MythBusters Tackles Four-Way Stop V. Roundabout Traffic Throughput

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Who doesn't love MythBusters? I mean, I'm still perplexed by the staying drier by walking and not running in the rain thing, but these are generally pretty fun. So Jamie and Adam recently took on the traffic throughput of a four-way stop intersection versus a roundabout. You'll have to check out the video above to the get the result, but one was 20% more efficient than the other. Of course for anyone intereseted in the four-way stop versus roundabout and how it actually works in a city, plenty of questions and issues were left undiscussed. This MythBusters was unfortunately only focused on the level of service for drivers.

How do the various intersections work for pedestrians? Bicycles? What about collisions in a roundabout? The traffic throughput may not even be the primary argument for roundabouts. Even if a roundabout handled the same or fewer cars, slowing traffic, and the elimination of dozens of points of conflict and the potential for head-on vehicle collisions would be a strong argument in their favor.

Recently we pulled together a compilation of videos showing how to navigate a roundabout and included images of the roundabout coming to University City: Nine Videos to Help You Navigate a Roundabout.

roundabout conflict points

Added graphic to highlight the vital importance of slowing traffic at conflict points:

These discussions regarding roundabouts never seem to cease. Anecdotes can be helpful, but in this case, there is a large body of study on the issue. Unless one asserts some type of driver exceptionalism (see incompetence), the evidence that roundabouts safer than traditional signalized or signed intersections is overwhelming. The Institute of Transportation Engineers (ITE), Federal Highway Administration and many state DOTs agree. If you're really into this, you can read the recent ITE here. A snapshot regarding previous research from around the world from that report:

In 1992, a before-and-after study was conducted in the Netherlands of 181 roundabouts that were previously stop controlled or signalized intersections. They found that the number of accidents in a year dropped by 51% on an average and the injury accidents decreased by an average forty four percent. 

A before-and-after study of 73 roundabouts in Australia conducted in the year 1981 showed a reduction of 74 percent in the casualty (i.e., fatality) accident rate and a 32 percent reduction in property damage accidents. 

In 1996, 34 modern roundabouts in Germany were studied. This study found that the number of fatalities and severe injuries decreased from 18 to 2. The number of accidents with heavy property damage decreased from 24 to 3. 

France studied about 83 roundabouts in the year 1986, and concluded that the transformation of regular intersections into roundabouts yielded significant safety benefits. While the fatalities reduced by 88 percent, the injuries fell by approximately 78 percent. Another study of 522 roundabouts in the year 1988 found that 90 percent of them had no injury accidents at all.

In Switzerland, two roundabouts built in 1977 and 1980 were studied for 4-8 years after they were converted as roundabouts from the conventional intersections. The findings of the study were that there were reductions of 75 percent in total accidents and 90 percent in the number of injuries. 

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  • wump

    roundabouts are horrible for pedestrians, at least in this CITY (dont know or care about the one in ucity), because drivers in this CITY dont respect yield signs in re pedestrians, they think those yield signs mean yield to other cars, not pedestrians. at a stop sign you have a chance that they will stop, but drivers dont yield to pedestrians because of a yield sign.

    • jhoff1257

      Yield signs do mean yield for other cars. By law drivers have to yield to pedestrians regardless of whether or not a yield sign is present. You can’t blame a type of intersection for a driver’s lack of intelligence.

      • wump

        they dont, thats reality, they do if there is a stop sign

        • Brian

          I walk 3.2 miles to work every day, Tower Grove South to CWE, and I can tell you that drivers yield to pedestrians at stop signs about 60% of the time. Typically, the rollers go through at 15 mph, the same speed they would be traveling going through a roundabout; some clowns go full-bore at 30-35 mph.

          • wump

            neece walk brohammer, do you take tower grove ave?

