Transportation Engineering

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Transportation Engineering

• Fundamentals of Traffic Signals

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Why Traffic Signals?

• Conflicting traffic movements, make roadway
  intersections unsafe for vehicles and pedestrians
• Intersections are a major source of crashes and
  vehicle delay (as vehicles yield to avoid
  conflicts with other vehicles).

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Why Traffic Signals?

• Most roadway intersections are not signalized due
  to low traffic volumes and adequate sight
  distances.
• At some point, traffic volumes and crash
  frequency/severity (and other factors) reach a
  level that warrant the installation of traffic
  signals.
• MUTCD def Any power-operated traffic control
  device other than a barricade warning light or
  steady burning electric lamp, by which traffic is
  warned or directed to take some specific action.
• Manual on Uniform Traffic Control Devices (MUTCD)

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Signalised Intersection

• Two simple examples in Dublin

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Signalised Intersection
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Traffic Signals
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History of Signalized Intersections

• 1868 - London, UK
• Manually operated semaphores (flags)
• 1914 - Cleveland, OH
• 1st Electric Signal
• 1917 - Detroit, MI
• Amber colour introduced

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Traffic Signals Advantages

• Ensures orderly movement of traffic in all
  directions
• Provisions for the progressive flow of traffic in
  a signal-system corridor
• Provisions for side-street vehicles to enter the
  traffic stream
• Provisions for pedestrians to cross the street
  safely
• Potential reduction of accidents, conflicts
  ensuring safety
• Possible improvements in capacity, and
• Possible reductions in delay

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Traffic Signals Disadvantages

• Large stop time delay
• Complex signal design problems
• The possible effects of a poorly-timed traffic
  signal
• increase in vehicle delay,
• increase vehicle crashes (particularly rear-end
  crashes)
• disruption to traffic progression

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Performance Measures
throughput
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Terminology 1

• Green time The time period in which the
• traffic signal has the green indication
• Red time The time period in which the
• traffic signal has the red indication
• Yellow time The time period in which the
• traffic signal has the yellow indication
• Cycle One complete rotation or sequence of
• all signal indications
• Cycle time (or cycle length) The total time
• for the signal to complete one sequence of
• signal indication.

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Terminology 2

• Permitted movement
• A movement that is made through a conflicting
  pedestrian or other vehicle movement.
• This is commonly used for right-turning
  movements where right-turn volumes are reasonable
  and where gaps in the conflicting movement are
  adequate to accommodate turns.
• Protected movement
• A movement that is made without conflict with
  other movements.
• The movement is protected by traffic control
  signal design with a designated green time for
  the specific movement.

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Terminology 3

• Effective green time The effective green time,
  for a phase, is the time during which vehicles
  are actually discharging through the
  intersection.
• Lost Time A portion of green or yellow time
  which is not utilised by traffic flow movements
  in a cycle.
• Start-up lost time At the beginning of each
  green indication as the first few cars in a
  standing queue experience start-up delays. This
  delay is measured as start-up lost time.
• The clearance lost time It is estimated by the
  amount of the yellow time not used by vehicles.

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Traffic Signal Times
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Basic Concepts of Intersection Signalisation

• Discharge headways
• Saturation flow at signalised intersections
• Delay
• Level of Service

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Discharge Headways

• Consider N vehicles discharging from the
  intersection when a green indication is received.
• The first discharge headway is the time between
  the initiation of the green indication and the
  rear wheels of the first vehicle to cross over
  the stop line.
• The Nth discharge headway (Ngt1) is the time
  between the rear wheels of the N-1th and Nth
  vehicles crossing over the stop line.

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Discharge Headways
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Discharge Headways

• The headway begins to level off with 4 or 5th
  vehicle.
• The level headway saturation headway

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Saturation flow rate

• In a given lane, if every vehicle consumes an
  average of h seconds of green time, and if the
  signal continues to be uninterruptedly green,
  then S vph could enter the intersection where S
  is the saturation flow rate (vehicles per hour of
  green time per lane) given by

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Saturation Flow Rate

• A variable directly related to discharge headway
• In a given lane, if every vehicle consumes an
  average of h seconds of green time, and if the
  signal continues to be uninterruptedly green,
  then S vehicles per hour (vph) could enter the
  intersection where S is the saturation flow rate
  given by
• In other words, s for a lane group/approach is
  the maximum number of vehicles from that lane
  group/approach that can pass through the
  intersection during one hour of continuous green
  time under the prevailing traffic and roadway
  conditions.

