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ID# C1040D1

Problem 4: Discussion

The decision to signalize an intersection brings with it many additional issues that also need to be addressed. Should the signal operate in a pre-timed, semi-actuated, or fully-actuated mode? Should it be coordinated with adjacent signalized intersections or should it be designed to operate as an isolated intersection? How should the left-turns be handled on each approach -- should they be permitted, protected, or both? What should be the phasing sequence? What are the appropriate settings for the various signal timing parameters, including in the case of actuated control the factors of minimum green time, maximum green time, and unit extension? The answer to each of these questions is affected by the answers to all the others, and so it is typical that multiple scenarios will need to be investigated before deciding upon a particular implementation plan.

A fairly comprehensive method is presented in Chapter 16, Appendix B of the HCM2000 for reasonably approximating traffic-actuated operation. This method takes explicit account of many phasing variables, detector design parameters, and controller settings. Its application in Problem 4 provided considerable insight into the relative merits of pre-timed versus actuated signal control at the U.S. 95/Styner-Lauder intersection.

The next problem steps away from the locale of the U.S. 95/Styner-Lauder intersection and recognizes that the U.S. 95 facility includes both urban and rural segments. In particular, Problem 5 focuses on a rural segment of U.S. 95 located south of Moscow, and identifies a type of two-lane highway analysis that is not explicitly addressed within the current version of the HCM.

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