Transcript Introduction to Traffic Signal Design

–
Lessons Learned from the Portland
Workshop - 2003
Best Practices for Signal Operations
Peter Koonce
TRB Annual Meeting
January 9, 2005
KITTELSON & ASSOCIATES, INC.
TRANSPORTATION PLANNING/TRAFFIC ENGINEERING
Presentation Overview
• Problem Statement
• Operational Assessment
• Parting Thoughts
Problem Statement:
Detection Layout and Location (Design)
• Briefly explain and provide (standard drawings,
graphs, or spreadsheets) your detection layout
approach.
• Discuss methodology used to determine the
number of detectors, what criteria is used?
Problem Statement:
Detection Timing (Operations)
• Briefly explain your approach to detector timing
• Discuss the timing functions used
– What are the basic parameters that are used for
detection timing and what is their purpose? Volume
density functions, Min Green, etc.
Problem Statement:
Define Detector Purpose
• Design for Safety and/or Efficiency
• Any Consideration of Timing Functions in Design
Phase or vice versa?
Safety
Minimum Green
Detector Switching
Volume-Density Function
Dilemma Zone Protection
Clearance Intervals
Efficiency
x
x
Future Topic
Both
x
x
Detector Functions and Timing (Operations)
• Considering the following questions
– How do different technologies change what you do?
– What effect does speed changes have on your
approach?
– Do you change parameters depending on the operation
of the intersection (isolated and coordinated or by
time of day)?
– How do additional through lanes (2 or more) effect
your approach to timing (gap settings, dilemma zone)?
– How much does the public effect what you do and
what effect exists?
– Do you use detector timing features and contrast that
with controller timing features?
Problem with the Traditional Approach
• One size fits most!
– Signal design phase may not
consider actual operations (or
vice versa)
• Session 1 – Design
– Consider layout and placement
of detectors
• Session 2 – Timing
– Consider operations of the
signal controller
Innovative Notion:
It is an Iterative Process
Are the detection needs different at…..?
• Type of conditions
– Urban conditions
– Rural conditions – high speed, mostly
uncoordinated/fully actuated
• Time of day
– P.M. Peak vs. Middle of night
• Coordinated vs. Uncoordinated
• High-speed (>35 mph) conditions
• High-speed coordinated
No
consensus
Practices of Signal Timing – New ITE Report
“The vehicle extension interval is one of the most
important actuated controller settings. Here again,
diverse techniques are used to select values. The
techniques reported include:
–
–
–
–
–
–
–
–
field evaluation,
speed and volume,
loop detector location,
Single standard value,
Fixed value, (2, 3, 3.5, and 7 sec were reported),
Length of detection zone,
Time for vehicles to cross intersection (minus 4 s), and
Time required for slow moving vehicle to pass over loops”
Graphical Representation of Vehicle Control Logic
Vehicle Control Logic
Uncoordinated Operation
Coordinated Operation
• Two separate cases from a signal control strategy
and how it affects detection on a phase
Purpose of Detection in Uncoordinated Operation
Vehicle Control Logic
Uncoordinated Operation
All phases
Initial Interval
Minimum Green
Variable Green
Passage Gap
Maximum Green
Coordinated Operation
• In high-speed uncoordinated
operation, detection settings
should not max green (for
safety)
• For efficiency, max greens or
detection should keep phase
time reasonable
Graphical Representation of Vehicle Control Logic
Vehicle Control Logic
Uncoordinated Operation
• Non-coordinated phase
or side street ends with:
