TRAFFIC RESPONSIVE
SIGNAL COORDINATION
TRB
TRAFFIC SIGNAL SYSTEMS COMMITTEE
MIDYEAR MEETING
JULY 25-27, 2003 – TORONTO, ONTARIO
DENNIS EYLER
VICE PRESIDENT
SRF CONSULTING GROUP, INC.
MINNEAPOLIS, MN
Purpose of Presentation
Provide an overview of the capabilities of traffic
responsive master controllers operating full traffic
actuated intersections
Present a few of the differences between adaptive
and traffic responsive
Suggestions for setting up a traffic responsive
system
TO GET THE OWNERS OF TRAFFIC
RESPONSIVE MASTERS TO USE THEM!
Definition of a traffic signal
A traffic signal is a device that
allows traffic engineers to leave
their intelligence at an intersection
to operate it in their absence.
My perspective
I have designed and operated over 40
traffic responsive arterial coordination
systems consisting of full traffic actuated
intersections. Most were on suburban
arterial roadways with speeds in the 50 to
60 mph range
Arterial coordination
systems in the
Minneapolis suburbs
50 +MPH
40- 45 MPH
Under 40 MPH
Audience poll
How many people here are operating or have operated
traffic responsive systems?
Are these systems also operating with full traffic
actuated intersection controllers?
How many are on roadways with speeds above 50
miles per hour?
Anyone used this type of system in an urban grid?
How many people here are currently running an
adaptive control system?
Why the poll?
I am always willing to stand trial before a
jury of my peers. However, I do want to
make sure that I am in the presence of one.
Definitions
- Traffic responsive master controller also
known as “closed loop”
– Has a library of prepared system timing plans – most
systems are capable of 100 + plans
– Variables
• Cycle length, Offset, Splits, Grouping
– Plan selection based on
• Volume levels, Directional distribution, Speed, Other inputs
• Full traffic actuated controller –
– Vehicular and pedestrian phases are enabled by
detection. Vehicular phases are also extended by
detection.
Definitions
• Coordination
– Merely a series of force offs and holds
applied in an organized (presumably logical)
manner to provide optimal flow through a
group of traffic signals
• Optimal
– Like beauty, it is defined by the “eye of the
beholder”
History of the traffic responsive &
adaptive systems (at least that part that I
can remember)
•
Early 1950’s – the 1022 controller with platoon carryover effect
–
•
1960’s – electro-mechanical technology
–
–
•
Mn/DOT insists on full actuated operation & uses leased phone lines for some twoway communication
Early 1970’s
–
–
•
Pre-timed controllers used as coordinators
Mn/DOT uses mutual coord device for 2 to 4 intersection systems, which creates a
virtual single controller
Late 1960’s - First solid state master controllers
–
•
Actually primitive adaptive control
Mn/DOT master controller cabinets are full ATR’s with data recording systems
Digital master controllers with digital coordinators
Late 1970’s – microprocessors and development of improved timing
software
–
–
Microprocessor master controllers and microprocessor coordinators
Type 170 controllers
History (continued)
• Early 1980’s
– Second generation microprocessor controllers with
internal coordination and precise time clocks also allows
wider use of time based coordination
– All temperature modems allow dial-up systems and give
us the “closed loop” system as we know it today
– Use of personal computers and Type 170’s as master
controllers
• Mid 1980’s
– Use of arterial masters in urban networks
• Duluth
• Peoria
• Others
History (continued)
• 1980’s and beyond - Adaptive control systems
are developed and deployed
–
–
–
–
SCOOT, SCATS, UTOPIA
TRAC
RT Tracks (OPAC, Rhodes and others)
City of Los Angeles
• After 1980 - for traffic responsive systems
– Development of area wide supervisory systems
– Increased system capabilities
Sample equipment ca. 1970
Comparison of
Systems
Arterial Signal Control Theory
Justification for “adaptable” operation
High
Isolated – full actuated
Responsive or adaptive
Rigid coordination
Low
1
2
3
4
5
6
Intersections per minute of travel time
7
Signal Control Theory
Justification for “adaptable” operation
High
Signal control is
adapted to traffic
conditions
Traffic is adapted
to signal control
Low
1
2
3
4
5
6
Intersections per minute of travel time
7
Responsive and Adaptive Objectives
• Adaptive control systems
– Minimize stop delay by optimizing splits and reducing cycle
lengths
– Stops are minimized through offset “optimization”
• Traffic Responsive systems
– Stop delay is reduced by cycle length selection and split control
– Stops (particularly high-speed) are minimized by strict offset
control and cycle length control
• Oversimplifications
– Adaptive minimizes stop delay, responsive minimizes stops
– Adaptive works best in an open network of “equal” roadways,
responsive works best on a high speed arterial or in a grid of
regularly spaced intersections
CYCLE LENGTH
Responsive and Adaptive
Adjustments
CYCLES
Understanding delay
(not handled adequately by the HCM)
• Delay is the time to traverse an area that is
addition to the time it would take at the normal
travel speed
• Delay consists of:
– Added path length (example: a loop ramp has a longer
travel path than a directional ramp)
– Geometric delay – traffic must slow because of
intersection geometry (example: a roundabout)
– Control delay – this has two components
• The initial imposition of the control (example: a stop sign)
• Delay because of a division of intersection capacity
– Congestion delay – travel time added because of the
interaction of the vehicles in the traffic stream
• Speed differentials – cars versus trucks
• Different driver behavior
Understanding delay
• Lost time for stopping
– A car - 30 MPH to stop to 30 MPH loses 12 to 15
seconds over traveling at a consistent 30 MPH
– A truck losses 30 to 35 seconds
– For 55 MPH a car loses 25 to 30 seconds
– A truck at 55 loses 60 to 80 seconds (as do any
vehicles behind that truck)
• If delay is $13 for cars & $21 for heavy commercial
vehicles at 7%, then a 30 MPH stop is worth $0.058
• A 55 MPH stop is $0.121
• Vehicle stopping costs are $0.045 and $0.15 for cars
and trucks at 30 MPH and $0.085 and $0.30 for cars
and trucks at 55 MPH
• Total cost of stop $0.11(30) and at $0.22 (55)
• Idling delay is $0.22/min and fuel adds another $0.03
for a total of $0.25
Implications
• At high speeds, reducing mainline stops by adding
delay to the side street is typically justified.
