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Roundabouts
 
 

 
 

Background 
 
The Oregon Department of Transportation, local jurisdictions and consultants continue to look for traffic control methods that provide ways to safety manage the flow of traffic, bicycles and pedestrians through intersections large and small. Roundabouts are one such method. 

Research 
 
ODOT's Research Unit has published the report "Modern Roundabouts for Oregon" (#98-SRS-522).

  • This report reviews current research and practice on modern roundabouts, both in the US and other countries.
  • The report compares the advantages and disadvantages of roundabouts, summarizes safety implications, and discusses pedestrian and bicyclist considerations.
  • Guidelines from other states on the geometric design of roundabouts are reviewed, as are studies and formulas used to evaluate roundabout performance, measuring capacity and delay. Software models for roundabouts are reviewed.
  • The report makes recommendations for considering and using roundabouts in Oregon, as well as identifying further research needs in this area.

Roundabouts in Brief 
 
The roundabout concept was first invented in the early 1900’s and deployed throughout Europe and America. During the 1950’s there was a loss of confidence in roundabouts, due mainly to the problem of traffic locking and the increasing number of accidents. Many were replaced by traffic signals.

In 1966, the offside priority rule (an entering vehicle gives way to vehicles in the roundabout) and the yield at entry operation enhanced roundabout capacity and safety performance. The success of the modern roundabout provoked renewed interest in roundabouts worldwide. 

Roundabout Images

Pictures

View photos of before and after pictures of the roundabout at Bend, Oregon.

Video Clips

The Federal Highway Administration has produced two informational videos about the use of roundabouts.


Presentations 

"Roundabouts: A Safer Choice" presentation from the 2010 Oregon Transportation Safety Conference.

Resources
 
Here are a few related sites and resources that may be of interest:

General Information and Driving Instructions (pdf)
 
Modern Roundabouts - Research Report (pdf)
 
New York's Roundabout Site

Single Roundabout Calculator (xlsx) - The Single Lane Roundabout Calculator employs HCM 2010 methodology. For each approach, the roundabout calculator produces conflict flow, entry flow, capacity, pedestrian impedance factor, v/c ratio, control delay, Level Of Service (LOS), and 95th percentile queue. Intersection delay and LOS are also calculated. Tabs are in order of use: notes, inputs, single lane, bypass lane, 2nd bypass lane, 3rd bypass lane, and 4th bypass lane. The Analysis Procedures Manual version 1 provides procedures for roundabout analysis in Chapter 7.