02/08/12

Traffic Circles and Roundabouts: They’re not the same thing…. Or are they?

From Steve Clark, Bicycling and Walking Program Manager

At a recent Saint Paul City Council meeting there was discussion about the safety of traffic circles, as part of the Council’s consideration of a bicycle boulevard on Griggs Street in Saint Paul. One council member had heard they were not very safe, and another suggested that he might be confusing traffic circles with roundabouts, which are much larger and found on busier streets.

Both council members were right, and both were wrong. If there is anything clear about this issue, it’s that there is much confusion. My hope is to reduce the confusion, but rest assured, I’ll be leaving plenty of room for disagreement!

Traffic circle in Seattle

Traffic circle in Seattle

This long and technical blog can be reduced to three statements:1) Well-designed modern roundabouts are extremely safe, 2) older style rotaries, or large traffic circles, were not very safe and, 3) residential traffic circles (the type that were approved as part of the Griggs Bikeway in Saint Paul) more closely resemble the modern roundabout than the older traditional large traffic circle. Hence, the residential traffic circle is also very safe.

Okay, now for nuanced, technical discussion: Some dictionaries do indeed claim that traffic circles, roundabouts, and rotaries all mean the same thing. But it is now generally accepted by U.S. traffic engineers that in a roundabout, entering traffic must always yield to traffic already in the circle, whereas in a traffic circle entering traffic is either controlled by stop signs or is not formally controlled. And in the case of the large traffic circles (the ones without a good safety record), vehicles could enter the circle without slowing down and, in many states, were given the right of way over traffic already in the circle.

The good news is those large traffic circles that were proven to be unsafe and quite confusing are no longer being built. The bad news is that their safety record was so poor that only recently has the public been receptive to the modern roundabout. In fact, the modern roundabout is now recommended by the Federal Highway Administration (FHWA) as a proven safety countermeasure, having been found to significantly reduce crashes and the severity of crashes for all road users.

Why the Safety Benefits

Modern roundabouts achieve their safety benefits by 1) managing speed (or “calming” traffic) and 2) preventing serious “T-boning” collisions. But to achieve these benefits, they need to be properly designed.

The keys to all well-designed roundabouts are the following:

  • Provide slow entry speeds and consistent speeds through the roundabout by using deflection
  • Provide smooth channelization that is intuitive to drivers
  • Provide adequate accommodation for the design vehicles
  • Design to meet the needs of pedestrians and bicyclists
  • Provide appropriate sight distance and visibility.

 

Traffic circle in Seattle

FHWA illustration of traffic circle with bike/ped

Now let’s take a look at the different types of modern roundabouts:

1) Residential Traffic Circle

These have been implemented in Minneapolis on several bicycle boulevard projects and have now been approved by Saint Paul City Council for the Griggs Street Bikeway. They are not new to either city, however, as neighborhoods have requested them for several decades. Like a modern roundabout, they require approaching traffic to enter at a slow speed and yield to any vehicle (including bicyclists) already in the circle. The intent is to keep traffic flowing in a counter-clockwise direction. Unlike a larger roundabout, the raised circle in the middle is relatively small, typically no more than 16’ – 24’ in diameter for residential roads 25’ – 36’ in width. The residential traffic circle is designed to be just large enough to force the motorist to travel beyond the adjacent curb line, which ensures a lower appropriate speed for navigating the intersection, but not necessarily a full stop. Seattle is perhaps the US city most famous for these traffic calming devices, having installed more than a thousand of them, while documenting a crash reduction of more than 70 percent!

2) Single-lane Roundabout

This is a type of circular intersection, designed to ensure low-speeds, reduce conflicts, and create predictable behavior through channelization. Single lane roundabouts are used in both rural and urban settings, on high speed and low speed roads, and are especially effective in creating a transition area that moves traffic from a high speed to a low speed environment. While they have been found to be much safer than either a signalized intersection or a 2-way-stop control situation, they are on par with an intersection where everyone has to come to a complete stop. However, because they keep everyone moving, they are superior to a 4-way stop in terms of operations. Bicyclists in particular benefit from roundabouts (including the residential traffic circle type described above) because they don’t have to expend the extra energy required when starting out from a complete stop.