– max green or
– Forceoff/split end
Coordinated Operation
Non-coordinated phase
Initial Interval
Minimum Green
** Efficiency under safety
– Safety is maintained if
you find a gap
Variable Green
Passage Gap
Maximum Green
Forceoff
Coordinated Phase
Graphical Representation of Vehicle Control Logic
Vehicle Control Logic
Uncoordinated Operation
Coordinated Operation
• Coordinated phase
Non-coordinated phase
– No dilemma zone possible
without actuated coord phase
** Efficiency over safety?
Coordinated Phase
Default Phase
Dwells in Coord Phase
Monitor Special
Zero Point
Yield Point
Graphical Representation of Vehicle Control Logic
Vehicle Control Logic
Uncoordinated Operation
1
2
3
4
Coordinated Operation
All phases
Non-coordinated phase
Coordinated Phase
Initial Interval
Minimum Green
Initial Interval
Minimum Green
Default Phase
Dwells in Coord Phase
Variable Green
Passage Gap
Variable Green
Passage Gap
Monitor Special
End Phase
Maximum Green
Maximum Green
Forceoff
Zero Point
No Dilemma
Yield
Point
Zone
Suggested Detection Placement
City of Portland Method
• Graphical method using safe stopping distance to
determine appropriate location of detectors
• Identify speed and distance relationship to
provide yellow as vehicle reaches the stop bar
(approximately 5-10 feet within)
• Largest gap setting is 2.5 seconds to insure
efficiency
• Use of presence mode for all detectors
Detection Design and Timing Philosophy
• If vehicle actuates the first advance loop, try to get
it through the intersection
– Use progressive loops to determine whether vehicles can
stop safely in advance of the intersection
– Use additional loops to minimize gap time
• Display the yellow as vehicle enters the intersection
(25 to 40 feet from stop bar at full speed)
City of Portland Detection Placement
0. Front loop standard is 60 feet because safe
stopping distance essentially ends at speeds
below 15 mph
1. Identify speed of facility and site first detector
at safe stopping distance using curve
2. Once vehicle leaves the back loop (back of
vehicle), project vehicle path to standard front
loop (at 60 feet) using the safe stopping distance
speed (25 fps) from front loop
3. Determine carryover by checking what minimum
gap setting is at front detector
Safe Stopping Distance Curve
2
vo
SSD  vo * tpr 
64.4( f  g )
Detector Location based on SSD - 35 mph case
Place Upstream Detection for Safety
1. Speed in this case is 35 mph, thus place back
loop at 183’
Detector Location based on SSD - 35 mph case
Determine whether additional detectors are needed
2. Vehicle moves from 167’ to next loop at 60’
– would require 4.2 sec at 25 feet per second
(25 fps is SSD from detector at 60’)
– use intermediate loop at 1.5 sec from upstream
Use a third detector to reduce gap setting
Detector Location based on SSD - 35 mph case
Additional detector for snappy timing
2. Split difference between first two detectors
Vehicle moves from 167’ to next loop at 115’
– would require 1.5 sec at 36 feet per second
– repeat for third loop
Detector Timing based on SSD - 35 mph case
Additional detector for snappy timing
2. Vehicle moves from 100’ to next loop at 60’
– would require 1.5 sec at 26 feet per second
Gap settings are very low, maintains efficiency
Detector Timing based on SSD - 35 mph case
Final Design
3. Determine carryover by checking what minimum gap setting
is at front detector
Vehicle trajectory carries vehicle through intersection at
safe stopping distance - 0.5 seconds is minimum gap at
front loop, carryover is 1.5 – 0.5 = 1.0 secs
Concept of Carryover
• Carryover defines the amount of time a call stays
active after the vehicle has left the detector
– Portland uses this to reduce the gap time in the
controller and maintain flow rates
– In this example, carryover is applied to back loops
Gap
Setting
Gap Setting +
Carryover
0.5 s
1.0 s + 0.5 s = 1.5 s
Detector Timing and Design - 35 mph case
Final Design
Detector locations: 182’, 118’, and 60’
Min Gap 0.5 sec, per City standard
Carryover from Back loops 1.0 sec
Posted or 85th
Percentile Speed
20 MPH
25 MPH
30 MPH
35 MPH
40 MPH
45 MPH
50 MPH
55 MPH
Front Loop
Placement
N/A
N/A
N/A
110'
160'
160'
190'
225'
Back Loop
Placement
120'
140'
180'
220'
320'
320'
380'
450'
Source: ODOT
Dilemma Zone Loop Placement
(ODOT Method)
Detector Timing based on DOT method - 35 mph
In this 35 mph design case with detectors at 220’ and 110’
there are two checks
The SSD is 37 mph from the advance and 24 mph from the
front loops
A car leaving from upstream detector travels 110’- (3’ loop + 15’
car length) = 92’ from stop bar
At 24 mph or 36 fps, the gap setting is 2.5 seconds
WSDOT Loop Detection
• Additional loops if gap setting is greater than 3
seconds
Operational Assessment
• A 2.5 second gap compared to a 1.5 second gap
– Losing one second every phase, four “phases” every
cycle
• What is the flow rate that will keep a phase
operational?
– On a 3-lane roadway: See example
– On a 5-lane roadway
– On a 7-lane roadway
Back to the ODOT 35 mph design case:
At 35 mph (51 fps) a vehicle will reach the first detector
(nose of vehicle @223’) and travel to over the second
detector (@92’) – 131’ at 51 fps = 2.6 seconds + additional
gap of 2.5 seconds
Gap
Setting on
Detector
Travel Time
to Next
Detector
2.5 s
2.6 s
Interval between successive vehicles – 5.1 seconds
Worst case flow rate: 706 vehicles per hour
37% of what is ideal saturation flow rate
Let’s try the PDOT 35 mph design case:
At 35 mph (51 fps) a vehicle will reach the first detector
(@183’) and travel over the final detector (@42’) – 141’ at
51 fps = 2.7 seconds
Gap
Setting on
Last
Detector
0.5 s
Travel Time
to Last
Detector
2.7 s
Interval between successive vehicles – 3.2 seconds
Worst case flow rate: 1,125 vehicles per hour
59% of what is ideal saturation flow rate
No Advance Loop Placement
• Proper use of gap timing settings to offer
– Efficient performance?
– Safety?
• Assume little safety benefit with stop bar
detection only
Observations from the Field
• Poor gap setting leads to inefficiencies at the
intersections, which suggest higher cycle lengths
are needed
• HCM analyses assume saturation flow in all
available lanes in theoretical calculations
– Does not occur in reality with sloppy gap timing
• Do we need different gap timing under congested
conditions?
• Do we need different gap timing to meet different
objectives?
Objective Statement for Gap Timing
Conditions
Rural
Off-peak
Urban
Congested
High-speed
approaches
Low Volume
High
Volume
Purpose
Settings
Safety
Longer
Gaps
Efficiency
& Safety
Lower
Gaps
Advanced
Questions from a Practitioner
• Are safety benefits possible with detector and
timing design?
• What is the importance of lane by lane detection?
• Is dilemma zone over multiple lanes a good idea?
• What maintenance issues need to be addressed?
• How do we communicate this to the practitioner?
Research Needs
• Quantify the operational benefits of an additional
detector(s) and lower gap settings
• Assess effects on performance under fully
actuated conditions (of 5- & 7-lane cross section)
• Quantify the benefits of advance detection
settings
• Consider use of a check-out detector at
intersection for improving performance