Transcript CIV 371 Transportation Engineering

CIV 475
Traffic Engineering
Reference:
1. National Institute for Advanced Transportation Technology NIATT
University of Idaho, Moscow, ID 83844-0901
http://niatt.uidaho.edu
2. Highway Capacity Manual, US Transportation Research Board
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When streets meet at an “level” intersection,
there is the potential for each vehicle to make
up to three (or four) “movements” they are:
go straight
turn left
turn right
U-turn (many areas make this illegal)
So for an intersection of two, two-way streets,
how many turning “movement” possibilities are
there?
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Conflicting traffic movements cannot share the
same space at the same time.
Because of their ability to separate traffic
movements in time, traffic signals are one of
the most common regulatory fixtures found at
intersections.
Other options include:
sign control (stop, yield)
“separation by space” (overpass / underpass)
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Traffic signals consisting of manually operated
flags were used in London, England as early as
the 1860’s
The first American electric signals were installed
in Cleveland, Ohio in 1914
By 1917, several signals were interconnected in
Salt Lake City, Utah in the first American traffic
control system
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Signal Control Types
Pretimed signals
assign right of way according to a predetermined
schedule (or timing plan)
the length of each time interval is fixed, usually
based on periodic intersection traffic counts
Actuated signals
have varying time intervals based on actual traffic
conditions
traffic conditions are “read” by the signal controller
by the use of “vehicle detectors”
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Inductive Loop Detectors IDL
Most common form of traffic detection
A loop of metal wire is placed in a saw cut in the
pavement
Unfortunately saw cuts have been found to
undermine the structural stability of the
pavement in some cases.
Loop detectors operate on the principle of
inductance, the property of a wire or circuit element
to "induce" currents in isolated but adjacent conductive
media.
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IDL
An IDL detector consists of:
an insulated electrical wire, placed on or below the
road surface, attached to a signal amplifier, a power
source, and other electronics.
Driving an alternating current through the wire
generates an electromagnetic field around the loop.
Any conductor, such as the engine of a car, which
passes through the field will absorb electromagnetic
energy and simultaneously decrease the inductance
and frequency of the loop.
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 For most conventional installations, when the
inductance or frequency changes a preset threshold in
the actuate detector electronics, this indicates that a
vehicle has been detected.
 IDL’s are installed in a variety of shapes such as square,
rectangle, diamond, circular and octagonal, though each
configuration produces a different electromagnetic field.
For instance, diamond loops reduce the probability of
detecting vehicles in adjacent lanes.
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 The controller electronics, usually housed in a rugged
cabinet at the roadside, detect, amplify, and process
loop signals.
 The controller orchestrates loop operation and provides
power.
 A typical controller can handle up to forty loops, though
in practice will probably oversee far fewer.
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Detectors operate in either the pulse or
presence mode. Presence operation, often
used with traffic signals, implies that detector
output will remain "on" while a vehicle is over
the loop. Pulsed detection requires the detector
to generate a short pulse (e.g. 100 to 150 ms)
every time a vehicle enters the loop, regardless
of the actual departure of the detected vehicle.
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 Rough component costs are given below.
 Loop with amplifier (purchase and installation) - $700
per loop
 Controllers - $2500 per unit
 Controller Cabinet - $5,000 per unit
 Fiber optic cable (purchase and installation) - $300,000
per mile
 Annual maintenance costs average around 10% of the
original installation and capital cost, adjusted for
inflation.
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Historically the inductive loop detector has been
a principal element of freeway surveillance
and incident detection systems.
However the information loops supply is limited,
and they alone can not provide comprehensive
freeway surveillance. Similarly, the precise
nature of a loop detected incident can not be
ascertained, and loops become less effective for
incident detection in low volume conditions.
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Advances in vehicle sensors and more
sophisticated detection algorithms present
transportation authorities with the opportunity
to implement or enhance automatic incident
detection systems. Promising technologies
include video image processing, doppler radar
detectors, cellular phones, and neural network
algorithms.
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Earlier preemption systems
 The unique benefits of Priority One are best
viewed in context of three earlier preemption
systems, each
 with its own advantages and limitations.


1.Strobe-based systems
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First introduced in the 1960's, these use a
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Priority One uses the satellite-based Global
Positioning System (GPS) to determine exact
vehicle
 position, direction of travel and speed, as well
as the exact time of day. The same advanced
12-channel
 GPS receiver is installed at each intersection
and in each vehicle. Each intersection serves as
a GPS base
 station with a known
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differential corrections via two-way radio to the
Since traffic signals are so common, professional
civil engineers (even non-transportation types!)
are often expected to know the basics of Signal
Timing Design.
