By Bryant Ficek, PE, PTOE
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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