Why Build Roundabouts?

Most people fall into one of two camps regarding roundabouts – they love them or hate them. In many cases though, the dislike of roundabouts can be traced back to misconceptions and images of old rotaries or traffic circles.

The modern roundabout is distinguishable from other types of circular intersection primarily by two key features:

  • Speed – a modern roundabout uses its smaller size and roadway curvature to keep vehicle speeds around 20-25 mph through the roundabout.
  • Yield at Entry – drivers in the circle have the right-of-way and any motorists approaching the roundabout must yield to those drivers.

roundabout_design2Besides confusing modern roundabouts with other circular intersection types, fear of the unknown or of change can also create a skeptical crowd when a roundabout is first proposed. But roundabouts will continue to be proposed and will have more widespread use because they work.

A few benefits of roundabouts include:

  • Safety. One-way operation around the central island eliminates T-bone and head-on crashes that can be common at traditional intersections. Slower speeds also mean that a crash, if it occurs, will result in fewer and less severe injuries. Compared to traffic signals, the research is indicating roundabouts can lead to about a 90% reduction in fatalities and a 75% reduction in crashes with injuries. The safety benefits are amazing, particularly with single-lane roundabouts.
  • Efficiency. A roundabout does not require a driver to stop unless there is traffic already in the circulating lanes. Theoretically, four motorists arriving at a roundabout from different directions at the same time could each continue without stopping or impacting the travel of the others. Four drivers from four different directions could never go at the same time under stop sign or traffic signal control. Thus, with the same amount of traffic, a roundabout can usually handle traffic with less delay compared to a stop sign or traffic signal controlled intersection.
  • Environmentally Friendly. Ever had to wait at a red light at night with no other traffic around? That doesn’t happen at a roundabout with its yield at entry control. A driver enters the roundabout when there’s a gap in traffic and they don’t have to wait for a light to turn green or have to come to a complete stop if there’s no other traffic. Roundabouts also need less pavement than traditional intersections, which leads to an improvement for water run-off.

Research suggests a single lane roundabout can accommodate up to 25,000 vehicles per day. A multi-lane roundabout with two entry lanes can accommodate up to 45,000 vehicles per day. Like other traffic engineering tools, a roundabout works better with certain characteristics.  The ideal conditions for a roundabout are:

  • Balanced traffic flow between all four legs. An unbalanced intersection with 90 percent of the volume on the major street may not see the same benefits as other more balanced locations.
  • High left turn movements. Left turns, and U-turns, are accommodated extremely well by a roundabout.
  • High crash history. Roundabouts can help reduce the severity of crashes and sometimes the overall number of crashes.
  • Complex geometry. A roundabout can often accommodate more than four legs and/or skewed intersections better than stop signs or a traffic signal.

Bicycle/Pedestrian Movements at Roundabouts

As with other types of intersections, pedestrians are provided crosswalks across the driving lanes. The similarities stop there as roundabouts have key differences from traditional intersection. The crosswalk is set back from the yield line so drivers can focus on pedestrians before focusing on other cars and entering the roundabout. A pedestrian crosses one direction of traffic at a time. In comparison to some intersections with multiple turn lanes, a pedestrian will only need to cross one or two lanes of traffic at a time before reaching that refuge spot. Slower vehicle speeds also improve the walk ability of a roundabout.

It should be noted that roundabouts are more difficult to cross for pedestrians who are blind or have low vision.  Pedestrians with visual-impairments have more difficulty with wayfinding, alignment, and gap/yield detection.  The Public Rights-of-Way Accessibility Guidelines (PROWAG) provide minimum standards to accommodate pedestrians.  However, research is still ongoing in terms of additional needs to assist visually-impaired pedestrians cross at a roundabout.

Bicyclists have the option of traveling through a roundabout as a pedestrian or as a car. The slower vehicle speeds are at the approximate speed of a bicyclist.  Bike ramps are typically provided before and after the roundabout, to allow bicyclists the option of using the crosswalks if they aren’t comfortable driving on the road within the roundabout.

