The Nobel Prize in Physics and Traffic Priority at Roundabouts

traffic roundabout

(Photo: Andrew Skudder)

The 2013 Nobel Prize in Physics was recently awarded for symmetry breaking and its consequence, the Higgs boson—a particle so well known that, according to the president of the American Physical Society, “[i]f you’re a physicist, you can’t get in a taxi anywhere in the world without having the driver ask you about the Higgs particle.” Teaching the symmetry unit in my own course this semester, I couldn’t help wondering about symmetry as I drove through an apparent example of symmetry: roundabouts or traffic circles.

Roundabouts use two complementary systems for controlling traffic flow: (1) Traffic in the roundabout has priority, or (2) traffic entering the roundabout has priority. The choice seems so symmetric, like choosing right- or left-hand traffic. In the United Kingdom, traffic in the roundabout has priority. In contrast, on many Massachusetts roundabouts, including one on my commute, entering traffic has priority. At rush hour, as drivers in the roundabout yield to the incoming traffic, the roundabout gets more and more packed, until the flow locks up and spreads the traffic jam to the surrounding roads.

The symmetry, in contrast to right- and left-hand traffic but like the broken symmetry producing the Higgs boson, is broken: There is more space outside than inside the roundabout. Giving priority to drivers exiting the roundabout better uses its limited space. Similarly, passengers exiting an elevator or subway car should have priority over entering passengers—although subway systems, with the incessant announcements to “please let passengers exit the train first,” must loudly remind passengers of the broken symmetry.


Why would anyone entering a roundabout have priority? It makes no sense at all...


In (at least some) roundabouts in Pennsylvania entering traffic has the right of way, the philosophy being that vehicles to the right generally have the right-of-way unless signage indicates otherwise. The real problem isn't that there are differing philosophies so much as that most people have no idea who has the right of way, a problem that is defintely not limited to roundabouts (at least in the US).


"...the philosophy being that vehicles to the right generally have the right-of-way unless signage indicates otherwise."

But is that true? I can't think of any instances where it is, but several where it isn't:

- On the freeway, entering traffic (typically from the right) does NOT have right of way.
- When you turn right at a red light, you are to the right of both the cross-traffic and the oncoming left-turning traffic (if they have a turning light), and you do NOT have right of way in either case.


I grew up and learned to drive in Massachusetts and was taught that traffic in the rotary (as they are properly called in Massachusetts) has the right of way. Are there specific rotaries with signage that indicate the opposite is true? If so, I'd be curious where exactly they are.

Now, it wouldn't surprise me if the practical behavior of traffic in certain spots was as if the law was that traffic entering the rotary has the right of way but it would surprise me if that were codified anywhere.

(BTW, what happened to the checkbox that allowed us to get updates on the comments? It seems to have disappeared.)


This contradicts you:

As well as this:


I think he's probably referring to the unwritten law of the Masshole driver. Sarcasm font intended.


The traffic circles in the Boston area have been altered over the years to make it easier for cars to enter. They had difficulty understanding how to manage rotaries with multiple streets entering. Back when traffic flows weren't as heavy, they were trying to make it easier for certain moves - like enter and veer to the right to get off immediately becomes an almost unsignalized move - and didn't recognize they were degrading the entire intersection. For example, Horace James Circle in Newton was opened up to allow drivers essentially to merge with barely a stop on to Hammond and then to Rt. 9. The system of traffic design then seems to have been largely ad hoc.

When I moved to Boston in the 1980's, I was floored by the terrible quality of much of the traffic design and engineering work. I've since worked with a number of the state's engineers and know they're good at their work, but much of what they do is remedial because so much was done poorly. My favorite sign for many years was the exit from the now gone Central Artery for the Faneuil Hall/Quincy Marketplace attraction was 'Dock Square'. Dock Square is a completely unremembered area which is now a brick patio of no interest whatsoever. The message was very clear: if you aren't from here, we aren't going to help you.

Some work remains hideous. When they rebuilt Kenmore Square, for example, they set up a left turn light (that takes you right to Fenway Park) with 2 lanes next to it that went straight. They then installed 2 left turn lights - not 1 - so every day cars would try to turn left from the center, through lane. They also installed a parking space on the other side of the intersection without realizing a car parked there blocked the travel lane. The wrong light signal lasted for over 2 years. It only takes changing the lens part.