      • Alex Ihnen

        Regardless, cars must slow down to enter a well-designed roundabout. In many areas, this means reducing vehicle speed from 40mph to 15mph or less. This saves lives. And cars can’t run a roundabout like they can blow through a stop sign.

        • KW

          Oh they certainly can. Within roundabouts divided up by a traffic island, I’ve personally observed lifted Ford pickups straight up drive through the island to go straight through.

          • Alex Ihnen

            That’s not a well-designed roundabout.

      • Eric Bunch

        Of course you can blame the intersection if it encourages poor driver behavior. Good engineering makes it easy for drivers to follow the law without relying on the safety net of traffic enforcement.

        • jhoff1257

          Well when you find the perfect intersection that negates the need for traffic enforcement, you let us know. I’ve lived in St. Louis for 20 years, no intersection is going to solve people’s inattentiveness.

          • Eric Bunch

            I’m not saying there is a perfect intersection, just that if a certain design elicits poor driver behavior it might be time to rethink the particulars. Apply the 85th percentile rule: do 85% of street users disobey the posted laws? If so, then it’s time to do something different. I would never say that any one design could totally negate the need for enforcement. Just saying that streets should be engineered in a way that makes it as easy as possible to follow the law. And if a great number of people don’t follow the law then the design IS to blame.

            Engineer the streets for safe behavior and there will be less reliance on enforcement. But by all means, we should still be dolling out traffic tickets to bad drivers.

          • Eric Bunch

            And I’ve never lived in St. Louis, but I can tell you that it is a universal axiom that bad street engineering begets bad drivers. Poorly marked crosswalks across poorly designed streets get almost no driver compliance here in Kansas City. And believe me, KCPD will be the first to tell you that they have much bigger problems to deal with than traffic compliance.

  • http://donspoliticalblog.blogspot.com Don

    The roundabout is 20% more efficient on moving cars through an intersection.

    To save others time.

    Roundabouts still require stop lights in high pedestrian areas which would seem to strip away much of the efficiency.

    • Alex Ihnen

      Don’t think I’ve ever seen a roundabout with a stop light. There are traffic circles (not to nitpick), but the terms used are important. Regardless of throughput, as a cyclist and pedestrian (and driver), I love roundabouts. To look at the example in U-City, cars routinely drive through that intersection at 30-40mph. A collision at 40mph gives a pedestrian a 10% chance of survival. At 20mph, that number of 90%. (added graphic to story)

      • http://donspoliticalblog.blogspot.com Don

        At about the 37 second mark in the above video is what I call a roundabout with a stop light (looks like London) that appears to be specifically for the benefit of pedestrians.

        I admit I don’t know traffic circles from roundabouts.

        • Alex Ihnen

          Yeah, just semantics, I guess. It seems no two roundabouts or traffic circles are alike!

          • Presbyterian

            I can nerd out on this. Though the terms can be used interchangeably, traffic circles typically are older and larger than roundabouts — and usually are signalized. The ones built in the nineteenth century (Paris, Washington) were mainly for placemaking, with parks and monuments at the center. Baron Haussmann laid out the Place de l’etoile in 1855. The DC circles were built in the 1870s as modifications to the rectangular plazas envisioned by l’Enfant’s original design.

            Early twentieth century circles were designed for high speed and ended up being signalized due to their danger.

            Modern roundabouts are smaller, non-signalized and operate at much slower speeds. Their design is for safety and efficiency.

            [Nerding out complete.]

          • samizdat

            Ah, the memories of negotiating DuPont Circle in DC…

          • Presbyterian

            Yes. But it is pretty.

          • samizdat

            ‘Tis true.

          • John R

            Center Cross in Tower Grove Park has the Shakespeare statue. Anywhere else in town that has a similar place-making roundabout/traffic circle?

          • Presbyterian

            Jackson Place Park in Old North was designed as a circular park. It was first set aside for such use in 1816. It’s still there, though Interstate 70 just about killed it. Here’s an old image:

          • Presbyterian

            Also North Grand water tower at 20th St. Maybe the Bissell Street water tower. Perhaps Pagoda Circle in Forest Park, though it’s blurring the lines.

