am sure their effects on safety and (traffic flow) have been extensively researched. (If you are searching for the data on them, they are almost always called roundabouts in Britian, so do not search for “traffic circles”.)
I also doubt that it is really true that American drivers are too stupid to quickly get the hang of roundabouts. They naturally slow traffic due to their structure (unless you are the kind of driver who does not slow down on curves, and ignores cross-traffic - but I don’t suppose drivers like that stay alive long anyway), but generally keep the flow of traffic moving better than lights or stop signs. I do not really understand why they have not long been much more common in America.
Here’s a PowerPoint on the subject.
Here’s a more comprehensive FHWA study. Look at the crash statistics on pp. 112-113.
A quick google image search for traffic circle lane patterns shows all kinds of arcane designs. Some of have a sort of spiral pattern: There are two lanes in the roundabout, but in each quadrant, the right lane exits to the road there, the inner lane shifts to the outer lane, and a new inner lane appears where cars enter at that point.
Or something like that.
This has the effect that cars can only stay in the roundabout for three-quarters of a full 180 (which results in a left turn, if you drive on the right American style), but you can’t make a full 180-degree turn in the roundabout.
From the Wiki page Roundabout, in the section on Turbo Roundabout:
Traffic flow diagram for that. Note how each inner lane spirals outward by the third exit around. Notice also that the various entrances illustrate different rules about which lanes you need to be in, in order to go which different ways.
That diagram shows that if you are on the larger road (the left/right one), you can do a 180, but not from the smaller (up/down) road. If we had more roundabouts in this area, I believe that blocking the u-turn route would be a bad idea: there is so much traffic at times that the steady flow would make some left turns nearly impossible, effectively negating some of the benefit of having the roundabout.
I hear that in some European states, they have some opposite-rule round-abouts, where cars in the circle must yield to incoming cars. I would be interested to know how well that works.
Of course they aren’t. We only have a handful of them in this area (10 in the entire county) but despite their rarity, I don’t see any significant problems with people using them. One in particular recently replaced a three-way intersection where previously one of the roads was always horribly backed up during evening rush hour. Now, no backup and cars go through it about as well as they do in countries with many more roundabouts.
I agree. There’s at least a couple dozen intersections in the areas I frequent that would benefit greatly from them. I’d suggest them to the county traffic planners, but I doubt it would do much good. Yes, they do occasionally put them in, but the key word is “occasionally”. And it takes them forever to approve them. The recent one I mentioned above was put in a year and a half ago. They haven’t put another one in since.
A roundabout effectively turns an intersection into a set of ‘T’ junctions. Much easier than crossroads to deal with where traffic is heavy. It has become more common here to put traffic lights on the approach roads, especially on large roundabouts where traffic might be moving too fast to allow people in.
The spiral markings are also seen more and more lately.
If you want to experience a roundabout with high volumes of traffic and a rule where entering traffic has right of way then go to Paris and drive round the Arc de Triomphe. There are (I think) 10 approach roads and no marked lanes or traffic lights. Amazingly there are few accidents.
When it comes to managing traffic - is this the beginning of the end for white lines, traffic lights and stop/go signs:
It’s hard to compare UK and US traffic. From the article:
In Ann Arbor, we’ve probably got thousands of intersections with lights, and our population is 114,000, a little over double theirs. At $1 million+ per roundabout (the cost around here), I don’t think we’re replacing all of our traffic lights with roundabouts any time soon.
“Common knowledge” at our Wisconsin DOT is that roundabouts are safer than traffic lights.
Wis DOT plans to replace 3 of our local intersections with them. Two are “T” intersections, one is a 4-way cross.
Right now, traffic can continue thru an intersection at full speed, or must come to a complete stop, depending on the light. With roundabouts,* all* traffic will have to slow down, always, but rarely stop. I’m not so sure this is a good thing, but time will tell, I guess.
I predict that 10 years from now, someone will discover a better way to manage traffic, and call it a “traffic light.”
We’ve had them in New England for a long time. We call 'em rotaries.
Drivers occasionally fail to heed traffic lights, either from haste or from distraction. My best friend was involved in a head-on collision while sitting at a red light. Nose-to-nose collisions simply cannot happen in a roundabout, and drivers are forced to pay attention to traffic instead of trusting the light to manage the intersection.
Around here, streets seem to keep getting wider and wider (more lanes). In most cases, the increased capacity is there to deal with cars waiting for the lights to change. With a roundabout, given ample time for people to adapt, most of the extra lanes can go away, because traffic does not stack up and wait. This is good in terms of fuel consumption and anxiety, but more importantly, stopping at a light allows the driver some time to do other things (e.g., text on a phone) – a roundabout would demand that they keep their mind on driving because there is no guaranteed stop-time. That, I think, would also be a net positive. When you are on the road, moving always feels better than not moving.
Safety can be measured objectively – how many accidents of what severity in a given time period – but as for sheer terror, nothing I have ever experienced was as demoralizing as the big four-lane traffic circles in Tijuana’s Paseo de los Heroes. I very quickly learned how to take side streets to avoid these monsters!
Here in hickville Arkansas, there are more & more circles going in, One & two lane mostly. All make A positive improvemnt in both tot
Typing over a cat makes me so slow I can’t make the time limit…
"Here in hickville Arkansas, there are more & more circles going in, One & two lane mostly. All make A positive improvement in both total travel time & safety for that intersection.
One of the problems with retrofitting intersection is a lack of room but on normal residential sized streets, mini-roundabouts are proving a good idea and do not cost a Million.
Really big ones in older towns with 4 lane in each direction are not possible because of space and cost needed to make the space. Big business do not like their iconic buildings torn down for traffic circle.
YMMV "