Signal Timing Design, at its simplest level,
involves finding the appropriate duration for all
of the various signal indications.
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The various movements allowed to move in
turn, or in phases.
In some cases, a single movement is given a
phase; or movements can be collected and that
grouping given a phase
Collected movements are those that can
proceed concurrently without any major conflict.
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Campus Dr. & Alameda
For example, the straight-through and right-turn
movements of Campus Drive are collected and
permitted to use the intersection simultaneously
without any serious danger to the motorists
involved.
This is one phase of a multi-phase cycle.
What about danger to pedestrians, are they
“permitted” to make any conflicting movements
during this phase?
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The reality is that some movements are allowed
to proceed during a phase even though they
cause conflicts.
Is this ethical… pedestrian volumes are very low
at most intersections
Those movements that are allowed but share
the “space” with other movements are called
permitted,
Those without any conflicts are called
protected movements
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The basic timing elements within each phase
include:
the
the
the
the
the
the
the
green interval,
effective green time,
yellow or amber interval,
all-red interval,
intergreen interval,
pedestrian WALK interval,
pedestrian crossing interval.
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The green interval is the period of the phase
during which the green signal is illuminated.
The yellow or amber interval is the portion of
the phase during which the yellow light is
illuminated. 2-5 seconds
The effective green time is contained within the
green interval and the amber interval.
The effective green time, for a phase, is the
time during which vehicles are actually discharging at
the design rate through the intersection.
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Graph of Flow vs. time for one movement at a
signalized intersection
Showing the need for use of Effective Green
Effective Green Time
Flow
Saturation
flow rate
(veh/hr)
Red
Green (shown in intervals)
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Amber
Red
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The all-red interval is the period following the
yellow interval in which all of the intersection's
signals are red. Some suggest 1sec. min
The intergreen interval is simply the interval
between the end of green for one phase and
the beginning of green for another phase.
It is the sum of the yellow and all-red intervals.
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The pedestrian WALK interval is the portion of time
during which the pedestrian signal says WALK. This
period usually lasts around 4-7 seconds and is completely
encompassed within the green interval for vehicular traffic.
Some pedestrian movements in large cities are separate
phases unto themselves.
Finally, the pedestrian crossing time is the time
required for a pedestrian to actually cross the
intersection.
This is used to calculate the intergreen interval and the
minimum green time for each phase.
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Intergreen Time
The intergreen period of a phase consists of
both the yellow (amber) indication and the allred indication (if present).
This length of the intergreen phase is governed
by three separate concepts:
stopping distance,
intersection clearance time,
pedestrian crossing time, (if there are no pedestrian
signals)
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The amber signal indication serves as a warning
to drivers that another phase will soon be
receiving the right-of-way.
The intergreen interval, therefore, should be long
enough to allow cars that are greater than the
stopping distance away from the stop-line to brake
easily to a stop.
The intergreen interval should also allow vehicles
that are already beyond the point-of-no-return to
continue through the intersection safely.
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 If the intergreen time is too short, only those vehicles
that are close to the intersection will be able to continue
through the intersection safely.
 In addition, only vehicles that are reasonably distant will
have adequate time to react to the signal and stop.
 This issue is called the"dilemma zone" concept.
 Those who are in between will be caught in a "dilemma”
and won’t have enough time to stop or safely cross the
intersection.
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The only responsible thing to do, it seems, is to
eliminate the dilemma zone.
This would allow any vehicle, regardless of its
location, to be able to safely stop or,
alternatively, safely proceed during the
intergreen period.
This is done by making sure that any vehicle
closer to the intersection than its minimum
braking distance can safely proceed through the
intersection without
speeding.
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First, calculate safe stopping distance
v2
S  vt 
2g( f  G)
v  speed
t  perception reaction tim e
g  acceleration
f  friction factor
G  grade(%)
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Next, we calculate the time required for a
vehicle to travel the minimum safe stopping
distance and to clear the intersection.
This is for the vehicle who has just passed the safe
stopping point (in his mind at least!)
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S  L W
T
v
T  clearancetim e
S  safe stoppingdis t ance
L  length of vehicle
W  width of int er sec tion
v  velocity
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Now that we’ve determined the first two
elements of the intergreen period length—
stopping distance and intersection clearance
time— we may be done (on those intersections that
have pedestrian lights)
The intergreen time for intersections that have
signalized pedestrian movements is the same as
the intersection clearance time.
