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21st Century Operations Using 21st Century Technologies

Traffic Signal Timing Manual

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This publication is an archived publication and replaced with the Signal Timing Manual - Second Edition.


Example 3-1

Long Description

The following information was provided in Example 3-1:

Example 3-1: Critical Movement Analysis

Step 1: Lane configuration, phasing, and volumes:

The figure shown here illustrates the lane configuration and desired phasing in this example. The East and West approaches each have three lanes: a dedicated left turn lane, a through lane, and a right turn lane with permitted through movement. The North and South approaches each have two lanes: a dedicated left turn lane, and a through and right turn lane. All left turns are intended for protected left turn phases. The corresponding turning movement volumes for each lane group are shown in the figure to the right.

Steps 2, 3, and 4:

Check east-west critical lane volumes to determine the pair of consecutive movements that requires the most time:

EB LT + WB TH/RT = 120 + (360 + 110)/2 = 120 + 235 = 355
WB LT + EB TH/RT = 170 + (690 + 280)/2 = 170 + 485 = 655 ← critical

Check north-south critical lane volumes to determine the pair of consecutive movements that requires the most time:

NB LT + SB TH/RT = 80 + 400 = 480 ← critical
SB LT + NB TH/RT = 120 + 210 = 330

Identify critical movements: WB LT, EB TH/RT, NB LT, SB TH/RT

Determine the sum of critical movement volumes: CS = 655 + 480 = 1135

Steps 5, 6, and 7:

Determine the reference sum (capacity): RS = 1530 × PHF × fa = 1530 × 1.00 × 1.00 = 1530

PHF = Peak Hour Factor (assumed to be 1.00 in this example)

Adjustment factor for area type, fa = 0.90 for Central Business District, 1.00 for other (assumed to be “other” in this example)

Identify cycle length: C = 120 s (assumed)

Determine total lost time: L = 4 critical phases × 4 s per phase = 16 s

Calculate and assess intersection volume-to-capacity ratio:

Xcm = ( CS / ( RS( 1 - (L / C) )) ) = ( 1135 / ( 1530( 1 - (16 / 120) )) ) = 0.86 → Near capacity