Star Sign up now to gain access to all the features of this website. NEW — access is now open to ALL USERS! — Already registered? Please log in.

Phase duration based on ped considerations: which equation to use?

Steven Lavrenz
Wayne State University
Phase duration based on ped considerations: which equation to use? was posted on November 16th, 2019 at 2:13 PM EST
regarding Chapter 31: Signalized Intersections: Supplemental

In chapter 31, the HCM provides 3 different equations to calculate phase duration based on pedestrian demands. Equation 3-70 uses a minimum perception-reaction time + time to cross the road, while equations 3-71 and 3-72 calculate a phase duration based on "cyclic" pedestrian demand.

The section (page 31-33) is silent on when to use 3-70 vs. 3-71/72...is it that both of them should always be applied? A quick calculation shows that equations 3-71 and 3-72 will return phase durations longer than 3-70 when ped demand is greater than about 14 peds per cycle...why isn't this information included somewhere?

Some guidance/clarification around how these equations are supposed to interact would be very helpful.

(Edited July 16th, 2020 at 1:31 PM EDT)
Jim Bonneson
RE: Phase duration based on ped considerations: which equation to use? was posted on November 25th, 2019 at 1:28 PM EST

This comment is directed to the section titled, Determining Phase Duration on the Basis of Pedestrian Considerations in Chapter 31. At the start of this section, it states “Two pedestrian considerations are addressed in this subsection as they relate to pretimed phase duration. ... When available, local guidelines or practice should be used to establish phase duration on the basis of pedestrian considerations.” The two considerations are identified as:

  1. the minimum green time a pedestrian needs to perceive the signal indication and traverse the crosswalk.
  2. the minimum green time required to serve pedestrian demand.

For each consideration, the HCM then offers an equation for computing the minimum green interval duration.

The quoted guidance defers the reader to consult local guidelines or practice to determine whether one consideration or both considerations should be used for a given intersection. If this local preference is not available, then the reader can compute the minimum green time associated with each consideration and use the longer of the two values as the minimum green interval.

(Edited July 25th, 2020 at 1:32 PM EDT)
Steven Lavrenz
Wayne State University
RE: Phase duration based on ped considerations: which equation to use? was posted on November 25th, 2019 at 1:41 PM EST

Thanks, Jim...so it sounds like it's on the local agency to adopt a policy one way vs. the other. Your explanation would be make a great addition to the text of the HCM clarifying this point.

(Edited July 25th, 2020 at 1:32 PM EDT)

Sign in to add a reply. Don't have an account? No problem, sign up for free.