          • John R

            Super appreciated, Presbyterian urbanist nerd!

          • Presbyterian

            :-)

  • Jeff

    I think your assumption that there are 24 points of pedestrian conflict on a four way stop at intersecting two way roads is flawed and a bit self serving. You’ve got the pedestrian crosswalks pushed up to the very corners of the road and then cars starting their turn way too far away from the intersection in order to create the angles necessary to achieve this number. There’s only so much space for a car to maneuver within the confines of a right angle intersection (depending on available curb space of course) and the way you’ve got the right hand turn paths drawn would have cars “curbing out” their tires just about every time. Whether the car is making a left, going straight, or making a right only changes the potential point of impact with a pedestrian a few inches to a couple of feet apart.

    Additionally, you fail to take into account that one car can only occupy one position at that point in the lane at a time. So even if those three points in each lane were separated by enough space to make them legitimate individual points of conflict, they would be an “either, or” type of situation. Either the car would strike a pedestrian in the center of the lane, slightly to the left of center, or slightly to the right, but clearly not three cars striking three pedestrians in the same cramped space simultaneously.

    Also, as at 4 way stops you’ll never have all points of conflict active at once as there are generally at most two cars entering the intersection at once regardless of the number of pedestrians. In the roundabout however, I can actually see how all points of conflict may be active at any given time as the traffic is constantly moving and the points are spaced out enough to allow multiple vehicles to cross each point of conflict simultaneously.

    At roundabouts drivers tend to be less aware of pedestrians and more aware of other traffic as the whole point of the roundabout is to keep traffic flowing. Forcing cars to stop however tends to give drivers an extra bit of time to scan the horizon and locate pedestrians about to enter the intersection as well as give pedestrians an opportunity to enter the intersection safely while vehicles are stopped. A busy roundabout may not see a safe break in vehicular traffic for several minutes.

    You also assume that pedestrians always follow the rules and would never dare to cut across the roundabout at a diagonal using the “bubble” in the center as a way point. This conveniently eliminates at least 4 (and likely more) points of pedestrian conflict from the roundabout model.

    There are far better intersection designs than the roundabout. Even those that incorporate bicycle traffic into the equation as the roundabout seems not to…

    http://www.youtube.com/watch?v=FlApbxLz6pA

    • Alex Ihnen

      I would disagree on a number of points. The points of conflict are important as displayed because a pedestrian needs to look for a right-turning car, a car going straight through an intersection and a left-turning car. There are three different traffic movements to be aware of. In a roundabout there is just one. A car will be coming from a predictable direction and moving in a predictable direction. For very small intersections, I think you’re right. The hard part about the roundabouts are great or awful argument is that these are all contextual issues. The speed of the cars is important too. Beyond comparing the roundabout to a four-way stop, many are being built at signalized or other intersections. The one proposed in University City will greatly reduce vehicle speed, making the area safer for everyone.

      In theory, a car that stops takes an extra moment to look for pedestrians and others, but many glide through intersections anyway, and also pay less attention. After more or less stopping at the sign, the assumption is that everyone else will stop. The most deadly accidents occur when this doesn’t happen. And vehicles must yield to pedestrians at roundabouts even when there’s heavy traffic. It’s not the case that cars simply continue to enter a roundabout and pedestrians stand helplessly waiting for a break.

      • dempster holland

        Vehicles “must yield” to pedestrians when there is heavy
        traffic. Who defines heavy traffic and who tells vehicles
        that there is heavy traffic. Knowing st Louis drivers, I see
        a good chance of them not yielding to pedestrians no
        matter what or, at the least, no cautious pedestrian would
        count on vehicles yielding

      • Eric

        “a pedestrian needs to look for a right-turning car, a car going straight through an intersection and a left-turning car.”

