Roundabouts Are Not a Silver Bullet

How to Know When a Roundabout Is Not the Right Design

Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.

Let’s start with the fact that roundabouts do indeed have many great characteristics and benefits. It’s undeniable that roundabouts can do many positive things.

The Benefits of a Roundabout

  • Increase traffic safety
    • Numerous studies have confirmed a significant decrease in injury crashes, particularly fatal and severe, incapacitating injury crashes. Roundabouts, in general, have less conflict points and slower speeds that lead to this decline. A December 2014 study by the Minnesota Department of Transportation concluded that roundabouts reduced all crashes by over 30 percent, with a decline in injury crashes by over 65 percent and a decline in the most severe injury crashes (including fatal crashes) by over 82 percent.
  • Decrease vehicle delays
    • When operating within their capacity, roundabouts will typically have lower overall delay compared to traffic signal or all-way stop control. Or, to reverse the thought, at the same Level of Service, roundabouts will accommodate more traffic compared to a traffic signal or all-way stop control. One way we like to think about this is to envision the typical four-legged intersection. Under a traffic signal or all-way stop control, the different approaches and movements must take turns to travel through the intersection. At a roundabout, a vehicle on each approach leg could enter the intersection and complete their movement at the same time.
  • Provide a calming effect
    • By physically slowing vehicles, roundabouts can provide a good transition between high speed and low speed areas or help to slow vehicle speeds on a corridor.
  • Deliver environmental benefits
    • Roundabouts can provide environmental benefits by reducing, and in some cases, eliminating vehicle stops. Less stopping means less vehicle idling, which directly translates into fewer air emissions and less fuel consumption.

I’m sure others can add to this basic list of benefits and we have also written positively about roundabouts in the past.

Despite these clear advantages for a roundabout, there are some issues to be aware of.

Disadvantages of a Roundabout

  • Yield Confusion
    • Dual lane roundabouts, with two circulating lanes all the way around, do not have the same safety record as single-lane roundabouts. Although the injury crashes decrease, property damage crashes (non-injury) often increase due to confusion about yielding upon entry and exact vehicle path for each turning movement. In some cases, the increase in property damage crashes can result in an increase in the overall number of total crashes at a site.
  • Unbalanced Traffic Flow
    • Unbalanced traffic flow between the approaches can lead to a disrespect for the yield upon entry rules. If 90 percent of the traffic is north-south and usually never stops for an east-west vehicle, then drivers quickly learn to never stop even if that rare east-west car appears.
  • Bad Weather Navigation
    • High vertical grades on the roundabout entries can make the approach difficult to navigate, particularly in areas prone to lots of snow and ice. Stopping a passenger or truck in rough weather conditions is tough before adding in a slight curve in the road.
  • Higher Right-of-Way Space Needed
    • Roundabouts need more right-of-way at the intersections. The diameter of a roundabout can be up to 150 feet for a single-lane and 200 feet or more for a dual-lane. Typical right-of-way at an intersection may be as low as a 60-foot by 60-foot square for a local road up to a 120-foot by 120-foot square for an expressway. More room on the corners is likely necessary.
  • Emergency Vehicle Priority
    • No priority to emergency vehicles. Roundabouts give equal treatment to every approach. For emergency vehicles, their priority rests on drivers getting out of and staying out of the way.
  • Difficult Pedestrian Crossing
    • Difficult to cross for pedestrian with vision impairments. Individuals with low vision or that are blind will have difficulty determining when traffic is yielding to them to cross.
  • Vehicle Progression Disruption
    • The progression of vehicles on a through corridor can be degraded. Similar to emergency vehicles, the design of roundabouts does not provide an inherent priority for a through route over a side street. If placed within a coordinated signal corridor, roundabouts could disrupt the platoon of vehicles from one intersection to the next.