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If you have an intersection where the pedestrian
movements are not regulated by a separate
pedestrian signal, you need to protect these
movements by providing enough intergreen
time for a pedestrian to cross
the
intersection.
In other words, if a pedestrian begins to cross the
street just as the signal turns yellow for the vehicular
traffic, he/she must be able to cross the street safely
before the next phase of the cycle begins.
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W
PCT 
vp
PCT  pedestriancros sin g tim e
W  width of int er sec tion
vp  pedestrianvelocity (~ 4 ft / s,1.3m / s )
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 The intergreen time is equal to whichever is larger, the
pedestrian crossing time or the intersection clearance
time.
 As you know, the intergreen period is composed of the
yellow interval and the all-red interval.
 The allocation of the intergreen time to these separate
intervals is a question that is answered best by referring
you to your local codes.
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 In some areas, the yellow time has been standardized
for several speeds.
 This would make the all-red time the difference between
the standard yellow time and the intergreen time.
 One other option is to allocate all of the intergreen
period as calculated to the yellow interval.
 You could then tack on an all-red period as a little extra
safety. This, however, might increase delay at your
intersection.
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Pedestrian Crossing Time,
Minimum Green Interval
The pedestrian crossing time serves as a
constraint on the green time allocated to each
phase of a cycle.
Pedestrians can safely cross an intersection as
long as there are not any conflicting movements
occurring at the same time. (Permitted left and
right turns are common exceptions to this rule.)
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This allows pedestrians to cross the intersection
in both the green interval and the intergreen
interval.
Thus, the sum of the green interval and the
intergreen interval lengths, for each phase,
must be large enough to accommodate the
pedestrian movements that occur during
that phase.
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 At this point, two separate conditions arise.
 If you have an intersection in which the pedestrian
movements are not assisted by a pedestrian signal,
you need to make sure that the green interval that you
provide for vehicles will service the pedestrians as well.
In this case, the minimum green interval length is somewhere
between 4 and 7 seconds. You already took care of the
pedestrian crossing time considerations when you calculated the
intergreen period length.
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If, on the other hand, you plan to provide a
pedestrian signal, you need to calculate the
pedestrian crossing time as described below.
This will not only give you the information you
need to program the pedestrian signal, but it
will also allow you to find the minimum green
interval for your vehicular movements as well.
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We need a few assumptions to calculate the
pedestrian crossing time.
 The WALK signal will be illuminated for approximately 7
seconds.
 A pedestrian will begin to cross the street just as the
DON'T WALK signal begins to flash.
 Pedestrians walk at an average pace of 4 ft/s or 1.2m/s
 The WALK interval must be completely encompassed by
the green interval of the accompanying vehicle
movements.
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 The total time required for the pedestrian movements
(T) is the sum of the WALK allowance (Z) and the time
required for a person to traverse the crosswalk (R).
 R = width of intersection (in feet)
________________
4 ft/sec

T=Z+R
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 The pedestrian crossing time governs the minimum
green time for the accompanying phase in the following
way.
 If the time it takes the pedestrian to traverse the
crosswalk (R) is greater than the intergreen
time(I), the remainder of the time (Z+R-I) must be
provided by the green interval.
 Therefore, the minimum green interval length (gmin) for
each phase can be calculated using the equation below.
 gmin = T - intergreen time(I) or
gmin = Z + R – I
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 If this equation results in a minimum green interval that
is less than the WALK time (Z), the minimum green
interval length is equal to the WALK time (Z).

gmin = Z
 You now have the minimum length of the green interval
for the vehicular movements, as governed by the
pedestrian movements. The WALK interval for the
pedestrians is whatever you assumed, and the DON'T
WALK flashes for the remainder of the green and
intergreen intervals.
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Saturation Flow Rate and
Capacity
Saturation Flow Rate can be defined with the
following scenario:
 Assume that an intersection’s approach signal
were to stay green for an entire hour, and the
traffic was as dense as could reasonably be
expected. The number of vehicles that would
pass through the intersection during that hour is
the saturation flow rate.
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Obviously, certain aspects of the traffic and the
roadway will effect the saturation flow rate of your
approach.
If your approach has very narrow lanes, traffic will
naturally provide longer gaps between vehicles,
which will reduce your saturation flow rate.
If you have large numbers of turning movements,
or large numbers of trucks and busses, your
saturation flow rate will be reduced.
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The saturation flow rate is normally given in
terms of straight-through passenger cars per
hour of green.
Most design manuals and textbooks provide
tables that give common values for trucks and
turning movements in terms of passenger car
units (pcu).
