Traffic circles are raised islands, placed in intersections, around which traffic circulates. They are good for calming intersections, especially within neighborhoods, where large vehicle traffic is not a major concern but speeds, volumes, and safety are problems.

Advantages:

• Traffic Circles are very effective in moderating speeds and improving safety
• If designed well, they can have positive aesthetic value
• Placed at an intersection, they can calm two streets at once

Disadvantages:

• They are difficult for large vehicles (such as fire trucks) to circumnavigate
• They must be designed so that the circulating lane does not encroach on the crosswalks
• They may require the elimination of some on-street parking
• Landscaping must be maintained, either by the residents or by the municipality

Effectiveness:

• Average of 11% decrease in the 85th percentile travel speeds, or from an average of 34.1 to 30.2 miles per hour (from a sample of 45 sites)
• Including a large sample from Seattle, an average of 73% decrease in accidents, or from an average of 2.2 to 0.6 accidents per year (from a sample of 130 sites)
• Excluding the large sample from Seattle, an average of 29% decrease in accidents, or from an average of 5.9 to 4.2 accidents per year (from a sample of 17 sites)

Similar Measures:

• By placing a raised island in a midblock location, you have a Center Island Narrowing
• By enlarging the intersection and the center island, inserting splitter islands at each approach, setting back the crosswalks away from the circulating lane, and implementing yield control at all approaches, you have a Roundabout

Cost Estimate(s):

• Varies by materials used and the amount of area covered