TRAFFIC ENGINEERING COURSE

(PWE 8322)

SIGNALIZED INTERSECTIONS

Instructor: Usama Elrawy Shahdah, PhDLecture # 06

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Types of Traffic Signals

Pre-timed: Repeat a pre-set constant cycle.

Actuated:Respond to the presence of vehicles and pedestrians

Semi-actuated

Detectors placed only on the minor approach

Major approach is interrupted only if vehicle present at the minor

approach

Fully actuated

Detectors are installed at all approaches

Green time are allocated based on the incoming traffic on each approach

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Cycle length & Signal Phase

Cycle (cycle length or cycle time): time needed for a complete sequence of signal indications

Phase (signal phase):

the part of a cycle allocated to any combination of traffic movements receiving right-of-waysimultaneously during one or more time intervals,

consisting green, yellow (amber), and/or all-red

Split: a percentage of a cycle length allocated to each of the

various phases in a signal cycle

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Two-Phase Example

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Inter-green time

Change Interval and Clearance Interval

Change interval: identical to the inter-green interval (the time

between the end of a green indication for one phase and the beginning of a

green indication for another)

Clearance interval: identical to the all-red interval (the

display time of a red indication for all approaches)

All-red time

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Effective Green Time

Effective Green (g)=Green Time + Change Interval–Lost Time

Lost Time (LT)=Start-up delays

the time during a given phase in which traffic could be discharging

through the intersection, but is not. This is the period during the green

interval and change intervals that is not used by discharging traffic

Effective Red (r) = Cycle Length – Effective Green

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Capacity and Saturation Flow

Capacity: the maximum number of vehicles that can reasonably be expected to pass over a given roadway or section of roadway, in one direction, during a given time period and under the prevailing roadway, traffic, and signalization conditions.

Saturation flow: The maximum number of vehicles from a lane group that would pass through the intersection in one hour under the prevailing traffic and roadway conditions if the lane group was given a continuous green signal for that hour.

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Traffic Signal Timing Procedure

1. Develop a phase plan

2. Convert volumes to through-vehicle equivalents

3. Determine critical-lane volumes

4. Determine yellow and all-red intervals

5. Determine lost times

6. Determine the cycle length

7. Allocate effective green to each phase

8. Check pedestrian requirements

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Step 1: Develop a phase plan

Phase (signal phase): the part of a cycle allocated to any combination of

traffic movements receiving right-of-way simultaneously during one or more

time intervals, consisting green, yellow (amber), and/or all-red

National Electrical Manufacturers Association

(NEMA)

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Treatment of Left Turns

Left Turn protected phase should be considered if any of the following criteria is met:

1) More than one turning lane is provided;

2) left turn demand,

3) The cross product rule

Example

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Phase Arrows

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Two-Phase Signal

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Three-Phase Signal

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Exclusive Pedestrian Phase

Ring-and-Barrier Diagrams

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Modern U.S. practice for signal control organizes phases by

grouping them in a continuous loop (or ring) and separating the

crossing or conflicting traffic streams with time between when

they are allowed to operate, either by making the movements

sequential or adding a barrier between the movements. The ring

identifies phases that may operate one after another and are

typically conflicting phases organized in a particular order.

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Step 2: Convert volumes to through vehicle

equivalents

Through vehicle equivalents (tvu) to left

turning vehicles ELT

Through vehicle equivalents to Right

turning vehicles ERT

VRTE = VRT * ERT

VLTE = VLT * ELT

Example

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Step 3: Determine critical-lane volumes

The critical lane volumes

is the highest movement

volume in the phase,

divided by the number of

lanes.

The critical volume is the sum of

the critical lane volumes

231 or 263

Vc A= 263 tvu/h

472 or 516

Vc B=516 tvu/h

140, 351, 283, or

128

Vc C=351 tvu/hr

Vc = 263+516+351 = 1,130 tvu/h

Example

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Step 4: Determine yellow and all

red intervals

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)01.0*4.64(2

47.1 85

Ga

Sty

Change Interval (Yellow)

t = driver reaction time (sec)

a = deceleration rate (fps2)

g = grade of approach (decimal)

S85 = 85th percentile speed (mph)

Guideline for Yellow Interval

Yellow time: Minimum = 3 sec

Maximum = 5 sec

• 3 sec => 16 to 56 km/h

• 4 sec => 56 to 80 km/h

• 5 sec => greater than 80 km/hExample

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Clearance Interval (All Red): ITE

Recommended Practice

No ped. traffic

Significant

ped. traffic

r= Length of the all-re phase, second

L = length of a standard vehicle (20 ft)

w = distance from the departure stop line

to the farthest conflicting traffic lane (ft)

P = distance from the departure stop

line to the farthest conflicting ped.

crosswalk (ft)

S15 = 15th percentile speed (mph) of

approaching traffic, or speed limit

Some ped. Traffic

exists

If Yellow interval needed is

more than 5 seconds

All red must be increased to 2

sec

Example

If the average approach speed (S) is

known, we can calculate:

S15 = S -5

S85 = S + 5

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Step 5: Determination of lost times

Default values from HCM:

Start-up lost time: l1 = 2.0 s/phase

Motorist use of yellow and all-red: e = 2.0 s/phase

Lost time per phase

Clearance lost time: l2 = Y – e

Total length of change and clearance intervals: Y = y+ ar

Total lost time per phase: tL = l1 + l2

Lost time per cycle

Example

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Step 6: Determine the cycle length