3) Multi-lane Roundabout

At TLC we believe it is important to distinguish between the single lane roundabouts and multi-lane ones. Multi-lane roundabouts are more difficult to make work well for pedestrians and bicyclists and also are more confusing for motorists. Often an engineer will assume that because a street has 4 lanes of traffic, a multi-lane roundabout is required. Some of the most successful roundabouts in the nation, however, have been done in conjunction with “road diets,” where a 4-lane road is reduced to 2 lanes, but operates even more efficiently (in terms of traffic flow) than the prior configuration. Why? Because the single lane roundabout allows all traffic to keep moving, albeit at slow speeds. This is superior to the long delays experienced at a signalized intersection. Streets fail at intersections, not midblock.

If a multilane roundabout is built, it is better to encourage cyclists to blend with traffic, by taking the lane, rather than providing a separate bike lane. With design speed at 15-20 mph, taking the lane does not require a heroic effort and is consistent with traffic law. The need to ‘take the lane’ is also true for single lane roundabouts.

4) Mini-Roundabout

It may be that the differences between residential traffic circles and mini-roundabouts are so minor that we’ll someday drop the distinction entirely. But the official word from the FHWA is that mini-roundabouts are rare in the United States and cost more than the types of residential traffic circles we’ve been building here. The main difference is that mini-roundabouts will employ splitter islands as vehicles approach the roundabout, which is deemed unnecessary for residential streets with far fewer vehicles. Indeed, the FHWA says that mini-roundabouts are suitable for some arterials. Mini-roundabouts are basically designs that follow all of the principles of the regular roundabouts but “cheat” by using mountable circles in the center for larger vehicles that otherwise might not be able to make the turns. (Note: mountable circles are also frequently used in residential traffic circles.) Mini-roundabouts do not require as much right of way (as their big brother) and so they can often simply be “dropped in place” using the pavement already available at a conventional signalized intersection. This is also true for residential traffic circles.

Again, because they are used in areas with more traffic, mini-roundabouts are typically designed with splitter islands at all the approaches to help channel traffic smoothly into the roundabout. These splitter islands also provide a refuge area for pedestrians to cross. So here is what I propose: let’s start calling residential traffic circles as “mini-roundabouts sans splitter islands.” Okay, maybe not.

Growing acceptance for roundabouts

A 2007 survey among various municipalities revealed that prior to the construction of a roundabout public support ranged from 22% to 44%. Several years after construction of roundabouts, the support increased to the range of 57% to 87%.

The smaller residential traffic circles appear to have an even higher rate of acceptance, and once installed, popularity increases, causing pressure on a city to install more. Again, the City of Seattle is the best example of this, and due to the huge demand, now conducts a rigorous application process to make sure intersections with the greatest need (worst safety record) receive the funds for the transformation.

Traffic circle on 5th St NE in Minneapolis

Check out the new traffic circles on the 5th St. NE bicycle boulevard in Minneapolis. Later this year, ride the Griggs Street Bikeway for our region’s premier bicycle boulevard featuring traffic circles.

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Comments

Not Convinced

I'm not convinced that neighborhood traffic circles are in the same class as modern roundabouts (mini or otherwise). Slowing auto traffic to 20 mph or less is key to the safety of modern roundabouts and in Portland many cyclists complain that motorist generally speed up to go around cyclists when approaching a neighborhood traffic circle. Portland has also found that the slowing effect for such circles is limited to about 100 ft from the intersection. In terms of traffic calming, speed humps are much more effective and can be spaced along a street without regard to intersection location. Since most neighborhood streets don't have too many crashes to begin with, the crash reduction arguement breaks down rather quickly.