Chapter 7 Traffic Control and Analysis at Signalized Intersections

1
Chapter 7Traffic Control and Analysis at
Signalized Intersections

• Principles of Highway Engineering and Traffic
  Analysis, 2005
• Third Edition
• Fred Mannering, Walter Kilareski
• Scott Washburn

2
Basic Concepts
3
Signal Timing Terminology

• Indication illumination of signal lenses which
  informs the driver as to which movements are
  permitted or prohibited
• Cycle one complete rotation through all of the
  indications provided
• Cycle length time required to complete one
  rotation, given in seconds, C
• Yellow time the change interval, warns drivers
  that the signal is changing from green to red
• Clearance interval the all red indication
• Green time the go indication for a particular
  movement or set of movements
• Red time the stop indication for a particular
  movement or set of movements
• Phase a green interval plus the change interval
  and clearance intervals that follow it (typically
  related to a particular movement or approach)

4
Modes of Operation

• Pretimed Operation preset cycle lengths and
  intervals. 3-dial signal controllers allow for
  three different cycles throughout the day.
• Semi-Actuated detectors at minor approaches.
  Green for major street unless vehicle detected on
  minor street
• Fully Actuated every approach has detectors.
  Green time allocated based on vehicle detection.
  Each cycle different, limits placed on min/max
  green times and min gaps between vehicles to
  maintain green indication.
• Computer Controlled System wide control.
  Optimal progression patterns determined for
  system wide operation. In order to optimize,
  however, cycle lengths must be the same or
  multiples of a base to achieve optima performance.

5
(No Transcript)
6
Inductive Loop
7
Machine-Vision Camera Detectors
8
Left Turn Timing

• Permitted Left Turns drivers permitted to cross
  opposing traffic but must select their own gap
  (green ball on signal head)
• Protected Left Turns left turns made without
  opposing through vehicular traffic (green arrow
  on signal head)
• Protected/Permitted or Permitted/Protected left
  turns protected at the beginning of a phase, then
  permitted during through movement green time

9
Dual-Ring Configuration

• Allows for maximum flexibility to control phase
  duration and sequencing of intervals
• Best hardware to have when implementing
  fully-actuated signals
• See Figure 7.3
• Movements 1-4 can occur simultaneously with
  movements 5-8 (as long as the occur on the same
  side of the barrier)
• The dual ring ability allows for skipping of
  phases where there isnt a need for them due to
  low flow
• Allows the unused green to be allocated to more
  congested phases

10
(No Transcript)
11
Discharge Headway

• Discharge headway time passage between
  successive vehicles as they cross the curb line
  during a green phase. Measured at rear wheels of
  vehicles.
• First headway longer than others. Includes
  driver reaction time, and acceleration time.
• Second headway shorter, reaction and acceleration
  times overlap.
• Eventually headways level out, typically around
  4-5 vehicle.
• Once this occurs, saturation headway can be
  measured.

12
Saturation Flow Rate

• Saturation headway hheadway achieved by stable
  moving platoon of vehicles passing through a
  green indication
• Saturation Flow Rate every vehicle assumed to
  occupy h seconds of green time, and if signal
  always green, then s vehicles/hour could enter
  intersection
• s 3600/h
• If signal always green, could simply multiply by
  the number of lanes to estimate the capacity of
  the approach.
• units of measure vehicles per hour of green
  time per lane, (vphgpl)

13
Lost Time

• Time that is not effectively serving any movement
  or traffic
• Total lost time includes start-up and clearance
  lost times
• Start-up lost time signal indication turns from
  red to green and vehicles do not instantly move
  at the saturation flow rate
• Clearance lost time later portion of the yellow
  phase all red phase

14
Start-up Lost Time

• Need to account for the time lost when first few
  vehicles crossing intersection
• Start-up lost time (l) actual
  headway-saturation headway multiplied by number
  of vehicles (n) traveling at headways greater
  than saturation headway (h)
• Lost time also occurs when a movement is stopped
  (at the beginning of the clearance interval)

15
Total Lost Time
16
(No Transcript)
17
Effective Green Time

• Amount of time available to be used at a rate of
  one vehicle every h seconds
• giGi Yi AR tL
• gieffective green time
• Giactual green time for movement i,sec
• Yisum of yellow plus all red time for movement
  i,sec
• tLtotal lost time per phase, sec
• Total lost time includes start up time and
  clearance lost time (tLtsl tcl)

18
Green Ratio

• Ratio of effective green time to cycle length for
  a particular movement
• Simple capacity can be determined using
• cisi(gi/C)
• cicapacity of lanes serving movement i, vph
• CSignal cycle length, sec
• sisaturation flow rate for movement i, sec
• gieffective green time for movement i,sec

19
Effective Red Time

• Effective red time is the time in which the
  intersection is not being utilized by traffic

r effective red time for a traffic movement in
seconds R displayed red time for a traffic
movement in seconds tL total lost time for the
movement during a cycle in seconds
20
Simple Capacity Estimation

• Approach or movement capacity can be estimated
  through a simple relationship

c capacity of a lane group or approach that are
served by a particular g s saturation
flow rate in veh/hr g/C ratio of the effective
green time to the total cycle length
21
Using Simple Queuing Models to Estimate Signal
Performance

• Ex 7.1 in text
• Pretimed signal with sat flow rate of 2400 vph
• 24 sec of effective green time in 80 s cycle
• Flow at approach is 500 vph
• Estimate operational performance using D/D/1
  queuing.

22
Example Continued

• Put arrival and departure rates in similar units

23
Example Continued

• A variety of measures can be determined using
  equations 7.7-7.13

24
Estimating Delay at Real-World Signals

• The D/D/1 models are limited by their assumption
  of uniform arrivals
• For signals, non-uniform arrivals are much more
  likely than uniform arrivals
• To determine LOS, control delay is estimated for
  traffic signals and unsignalized intersections
• Control delay deceleration time, queue move-up
  time, stop time, and acceleration time through
  the intersection

25
HCM Delay Model

• Total Average Individual Stopped Delay for Random
  Arrivals (sec/veh)

26
HCM Delay Model

• daverage signal delay per vehicle in sec
• d1avg delay per vehicle due to uniform arrivals
  in sec
• PFprogression adjustment factor
• d2avg delay per vehicle due to random arrivals
  in sec
• d3avg delay per vehicle due to initial queue at
  start of analysis period in sec

27
HCM Delay Model

• Ccycle length, sec
• geffective green time for lane group in sec
• Xv/c ratio for lane group
• Tduration of analysis period in hours
• kdelay adjustment factor that is dependent on
  signal controller mode
• Iupstream filtering adjustment factors
• clane group capacity in veh/hr

28
Example

• An approach to a pretimed signalized intersection
  has
• s 2400 veh/hr
• 24 sec effective green
• C 80 sec
• Given flow 500 veh/hr no initial queue and
  flow accounts for 15-min period, determine
  average approach delay per cycle.

29
Example Continued

• Calculate uniform delay first

30
Example Continued

• Calculate Random Delay