Transit at Roundabouts

If the roundabout is designed with a tractor-trailer in mind, a typical transit bus should have no difficulty travelling through a roundabout.  If larger articulated buses are expected, their movements should also be checked during the design phase to preferably keep the bus off the truck apron for passenger comfort.  Crosswalk locations may need to be set back further from the roundabout to avoid a bus blocking the crossing while waiting to enter the roundabout.

Rail transit has also been successfully incorporated into roundabouts.  Typically, the train will run through the medians and directly through the center island.  Vehicle and pedestrian operations will either fully stop when a train goes through or only operate on a partial basis.

Stops or stations should be located outside the roundabout to avoid disruption to the overall operations.  Locations should consider things like the ability of a bus to merge back into the traffic stream and the connections to the sidewalk/trail system.

Single Lane versus Multi-Lane Roundabouts

Single lane roundabouts are relatively straight-forward for drivers to understand and travel through to their destination. Multi-lane roundabouts add another dynamic with lane choice. The two most common crashes at roundabouts are failure to yield and improper lane use, which are both more frequent at multi-lane roundabout.

The decision to provide a multi-lane roundabout instead of a single lane roundabout should be based on an operational analysis using current or near term volumes. Using long-term volumes (20+ year forecasts) can result in a larger roundabout than needed and lead to safety issues. A roundabout should be designed as small as needed for the near term. It is better to have a single lane roundabout and upgrade to multi-lane in five to ten years rather than providing a larger roundabout immediately. In most cases, the upgrade of a roundabout can be planned with the initial construction to minimize the cost and disruption of future expansion.

If a multi-lane roundabout is needed, public outreach is often necessary to reach motorists before they have to drive or walk through the roundabout. The key driving points to express are selecting the proper lane before entering the roundabout and yielding to both lanes. Signing and striping of the roundabout is also very important to re-enforce the public outreach and reduce potential issues associated with a multi-lane roundabout.

Design Guidelines for Roundabouts

The design of a roundabout is more complicated than a traditional intersection, as the vehicle speeds need to be controlled, entry paths can have subtle variations, signing and striping is very important in terms of effective operation, and safely accommodating bicyclists and pedestrians can be tricky.  In addition to making sure large trucks can safely and effectively more through the intersection, roundabouts are typically designed by engineers who specialize in them.

Rather than trying to summarize those design guidelines here, each state’s specific rules and regulations, as well as the national roundabout manuals, should be reviewed and understood. Many State DOTs also have a list of pre-qualified roundabout designers that have attained the appropriate level of experience and education to properly design roundabouts.

Drawbacks of Roundabouts

There are no silver bullets in transportation planning and engineering.  Roundabouts are a better choice than traffic signals at most locations, however there are a few limitations the design engineer should consider:

  • There may not be enough room to build the roundabout.
  • A corridor with a heavy commuter pattern (most vehicles going one direction in the morning and then returning in the evening) can sometimes provide less delay with coordinated traffic signals.
  • The overall distance pedestrians need to walk is often longer around a roundabout than a traffic signal controlled intersection.
  • More difficult for visually-impaired pedestrians to cross compared to traffic signals.
  • Roundabouts are static. Traffic signals can adapt to significantly different traffic patterns, like traffic letting out after a concert or football game at a stadium.
  • Multi-lane roundabouts can be difficult in terms of design and operation. The delay and safety experience at a multi-lane roundabout is sensitive to small geometric characteristics.

Resources

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One thought on “Why Build Roundabouts?

  1. “Speed – a modern roundabout uses its smaller size and roadway curvature to keep vehicle speeds around 20-25 mph through the roundabout.”

    Based on AASHTO’s minimum radii for low speed urban street design the required vehicle path curve would range from about 100 ft at 20 mph to 200 ft at 25 mph assuming a 2% negative cross slope for drainage of the roundabout. If the intersection were warped to drain to the center of the roundabout the radii could be reduced to 90 ft and 170 ft respectively.

    In either case the intersection area required would be significantly to enormously larger than that available with 60 ft ROW’s on the approach legs for the two lane by two lane situation posited in this paper.

    At a speed of 15 mph through the roundabout the minimum vehicle path radius would be 50 ft for negative cross slope and 44 ft for positive cross slope. The latter condition could be contained in the typical ROW envelope.

    To sum up I would suggest a little more skepticism towards this particular silver bullet, especially in urban settings.