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Determining the saturation flow rate can be a
somewhat complicated matter.
The saturation flow rate depends on roadway
and traffic conditions, which can vary
substantially from one region to another.
It’s possible that someone in the area has
already completed a measurement of the
saturation flow rate for an approach similar to
yours. If not, you'll need to measure it in the
field.
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 One other possibility, which is used quite frequently, is
to assume an ideal value for the saturation flow rate and
adjust it for the prevailing conditions using adjustment
factors.
 A saturation flow rate of 1900 vehicles/hour/lane, which
corresponds to a saturation headway of about 1.9
seconds, is a fairly common nominal value.
 Design manuals usually provide adjustment factors that
take parameters such as lane-width, pedestrian traffic,
and traffic composition into account.
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Capacity is an adjustment of the saturation flow
rate that takes the real signal timing into
account, since most signals are not allowed to
permit the continuous movement of one phase
for an hour!
If your approach has 30 minutes of green per
hour, you could deduce that the actual capacity
of your approach is about half of the saturation
flow rate.
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The capacity, therefore, is the maximum hourly
flow of vehicles that can be discharged through
the intersection from the lane group in question
under the prevailing traffic, roadway, and
signalization conditions.
 c = (g/C) · s
 c = capacity (pcu/hour)
 g = Effective green time for the phase in question (sec)
 C = Cycle length (sec)
 s = Saturation flow rate (pcu/hour)
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Capacity can be used as a reference to gauge
the current operation of the intersection.
For example, let us assume that you know the
current flow rate for a lane group and you also know
the capacity of that lane group. If the current flow
rate is 10% of the capacity, you would be inclined to
think that too much green time has been allocated to
that particular lane group. You'll see other uses for
capacity as you explore the remaining signal timing
design concepts.
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Peak Hour Volume, Peak Hour
Factor, Design Flow Rate
The peak hour volume is the volume of traffic
that uses the approach, lane, or lane group in
question during the hour of the day that
observes the highest traffic volumes for that
intersection.
For example, rush hour might be the peak hour
for certain interstate acceleration ramps. The
peak hour volume would be the volume of
passenger car units that used the ramps during
rush hour.
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Notice the conversion to passenger car units.
The peak hour volume is normally given in
terms of passenger car units, since changing
turning all vehicles into passenger car units
makes these volume calculations more
representative of what is actually going on.
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 The peak hour factor (PHF) is derived from the peak
hour volume.
 It is simply the ratio of the peak hour volume to four
times the peak fifteen-minute volume.
 For example, during the peak hour, there will probably
be a fifteen-minute period in which the traffic volume is
more dense than during the remainder of the hour. That
is the peak fifteen minutes, and the volume of traffic
that uses the approach, lane, or lane group during those
fifteen minutes is the peak fifteen-minute volume.
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( peak hour volum e)
PHF 
4 * ( peak15min volum e)
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The design flow rate or the actual flow rate, for
an approach, lane, or lane group is the peak
hour volume (flow rate) for that entity divided
by the peak hour factor.
A simpler way to arrive at the design flow rate is
to multiply the peak fifteen-minute volume by 4.
However you derive the value, most
calculations, such as those that measure the
current use of intersection capacity, require the
actual flow rate (design
flow rate).
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Critical Movement or Lane
While each phase of a cycle can service several
movements or lanes, some of these lanes will
inevitably require more time than others to
discharge their queue.
For example, the right-turn movement of an
approach may service two cars while the
straight-through movement is required to
service 30 cars. The net effect is that the rightturn movement will be finished long before the
straight-through movement.
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What might seem to be an added complexity is
really an opening for simplicity.
If each phase is long enough to discharge the
vehicles in the most demanding lane or
movement, then all of the vehicles in the
movements or lanes with lower time
requirements will be discharged as well.
This allows the engineer to focus on one
movement per phase instead of all the
movements in each
phase.
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The movement or lane for a given phase that
requires the most green time is known as the
critical movement or critical lane. The critical
movement or lane for each phase can be
determined using flow ratios.
The flow ratio is the design (or actual) flow rate
divided by the saturation flow rate.
The movement or lane with the highest flow
ratio is the critical movement or critical lane.
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Cycle Length
Determination
 Once you know the total cycle length, you can subtract the
length of the amber and all-red periods from the total cycle
length and end up with the total time available for green signal
indications.
 Efficiency dictates that the cycle length should be long enough
to serve all of the critical movements, but no longer.
 If the cycle is too short, there will be so many phase changes
during an hour that the time lost due to these changes will be
high compared to the usable green time.