        Exactly.

        Re vehicles looking for pedestrians: in roundabouts the pedestrian and vehicle crossing points are separate. So drivers only need to look for one at a time. They can still break the law and zoom through the crosswalk, but if they are law-abiding it is much easier for them to process all possible hazards.

        Roundabouts are inappropriate in two situations. In very high-traffic intersections, they become jammed up. In physically constrained intersections, there is not room for them and stop signs are necessary. For most medium-traffic intersections, though, they are the best place.

        • Eric

          I meant best choice, not best place, of course.

  • JustFlushIt

    I remember driving from the airport in Grand Cayman (on the left side of the road!) to Bodden Town on the other side of the island and it was all round abouts. From a cars perspective, the roundabouts also keep traffic MOVING.

    • samizdat

      “…keep traffic MOVING”.

      ^^^This. One other thing to keep in mind is that the stop sign (especially as they are deployed here in the City, where it seems that the only thing needed to get one installed is a lack of intelligence and common sense) is a tremendous waste of fuel, and further, causes greater deposits of both carbon dioxide and particulate emissions into the atmosphere, both of which are hazardous to human and other animal life. High rates of asthma, anyone?

      • T-Leb

        I have a lean burn vehicle… I can stop and go easily without polluting any extra. Tdi Diesel 40-50 mpg easily.

        • samizdat

          Cumulatively, your vehicle, my vehicle and the thousands of others driving City streets every day emit more pollutants as a result of stop/go traffic. And as far as I know, unless VW (or whomever makes your turbo direct injection diesel) has incorporated the same type of ignition mapping as Toyota has in their hybrids–stop/start–the engine is still running.

          What’s the http://en.wikipedia.org/wiki/Cetane_number of your regular fuel stop? That may be something to keep in mind, as well as fueling up at a high turnover location (truck stop), as particulate build-up will kill the fuel pump.

          Happy motoring. Diesels have come a long way since my moms ’79 Rabbit, and my ’78. Thankfully.

          • T-Leb

            Most city vehicles will be natural gas or electric in 10-15 years.

  • T-Leb

    I realized I went thru some kind of roundabout or giant circle at Carondelet Park yesterday.

  • Douglas Scott

    All the studies point to one result, greater safety for pedestrians. Let us not be arrogant Americans and think that only our point of view, based on hearsay, is valid.

  • D_the_FactMan

    Skeptically, in Sept 2013 I attended a seminar on roundabouts at the annual conference of the Tennessee Chapter of the American Planning Association. The findings were pretty much like those reported in this article and video. They are safer and much more efficient than four-way stops or stop lights. It’s important to remember, however, that roundabouts and traffic circles are not the same thing. Traffic circles are much more confusing.

  • Keith P.

    Roundabouts are the soup of the day for traffic engineers right now, but they are almost universally hated by drivers. Too tight, too much potential for collision, too many points of distraction. They are horrid.

    • Alex Ihnen

      Drivers don’t like anything that introduces uncertainty to driving. Paradoxically, this doesn’t mean that less uncertainty is more safe. The extreme is long straight stretches of highway, but we’ve widened urban streets into roads and highways, cleared all real and perceived obstacles and have produced faster traffic. This makes a place less safe for anyone other than someone in a car, decreases the value of adjacent land for small business and residential use, and produces more violent and deadly collisions. The opposite extreme is removing all signs and lane markings altogether. This has been done in some European cities with the result being a more effectively shared space resulting from uncertainty, slower speeds and the need to make eye contact with other users to navigate the space.

    • http://joshrestivo.com/ Josh Restivo

      How much actual experience have you had driving in areas that employ roundabouts? If you’re only experience has been with the half-dozen or so that have popped up in the St. Louis area, then I can understand a negative opinion. While I welcome those roundabouts, it’s a bit jarring to go from stop lights/signs to roundabouts and back again. There really needs to be a cluster of them for anyone to get comfortable with the experience.