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Saturation Flow Rate

• The saturation flow rate is normally given in
  terms of straight-through passenger cars per hour
  of green.
• Passenger Car Units A unit of measure whereby
  HOVs and turning movements are converted to
  straight-through passenger cars using
  multiplication factors. This allows to deal with
  mixed traffic streams more accurately than
  assuming that all vehicles are equal.
• Unit for a single lane vphgpl (vehicles per hour
  of green time per lane) Here, vehicle is
  equivalent to passenger car units (pcu)
• Unit for an approach vphg (vehicles per hour of
  green time) or pcuphg (passenger car units per
  hour of green time)

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Factors Affecting Saturation Flow

• Position of lane
• Lane width
• Approach grades
• Parking conditions
• Number of HOVs
• Number of turning vehicles
• Conflicting vehicular flow
• Conflicting pedestrian flow
• We will be only considering the geometric factors

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Non-opposed Streams

• For non-opposed streams saturation flow , s1 for
  individual lanes is given by,

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Opposed Streams
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Opposed Streams
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Example 1
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Example 2
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Example 2
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Capacity

• The maximum hourly rate at which persons or
  vehicles can be reasonably expected to traverse a
  point or uniform segment of a lane or roadway
  during a given time period under prevailing
  traffic and roadway conditions.
• (HCM 2000)
• Unlike saturation flow, capacity considers
  traffic signal conditions
• The formula for calculating capacity (c) is,
• c (g/C) s N
• Wherec capacity (pcu/hour)g Effective
  green time for the phase in question (sec)C
  Cycle length (sec)
• N no. of laness Saturation flow rate
  (pcu/hour)

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Capacity Calculation

• Capacity can be calculated for two categories of
  flow uninterrupted and interrupted.
• Uninterrupted-flow facilities
• no fixed elements external to the traffic stream
  to interrupt flow
• Motorways, multilane highway, dual carriageway
• Interrupted-flow facilities
• have controlled and uncontrolled access points
  that can interrupt flow
• signalised and non-signalised intersections,
  urban streets

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Delay

• Very important performance measure for
  intersection signal design
• A measure most directly related to Drivers
  experience
• Stopped Time Delay time a vehicle stopped
  waiting to pass the intersection.
• Approach Delay stopped time acceleration
  deceleration
• Travel Time Delay (actual travel time-desired
  travel time)
• Time-in-queue Delay Total time from joining a
  queue to passing the stop line
• Delay is affected by cycle time, green time and
  volume to capacity ratio

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Delay

• Total delay
• Stopped delay
• Time in queue delay

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Time-in-queue Delay
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Level of Service (LOS)

• The performance of a signalised intersection is
  measure considering the average stopped time
  delay.
• 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.
• The first step in the LOS analysis is to
  calculate the average delay per vehicle for
  various portions of the intersection. The LOS of
  can be calculated for an entire approach, or
  alternatively, for each individual lane.

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Level of Service (Highways)
LOS A
LOS B
LOS C
LOS D
LOS E
LOS F
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LOS

• Level of Service A - Operations with low delay,
  or delays of less than 5.0 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 5.1 and 15.0 seconds per vehicle. This
  LOS implies good progression, with some vehicles
  arriving during the red phase.
• Level of Service C - Operations with delays
  between 15.1 and 25.0 seconds per vehicle. This
  LOS witnesses longer cycle lengths and fair
  progression.

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LOS

• Level of Service D - Operations with delays
  between 25.1 and 40.0 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.
• Level of Service E - Operations with delay
  between 40.1 and 60.0 seconds per vehicle. This
  LOS is considered unacceptable 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.