Ha..."if you aren’t from here, we aren’t going to help you". So true. All of the signage is meant for people who have lived in the area all their lives.
I still find it shocking that one can drive down a street for blocks and not be able to find out what road you are on as they only tend to label a street if it crosses one that is deemed more important. Someone should do a study on the societal costs of printing and installing signs vs people driving around aimlessly trying to figure out what street they are on.


I moved to Greater Boston 7 years ago and was mystified by the weird behavior regarding rotaries. A native explained that legally one used to give way to those entering the rotary (counter to logic), but that they had changed the law to come in line with everywhere else in the world. Now, the schizophrenia in Massachusetts rotaries are generally a holdover from past practices and the general tendency towards "assertive" driving. In short, people don't really have a good script, so they just bull in.

I would also say that people in Massachusetts don't really understand the concept of staying towards the center of a rotary and then migrating to the outside as you near your exit. It only takes a small percentage of people to screw this up for everyone (by staying outside and trapping those on the inside), so even though I know the "right" way to do it, I tend to go with the flow to avoid going in circles.

I'm with Ray. I'd like to know which one you are referring to as I live just outside of Cambridge.



Buffalo, NY has a few traffic circles on the west side of the city. At least one of them is considered part of the major street which passes through (around?) it. Cars on that major street have priority but cars on the "side" streets defer to traffic in the circle.

Randy Hudson

The Massachusetts "rule" is just the lack of a special case for traffic circles to override the standard rule granting the vehicle on the right the right-of-way, when two roads join and there are no other traffic control indicators. Often, some or all entrance lanes are posted with "YIELD" signs, which do override that default and serve to give priority to traffic within the roundabout.


I agree with the other comments here. I also live in Massachusetts and know the law states you must yield to the traffic in the rotary. However as it happens some ppl are ignorant of the rules and plow on in as once again today I witnessed this particular event in concord center. I know of a few rotaries that are hard to judge due to the fact that they are failures of traditional design that give a preference to the incoming traffic over the traffic in the rotary by aligning the rotary entrances and exits in a straight line.

Joe J

Recently, Mythbusters did an episode on efficiency of roundabouts over 4 way stops, they were even with American drivers. Trying to remember if they mentioned who had priority in their roundabout test.


I had some kind of debate with someone over this question once. I don't remember what the outcome was, but I remember stumbling upon the realization that a key difference was one of linear vs. non-linear choices.

Taylor S. Marks

MA is the only place in the world that I'm aware of that actually gives priority to entering rather than exiting roundabouts. The worst part is that they actually think their backwards system is the way it's done in the rest of New England, so they continuously cut off people when they leave their state. MA drivers didn't get the nickname massholes for nothing.


Many people confuse older styles of circular intersections with modern roundabouts. East coast rotaries, large multi-lane traffic circles (Arc D’Triumph), and neighborhood traffic circles are not modern roundabouts. If you want to see the difference between a traffic circle, a rotary (UK roundabout) and a modern roundabout (UK continental roundabout), go to to see pictures. And here’s another site that shows the difference between an older rotary and a modern roundabout:
The FHWA ( has a video about modern roundabouts that is mostly accurate ( ).

Eric M. Jones

Massachusetts (where I live) has insanely poor traffic signage, traffic islands floating about, miserably poor drivers, the Big Dig (which hilariously cost taxpayers one-million dollars per foot...more than 2-1/2 Panama Canals would cost in today's money), bad attitudes and I could go on. But traffic circles?...entering traffic never has the right of way.

BTW: The Massachusetts State Troopers are smart, honest and helpful. All other local police are crooked.


You might want to try real modern roundabouts (ie. guided ones) like they are popular in the Netherlands:


In Washington DC where I live, we have few major roundabouts such as Logan and DuPont Circle fitted with traffic lights... over twenty of them! Getting sucked into these roundabout vortex, feels like a traffic black hole.
I always thought It is just a great example of the cost of stupidity and government arrogance that we all have to pay for in our daily life.