      I’ve driven a fair amount over in the UK, both in urban (London) and more rural areas. Roundabouts are ubiquitous there. At first they were maddening, though at least in part because of the right-side driving thing, within an hour or so, you adjust. After a couple of days of driving, I didn’t want to go back to driving in the U.S. Driving across town, or across country, while only occasionally having to come to a dead stop is nice. It’s real nice.

      The pedestrian thing is addressed there in a novel way which has economic benefits that I’ve yet to see mentioned in the St. Louis conversation. Rather than crossing at corners (which becomes dangerous with larger roundabouts), there are pedestrian crosswalks a little further into the blocks. There are flashing yellow lights to warn drivers. The law is simply that you stop your car at the crosswalk if you see pedestrians preparing to cross. Pedestrians *always* have the right-of-way. If there are no pedestrians crossing or standing in the little marked waiting zone, the driver may continue through without stopping.

      The economic benefits are realized in the slight detouring of people just a little further into each block. It’s not so much that it’s inconvenient to get anywhere. In fact, I’d estimate that the only people who would scream about inconvenience are those grasping for theoretical arguments against roundabouts and who never walk anywhere anyway.

      Rather than just seeing the QuickMoco or McJackintheBell at every corner as you walk down an avenue, pedestrians get pulled just a hair further into some of the cross-streets. Suddenly there is far more viable commercial (think street-level retail) space available. This is a real, observable phenomenon.

      In short, I’d really like to hear negative opinions (and reasons) from anyone who’s actually spent time driving in a street-grid that primarily employs roundabouts. Such opinions seem to be in short supply.

  • jasomm

    This “experiment” tests ONE type of 4 way stop, with ONE type of roundabout. Does not introduce pedestrians, nor drivers that are actually trying to get anywhere. This could have (and has) been done on a computer without wasting gas for hours and still given equally as meaningless results.

    Sometimes roundabouts are a better solution, sometimes stop signs or traffic lights are a better solution. But in general, the US under-uses roundabouts when they would improve things.

  • Craig Hoolihan

    lets face it, an intersection is only as good as its drivers… and well St. Louis drivers might have the worst.

    • Adam

      i don’t know… i think everybody thinks their city has the worst drivers. i just got back from denver and was horrified by how fast people drive–basically highway speeds on city streets (granted the streets are wide).

      • Presbyterian

        One list we didn’t make:
        Allstate Ranks Cities with Worst Drivers

        10. Arlington, VA
        9. San Francisco, CA
        8. Miami, FL
        7. Alexandria, VA
        6. Philadelphia, PA
        5. Glendale, CA
        4. Hialeah, FL
        3. Providence, RI
        2. Baltimore
        1. Washington, DC

        Hands-down, mid-Atlantic drivers are the worst, with southern Florida and California getting honorable mention. My first driving lesson at 15 was to circumnavigate the capital beltway during rush hour. Yes, the worst.

        http://www.weather.com/travel/driving-scenic-drives/worst-driving-cities-2013-20130829

        • Adam

          yep. highway driving around hear is laid-back compared to beltway driving.

        • Steve Kluth

          Makes sense. When I moved here from Wisconsin, I thought drivers here were awful. Then I moved to Norfolk VA and discovered how bad driving can be. Norfolk has young military hotshots in sports cars who like to party at all hours, military wives from countries where they never drove (S Korea, Philippines), lots of retirees enjoying the moderate climate, and religious wackos who apparently think Jesus will save them from their gawd-awful driving, mixed in with the typical urban mix. DC is similar except the diplomatically immune replace the military hotshots and their wives, and political appointees replacing the religious wackos.

    • Alex Ihnen

      I actually believe the opposite is true: drivers are only as good as the built environment. Build six lane “streets” in the city and people drive too fast, not primarily because they’re bad drivers, but because the street is built to go that fast.