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.

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  1. Miguel says:

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

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    • J1 says:

      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).

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      • NZ says:

        “…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.

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      • J1 says:

        You got me wondering. In urban areas, nearly all traffic conflicts are governed by signage or signals, so that type of rule might be one you rarely run into. But from my local driver handbook:

        “If traffic from another roadway is merging
        into the roadway you are traveling on, safely
        change lanes away from the merging traffic if
        possible. If it is not possible to change lanes
        away from the merging traffic, adjust your
        speed and vehicle position to safely allow
        the traffic to merge”

        “At intersections where there are no stop
        signs, yield signs or other traffic signals, if
        two vehicles come to the intersection at the
        same time, the driver of the vehicle on the
        left must yield to the driver of the vehicle
        on the right”

        “At a four-way intersection where all drivers
        are faced with stop signs, all drivers must
        yield to pedestrians; otherwise the vehicles
        should proceed through the intersection in a
        “first to arrive, first to proceed order.” If two
        vehicles reach the intersection at approximately
        the same time, yield to any vehicle on your right.”

        I guess the first could be considered a “left-of-way” too, in the case of a left freeway entrance, and I think it’s a localism – when I learned to drive in California that was definitely not the law. I’d consider your second example a case of signage indicating otherwise, though I should have been more general in that statement and included traffic lights Likewise, everwhere I’ve lived, where there’s a conflict between a vehicle turning left and a vehicle turn right, the vehicle turning right has the right-of-way unless signage/signaling indicates otherwise (the opposite applying if you live in a left-drive area).

        Don’t infer from any of my remarks an endorsement of the Pennsylvania system. Like you, I thought it was weird (I’d consider the circle a road I was merging onto), but that was the explanation.

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    • Abigail says:

      Priority for traffic entering the roundabout used to be the case in the Netherlands. The reason being that a main rule for determining who has priority on an intersection is “traffic yields to traffic coming from the right”. Giving priority to traffic on a roundabout means that you have to yield to traffic coming from the left!

      The Netherlands has switched, and while in many cases it makes sense, it does mean there’s an additional traffic rule one has to learn.

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    • Mike B says:

      A Traffic Circle is defined by the concept of entering traffic having the right of way and was invented in New Jersey in the 1920’s when there were few cars on the road. Traffic Circles also have the property of being physically large so that cars on the inbound routes can maintain speed and moreover they have multiple lanes of traffic inside the circle itself.

      When used in their original context of low traffic density Traffic Circles with entering right of way work flawlessly. Cars in the circle have room to make way for cars entering and it works like a standard cloverleaf interchange with one large merge zone. The problems occur when traffic density increased and the circles would jam up.

      There are still two “classic” traffic circles in New Jersey that work as intended. These are on Route 70 in the Pine Barrens at the junction of US 206 and NJ 72.

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  2. Ray says:

    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.)

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    • FIPer says:

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

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  3. jonathan says:

    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.

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    • John says:

      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.

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  4. John says:

    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.

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  5. Chris says:

    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.

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  6. Randy Hudson says:

    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.

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  7. CHRIS says:

    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.

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