This doesn’t mean we don’t like roundabouts or won’t recommend them in our work. Instead, we want everyone to recognize the full scope of pros and cons that roundabouts bring. With proper analysis, planning and design, and education for the public (particularly in areas where roundabout will be new), roundabouts have been and will continue to be highly successful in helping to keep people moving safely and efficiently. Arguably, roundabouts should be the first option considered for intersection control. But each case and intersection needs to be examined on its own merits. And don’t think blindly installing roundabouts everywhere is the easy answer to solve all our congestion issues.

NCHRP Report 672, Roundabouts: An Informational Guide, Second Edition, was referenced in developing this article.

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3 thoughts on “Roundabouts Are Not a Silver Bullet

  1. So, just where is that roundabout shown on the article? I can’t imagine how it could be more horribly designed – ok, that’s a little strong but really? Could we not have picked a more modern modern roundabout? What decade was that from?

    As for your article, I wrote a similar one back when I was getting my Ph.D. in the obvious -ok that’s a little strong, too but what I think we need to do is to encourage sustainable solutions and help those who continue to install signals when a roundabout either would do or do a better job – learn from their past.

    The only real advantage to stopping at stop lights is the ability to fire off a text or two. Corridors that need platooned flow and huge volumes also need the lights but sooo many signals are being installed or replaced in kind as they age with more signals. We’re still leaving many opportunities on the table.

    We can do more to get roundabouts installed but burning up print and media attention with an article stating that roundabouts aren’t always the answer simply isn’t helping as much as other topics like – how to use the advocate model to get more roundabouts in your areas where they make sense.

    If you would like me to co-author that topic with you, feel free to contact me. We’ll add some value

    • Hi DJ – thanks for the comment and nice catch on the original roundabout photo. We didn’t scrutinize it enough before posting and it has been replaced. The current one is a much better representation of a modern roundabout, courtesy of Joe Gustafson at Washington County in Minnesota.

      Modern roundabouts have been a key development in traffic engineering over my career with an obvious advantage in improving vehicle safety. My first experience was the analysis of and final design of a multi-lane roundabout at an interchange’s intersections back in 2006. Our understanding and design has continued to improve since that time.

      Though extremely valuable, roundabouts are a tool in the toolbox. Limited access intersection, separated left turn movements, traffic signals, and other alternative designs like hamburgers, continuous ‘T’s, and parallel flow intersections can all have a place on the system (see FHWA’s Alternative Intersections/Interchanges: Informational Report (AIIR), dated April 2010 for those who need a primer).

      I agree that too many older signals are replaced with another signal because “it’s already there.” But the flip side is also true – we should not replace every signal with a roundabout based on the generic comparison of roundabouts to signals. At least an informal review of pros/cons should be undertaken if not a more detailed study. As you stated, let’s get roundabouts installed “where they make sense.”

      I would love to do a follow-up article with you on this topic. Let’s connect outside this blog and make it happen.
      -BJF

  2. Hi Mike,

    One of the better advantages of roundabouts versus a signal intersection is that maintenance costs are lower. Also, as traffic assessments tend to focus on peak hours, and intersection that reaches capacity as a roundabout during peaks can justifiably be left as is, since signals would only be required in capacity terms for maybe 2 hours each day.

    A signal intersection might be more compact at the intersection itself but the approaches often need more width than a roundabout.

    In terms of safety issues due to yield confusion, this can be clarified/rectified by using spiral lane markings in the circulatory carriageway to separate out the turning movements (a similar concept is the turbo roundabout). This can also help address unbalanced flow as vehicles are in the correct approach lane for their destination and do not need to change lanes in the roundabout.

    If traffic flows are unbalanced to the point where the roundabout stops working, partial signalling of the intersection works well. This can often be done on a part time basis (however this isn’t ideal). I’m n the UK, fully signalled roundabouts are generally the highest capacity intersection type before grade separation.

    One more thing, roundabouts should never be placed in a signal corridor add the traffic platoons from a signal junction undermine the performance of the roundabout.