A desirable cycle length using default saturation

flow rate (1615 vph)

Where: Cdes = desirable cycle length

L = total lost time per cycle, s/cycle

PHF = peak-hour factor

v/c = target v/c ratio for the critical movement in the intersection

Example

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Step 7: Allocate effective green to

each phase

Splitting the Green

Available effective green time in the cycle

Effective green time for phase i

Where:

gi = effective green time for phase I, second

gTOT= total effective green time in cycle, second

Vci=critical lane volume for phase or sub-phase i, veh/h

VC= sum of critical –lane volumes, veh/h

Example

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Step 8: Determining pedestrian Signal

requirements

Where:

Gp = min ped crossing time, sec

3.2 = pedestrian start-up time, sec

L = length of the cross walk, ft

Sp = walking speed (4 fps)

Nped = # of ped during an interval

WE = effective crosswalk width

DON’T WALK = L/Sp

If Min. Ped Grn > Grn for φ ==> Grn for φ must be increased

(also increase total cycle length)Example

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Example: Signal Timing for Intersection of Major Arterials

1

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Step 1: Develop Phase Plan

Check left-turn against the criteria of slide 10

to determine whether or not it needs to be protected.

EB: VLT=35 < 200

Xprod = 35*(500/2) = 8,750 < 50,000

No Protection needed

WB: VLT=25 < 200

Xprod = 25*(610/2) = 22,875 < 50,000

No Protection needed

NB: VLT=220 > 200

Protection needed

SB: VLT=250 >200

Protection needed

As a result, an exclusive left Turn on NB – SB

Permitted left Turn on the EB - WB

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1

1

Step 2: Convert Volumes to Through Vehicle equivalent

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231 or 263

VcA= 263 tvu/h

472 or 516

VcB=516 tvu/h

140, 351, 283, or 129

VcC=351 tvu/hr

Vc = 263+516+351 = 1,130 tvu/h

Step 3: Determination Critical Lane Volumes

129

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Step 4: Determine Yellow and All-Red intervals

S85 = Speed limit = 45 mph

The all-red interval will reflect the need to clear the full width of the

street plus the width of the far crosswalk.

Phase A ------- P =60+10=70ft

Phase B -------- P = 60+10=70ft

Phase C -------- P=55+10=65 ft

Assume Vehicle length = 20 ft

ar A,B = (70+20)/(1.47*45) = 1.4 second

ar C = (65+20)/(1.47*45) = 1.3 second

SecondCBAy 3.4)0()10*2(

45*47.10.1,,

Note: As a Speed Limit =45 mph,

there will no differentiation between

the S85 and S15.

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Step 5: Determination Of Lost Times

Remember: l1 and e are both 2.0 seconds

Y A,B= tL A,B = 4.3 + 1.4 = 5.7 second

Y C= tL C = 4.3 + 1.3 = 5.6 second

Total lost time per cycle, L, is

L = 5.7 + 5.7 + 5.6 = 17.0 second

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Step 6: Determine the Desirable Cycle Length

ondCdes sec7.109155.0

17

90.0*92.0*1650

11301

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Assuming that this is a pre-timed

signal controller, a cycle length of

110 second would be selected.

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Step 7: Allocate Effective Green to Each Phase

Effective green time for all phases:

gtot = 110 – 17 = 93 second

gA = 93*(263/1130) = 21.6 sec

gB = 93*(516/1130) = 42.5 sec

gC = 93*(351/1130) = 28.9 sec

Check:

21.6 + 42.5 + 28.9 + 17 = 110 ------- ok

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Step 8: Check Pedestrian Requirements

Note: pedestrian will be permitted to cross the E-W only during phase B

Pedestrian will cross the N-S during phase C

Pedestrian Volume = 200 peds/h ( for moderate activity)

Number of cycles per hour = 3600/110 = 32.7 cycles/h

Nped = 200/32.7 = 6.1 peds/cycle.

Required pedestrian green times are:

Gp B = 3.2 + (60/4) + (0.27*6.1) = 19.8 second

Gp C = 3.2 + (55/4) + (0.27*6.1) = 18.6 second

Compare with sum of ( green, yellow, and all-red times), we get:

Gp B = 19.8 s < GB +YB = 42.5 + 5.7 = 48.2 s ----- ok

Gp C = 18.6 s < GC + YC = 28.9 + 5.6 = 34.5 s ----- ok

Therefore, no changes to the vehicular signal timing are required to accommodate pedestrian safely

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For major arterial crossings, pedestrians signal would normally be provided.

During phase A,

all pedestrian signals would indicate “DON’t WALK”.

During phase B,

the pedestrian clearance interval (the flashing DON’tWalk) would be L/Sp or 60/4 = 15.0s

The WALK interval is whatever time is left in G+Y, counting from the end of Y: 48.2 – 15 = 33.2s

During phase C

L/Sp is 55/4 = 13.8s,

WALK interval = 34.5 – 13.8 = 20.7s

Step 8: Check Pedestrian Requirements

Home Reading35

"TRAFFIC SIGNAL TIMING MANUAL", FHWA,

Publication Number: FHWA-HOP-08-024

Available online at:

http://ops.fhwa.dot.gov/publications/fhwahop08024/

fhwa_hop_08_024.pdf

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