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Roundabout Control Delay

Joe Olson
City of Fort Collins
Roundabout Control Delay was posted on March 19th, 2012 at 2:52 PM EDT
regarding Chapter 21: Roundabouts

The HCM defines control delay as "the delay brought about by the presence of a traffic control device. Control delay includes delay associated with vehicles slowing in advance of an intersection, the time spent stopped on an intersection approach, the time spent as vehicles move up in the queue, and the time needed for vehicles to accelerate to their desired speed."

The HCM also notes other types of delay including geometric delay: Delay caused by geometric features causing vehicles to reduce their speed in negotiating a system element (such as delay caused from negotiating a roundabout). NCHRP 672 further discusses geometric delay at roundabouts and confirms that geometric delay is not considered as part of control delay at roundabouts.

Could someone provide clarification on this for me? This has implications when comparing a roundabout alternative to other intersection control alternatives i.e. 2-way STOP or signal. It seems that a more useful term would be "intersection delay" which would be defined as time lost negotiating an intersection compared to if the intersection did not exist and flow was not interrupted. This would allow for more of an "apples to apples" comparison of roundabouts to other forms of control.

Erik Ruehr
VRPA Technologies
RE: Roundabout Control Delay was posted on May 28th, 2012 at 9:24 PM EDT

I agree that "intersection delay" or some other term that would provide an "apples to apples" comparison would be useful. For now, the only way to get that kind of an answer would be for the user to calculate the geometric delay for different alternatives and add it to the control delay. This is probably something to consider for future improvements to the HCM.

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