 But if the cycle is too long, delays will be lengthened, as
vehicles wait for their turn to discharge through the
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Webster’s equation
 Several methods for solving this optimization problem
have already been developed, but Webster’s equation is
the most prevalent.
 Webster's equation, which minimizes intersection delay,
gives the optimum cycle length as a function of the lost
times and the critical flow ratios.
 Many design manuals use Webster's equation as the
basis for their design and only make minor adjustments
to suit their purposes.
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Co=

1.5L + 5
1 - S (V/s)
 Co = Optimum cycle length (sec)
 L = Sum of the lost time for all phases, usually taken as
the sum of the intergreen periods (sec)
 V/s = Ratio of the design flow rate to the saturation flow
rate for the critical approach or lane in each phase
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 After you have calculated the optimum cycle length, you
should increase it to the nearest multiple of 5.
 For example, if you calculate a cycle length of 62
seconds, bump it up to 65 seconds.
 Once you have done this, you are ready to go.
 If you know the intergreen times for all of the phases,
you can then calculate the total time and allocate it to
the various phases based on their critical movements.
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Green Split Calculations
Once you have the total cycle length, you can
determine the length of time that is available for
green signal indications by subtracting the
intergreen periods from the total cycle length.
But, the result is useless unless you know how
to allocate it to all of the phases of the cycle.
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Green time is allocated using a ratio equation.
Each phase is given a portion of the available
green time that is consistent with the ratio of its
critical flow ratio to the sum of all the critical
flow ratios.
The proportion of the available green time that
should be allocated to phase "i" can be found
using the following equation:
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v
( )i
s
gi 
* GT
v
( )
s
gi  length of green int erval for phase" i"
GT  the avilable green tim e for the cycle
v
( )i  critical flow rate for phase" i"
s
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Timing Adjustments
Once you have calculated the lengths of the
minimum green intervals, green intervals, and
intergreen intervals, as well as the design flow
rates and capacities for each of your phases; it
is time to ask yourself whether or not your
results actually work.
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The first and most obvious check involves the
green intervals.
Check the length of the green interval for each
phase.
If it is not greater than the length of the phase's
minimum green interval, you need to bump up
the cycle length and add green time to that
phase until the green interval is equal to or
greater than the minimum.
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The second check involves capacity.
If the capacity of a particular phase is below the
design flow rate for that phase, you should
back-calculate the effective green time that
would allow the phase to run at the design
flow rate.
Once again, simply increasing the cycle length
and allocating more time to the green interval of
the troubled phase will solve the problem.
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Webster noted that the cycle length can vary
between 0.75Co and 1.5Co without adding
much delay, so don't worry too much about
adding a few seconds to the nominal cycle
length.
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Computing Delay and LOS
One way to check an existing or planned signal
timing scheme is to calculate the delay
experienced by those who are using, or who will
use, the intersection.
The delay experienced by the average vehicle
can be directly related to a level of service
(LOS).
The LOS categories, contain information about
the progression of traffic under the delay
conditions that they
represent.
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LOS based on Highway Capacity Manual 2000
edition (HCM 2000)
Level of Service A Operations with low delay, or delays of less than
10 seconds per vehicle.
This LOS is reached when most of the oncoming
vehicles enter the signal during the green phase, and
the driving conditions are ideal in all other respects
as well.
Level of Service B Operations with delays between 10 and 20
seconds per vehicle. This LOS implies good
progression, with some vehicles arriving during the
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Level of Service C Operations with delays between 20 and 35
seconds per vehicle. This LOS witnesses longer cycle
lengths and fair progression.
Level of Service D Operations with delays between 35 and 55
seconds per vehicle. At this LOS, congestion is
noticeable and longer delays may result from a
combination of unfavorable progression, long
cycle lengths, and high V/c ratios.
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Level of Service E Operations with delay between 55 and 80
seconds per vehicle.
This LOS is considered acceptable by most drivers.
This occurs under over-saturated intersection
conditions (V/c ratios over 1.0), and can also be
attributed to long cycle lengths and poor progression.
Level of Service F – delay >80 seconds/vehicle
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The first step in the LOS analysis is to calculate
the average delay per vehicle for various
portions of the intersection.
You might be interested in the LOS of an entire
intersection, an approach (say northbound), or
you might be interested in the LOS of each
individual lane within an approach.
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Average Stopped Delay Per
Vehicle:
d=d1*PF + d2 + d3
d1= uniform control delay
d2= incremental delay
d3= initial queue delay
PF = progression factor
See HCM handout for details
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