Decision-Making Guide for Traffic Signal Phasing (2020) / Chapter Skim
Currently Skimming:
Pages 25-39
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 25... ...
17 4 Left-Turn Phasing Mode Left-turn phasing mode is a key component of the design and operation of a traffic signal, with impacts on safety and delay for all users of an intersection. This chapter of the Guide is intended to address accommodations for left turns from two-way streets as well as U-turns to the left.
|
From page 26... ...
18 Figure 7 - Typical Phasing Diagram for Permissive-Only Phasing (Adapted from NCHRP 2015) Figure 8 - Typical Phasing Diagram for Protected-Only Phasing (Adapted from NCHRP 2015)
|
From page 27... ...
19 Figure 9 - Typical Phasing Diagram for Protected-Permissive Phasing (Adapted from NCHRP 2015) Figure 10 - Typical Phasing Diagram for Split Phasing on the Minor Street (Adapted from NCHRP 2015)
|
From page 28... ...
20 4.1.1 Signal Faces Left-turn movements are controlled by one of two types of signal face: β’ Shared signal face, where the same signal face controls the left-turn movement and another movement (usually the through movement) on the same approach.
|
From page 29... ...
21 4.2 Left-Turn Phasing Mode Considerations Typically, protected-only phasing mode is operationally less efficient than permissive-only and protectedpermissive modes in most cases because motorists cannot use available gaps to turn left. Rather, green time must be allocated to the left-turn movement, which reduces the time available for other movements, increasing delay for all movements.
|
From page 30... ...
22 each additional opposing lane, passenger car drivers need an additional 0.5 second time gap.1 Sight distance can be calculated using the following formula: πΌππ· = 1.47 β π β π‘ where: ISD = Minimum required intersection sight distance (feet) S85 = Speed (mph)
|
From page 31... ...
23 point may also be desirable if drivers tend to wait at a different point, but the left-turn signal face should be visible from any point where a measurement is made. For adequate sight distance to exist, an opposing vehicle must be continuously visible in any approach lane for the distance specified in the Green Book.
|
From page 32... ...
24 La ne C on fig ur at io n (C on tin ue d) The roadway used by vehicles departing the left-turn movement may impact the choice of left-turn phasing mode.
|
From page 33... ...
25 count from published industry state of the practice. Chapter 11 provides more information about combining the impacts of safety and operations at individual intersections.
|
From page 34... ...
26 Left-Turn Volume Le ftTu rn V ol um e Permissive-only phasing mode is efficient when the number of sufficient gaps available to make a left turn exceeds the number of vehicles making the turn. In this context, a gap is usually measured in units of seconds.
|
From page 35... ...
27 Le ftTu rn V ol um e (C on tin ue d) πΏ = π β ππ where: VLT = number of vehicles turning left during the intersection's peak hour VOT = number of opposing through vehicles during the same hour NL = number of opposing through lanes It may be desirable to include opposing right-turn volume in VOT and opposing right-turn lanes in NL where right-turn volume is heavy or where conflicts with left turns are unusually high, such as where dual right-turn lanes exist.
|
From page 36... ...
28 Left-Turn Delay Le ftTu rn D el ay Permissive-only phasing mode may result in excessive delay for left-turning vehicles even where the volume criteria above indicate that permissiveonly mode provides acceptable operations. This is particularly true when left-turning vehicles arrive nonuniformly during the peak hour.
|
From page 37... ...
29 4.2.3 Other Considerations At intersections with a high volume of pedestrians, conflicts between permissive left turns and crossing pedestrians may make protected-only left-turn phasing mode desirable. Some intersection configurations may require protected-only phasing mode for reasons not already indicated here.
|
From page 38... ...
30 β’ An approach where a significant portion of crashes occurs at a certain time of day. During highcrash times, protected-only phasing mode may be appropriate even though protected-permissive mode is used at other times.
|
From page 39... ...
31 configuration is more intuitive to drivers than an MUT because left turns are served from their typical position in the left lane. β’ Jughandle configurations typically route left-turning vehicles onto the side street, where they join the stream of side street through traffic.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.