Introduction to

Signal Operation

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Timing Concepts/Terms
• Interval: A period of time during which all signal indications
(greens, yellows, reds) remain the same for all approaches.
• Indication: The illumination of one or more signal lenses
(greens, yellows, reds) indicating an allowed or prohibited
traffic movement.
• Cycle: One complete sequence (for all approaches) of signal
indications (greens, yellows, reds).
• Cycle Length: The total time for the signal to complete one
cycle (given the symbol C and usually expressed in seconds).

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Timing Concepts/Terms
• Green Time: The amount of time within a cycle for which a movement or
combination of movements receives a green indication (the illumination of a
signal lens). This is expressed in seconds and given the symbol G.
• Yellow Time: The amount of time within a cycle for which a movement or
combination of movements receives a yellow indication. This is expressed in
seconds and given the symbol Y. This time is referred to as the change interval, as
it alerts drivers that the signal indication is about to change from green to red.
• Red Time: The amount of time within a cycle for which a movement or
combination of movements receives a red indication. This is expressed in seconds
and given the symbol R.
• All-Red Time: The time within a cycle in which all approaches have a red
indication (expressed in seconds and given the symbol AR). This time is referred to
as the clearance interval, because it allows vehicles that might have entered at
the end of the yellow interval to clear the intersection before the green phase
starts for the next conflicting movement(s).

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 Timing Concepts/Terms
•Phase: The sum of the displayed green, yellow, and red times for a
specific movement defined within the signal control logic.
•Timing Stage: Consists of the time during which the same combination
of movements are moving simultaneously. The sum of the timing stage
lengths (in seconds) is the cycle length.

Timing Stage

Phase Phase

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Timing Concepts/Terms
• Protected movement: A movement that has the right-of-way and does not
need to yield to conflicting movements, such as opposing vehicle traffic or
pedestrians.
• Through movements, which are always protected, are given a green full circle
indication (or in some geometric configurations, a green arrow pointing up).
• Left- or right-turn movements that are protected are given a green arrow
indication (pointing either left or right).

• Permitted movement: A movement that must yield to opposing traffic flow

or a conflicting pedestrian movement. This movement is made during gaps
(time headways) in opposing traffic and conflicting pedestrian movements.
• Left- or right-turn movements with a green full circle indication (or flashing yellow
arrow for left) are permitted movements. Left-turning vehicles in this situation
must wait for gaps in the opposing through and right-turning traffic before making
their turns. Right-turning vehicles must yield to pedestrians in the adjacent
crosswalk before making their turns.

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Typical Controller
Operation Modes
• Pre-timed
• Semi-actuated
• Fully-actuated

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Pre-timed Control
• Cycle length, timing stages, interval lengths all fixed for
specific time periods
• Good choice when traffic demands are fairly constant
• Good choice for coordinated systems
◦ when progression is needed on more than one phase
◦ when early return of coordinated phases would interfere with
progression

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Semi-actuated Control
• Services minor approaches only when demand
exists
• Need detection on minor street approaches
• Cycle length and timing stage lengths variable
• Good choice for use in coordinated systems
• Generally not recommended for use under
isolated control

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Fully-actuated Control
• Responds to traffic conditions on all approaches
• Need detection on all approaches
• Cycle length, timing stage lengths and phase order
variable (potentially every cycle)
• Generally best choice for isolated (non-coordinated)
control

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Saturation Flow Rate
The saturation flow rate is the maximum hourly volume that can pass
through an intersection, from a given lane or group of lanes, if that lane
(or lanes) were allocated constant green over the course of an hour.

3600 (Eq. 7.1)

s
h

s = saturation flow rate in veh/h,

h = saturation headway in s/veh, and
3600 = number of seconds per hour.

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Adjusted
Saturation Flow Rate
Base value, per the Highway Capacity Manual, is
1900 pc/h/ln
This value is then adjusted for a variety of roadway
and traffic related conditions.
Factors such as lane width, grade, heavy vehicle
percentage, bus activity, parking activity, pedestrian
and bicyclist activity, and turn movement
geometry…

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 Lost Time
• Time that is not effectively serving any movement of traffic
• Start-up lost time: Occurs when a signal indication turns from red to green, drivers in
the queue do not instantly start moving at the saturation flow rate; there is an initial
lag due to drivers reacting to the change of signal indication and the time it takes for
vehicles to accelerate to their saturation-flow speed.
• Clearance lost time: Occurs when the signal indication turns from green to yellow, the
latter portion of time during the yellow interval is generally not utilized by traffic.
Additionally, if there is an all-red interval, this time period is generally not utilized by
traffic. Typically, the last second of the yellow interval and the entire all-red interval
are included in the estimate of clearance lost time.

tL = tsl + tcl (Eq. 7.2)

tL = total lost time for a movement during a cycle in seconds,

tsl = start-up lost time in seconds (typical value of 2 seconds), and
tcl = clearance lost time in seconds (typically the last second of the
yellow interval plus the entire all-red interval).

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Effective Green and Red
Times
g  G  Y  AR  t L (Eq. 7.3)

g= effective green time for a traffic movement in seconds,

G= displayed green time for a traffic movement in seconds,
Y= displayed yellow time for a traffic movement in seconds,
AR = displayed all-red time in seconds, and
tL = total lost time for a movement during a cycle in seconds.

r  R  tL (Eq. 7.4) Or r Cg (Eq. 7.5)

r = effective red time for a traffic movement in seconds,

R = displayed red time for a traffic movement in seconds, and
tL = total lost time for a movement during a cycle in seconds.

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Capacity
Capacity is determined on a lane group basis

c = s  g/C (Eq. 7.6)

Where:
s = adjusted saturation flow rate
g/C = eff. green to cycle length ratio

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Calculate Analysis Flow Rates
Adjust hourly volumes for RTOR
Adjust for peak hour factor (PHF)

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Phasing Considerations
Need to balance safety and movement efficiency
Generally, fewer phases--better service
Generally, more phases require longer cycle lengths
Generally, more phases results in overall reduction of intersection
capacity

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Left Turn Phase
Considerations
Permissive most desirable
Prot/perm generally better than just prot
Prot usually because of accident problem, multiple left-turn lanes, view
of oncoming traffic blocked

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Left Turn Phase
Considerations
Lead/Lag?
◦ Leading most common, and usually preferred
◦ Lagging may be better when time is needed for left-turn vehicles to
queue
◦ Lagging my be needed when LT bay gets blocked with thru vehicles
◦ Lagging can lead to greater accident potential if not careful with other
phasing

Can do one approach lead and opposing approach lag if

operationally better

https://www.youtube.com/watch?v=EkLPuaK9CQk

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Right Turn Phase
Considerations
Usually permissive (pedestrians)
RTOR is commonly accepted
May sometimes overlap with LT phase of approach to right (protected)

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Actuated Timing Terms
Minimum green
Maximum green
Unit extension interval
Maximum extension
Passage time
Minimum gap
Time to reduce
Time before reduction

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