• Early versions of the HCM virtually ignored lost
time due to stops and assigned it a value of 30%
of other intersection delay.
• At 55 MPH and with V/C ratios of .5 to .6
“snappy timing” can cause lost time due to
stopping to be 2/3 of the total delay. For a highspeed approach near capacity, lost time for
stopping would be still be over 40%.
Traffic responsive system capabilities
• Master controller:
– Uses a library of prepared system timing plans
• Most systems are capable of 100 + plans
– Variables
•
•
•
•
•
Cycle length
Offset
Splits – real time with actuated controllers
Grouping
Crossing artery synch
– Plan selection based on
•
•
•
•
Volume levels & directional distribution
Speed
Time of day
Special detection & other inputs
– Serves as a communication hub and allows remote
intersection monitoring and timing plan changes
Adaptive system capabilities
• Central controller:
– Processes data and is home to the “algorithm”
– Coordination is “real time”
• Infinite plans
– Communication hub with some monitoring
– May have an emergency backup fixed plan
– Variables
•
•
•
•
Splits
Cycle length
Offset
Grouping
– Adjustments are based on prediction of arriving traffic:
• Size of platoon
• Turn percentages
• Arrival time
Adaptive – Responsive
infrastructure comparison
Items
Adaptive
Responsive
Preliminary efforts
Training, setup and
Training and development of
timing plans
calibration
Central control system
Central computer hosting
algorithm
PC and on street master
Communication
Dedicated
Dial up
Detection
Depends on system
Normal intersection
Controllers
Depends on system
Off the shelf
Software
Proprietary license fees or
FHWA
Competitive - NEMA or 170
In operation
Set and forget ???
Periodic plan updates
Incident management
Adapts to handle
Call for special plans
Adaptive – Responsive
comparison of operation
Items
Adaptive
Responsive
Cycle lengths
Infinite
6 or more
Splits
Infinite, but small
adjustments per cycle
Multiple, plus add’l max. plus
queue response
Minimum split
Peds plus yellows & all reds
Peds treated as exceptions
Offsets
Infinite
5 or more
Dilemma zone
protection
Not with SCOOT & SCATS
With actuated operation
Phase order changes
Difficult or impossible
Readily changed
In operation
Set and forget ???
Periodic plan updates
Incident management
Adapts to handle
Call for special plan(s)
Transit priority
Priority control required
Great for timetable operation
High speed flow
All vehicles are equal
Coordination favors the mainline
Arterial time-space diagram
¼ mile spacing - 45 mph progression speed – 120 second cycle
Lead – lag
lefts
Lead lefts
Lag - lead
lefts
Arterial time-space diagram
1/2 mile spacing - 50 mph progression speed – 75 second cycle
Time-space (busway)
N
Stated objections to traffic responsive
control
•
•
•
•
•
•
Labor to develop and maintain timing plans
Expertise required to setup system
Rigid cycle lengths
Slowness of response to changing conditions
Early releases causes coordination problems
Funding is available to install adaptive control,
funding may not be available to hire staff for
operating a traffic responsive system
Actuated mainline green in coordination
Added mainline green from
no call on following phases
and force offs held in place
Mainline extension
Coordinated green
Added front end green from
unused phase time and force-offs
moved forward
TR Systems Setup Issues
• Understand what detection your master
controller will have available to make its
plan selection
– Where is capacity an issue
– Where is directionality of flow an issue
– Detection to determine offset and cycle length
may be at different locations
TR Systems Timing Issues
• System timing plans should cover a range
of representative conditions, not be a
collection that is simply created from
computer solutions that are based on
data snapshots
–
–
–
–
Round off traffic data
Chart the day’s expected flows and fluctuations
Check the “natural” cycle lengths
Constrain software for best solutions within
cycle length ranges
TR Systems Timing
• Create plan library to handle the range of traffic
conditions
• Create timing plans for saturated conditions
• Create timing plans for incidents
• Determine the “free to coordinated” threshold (about
100 vehicles per lane per hour)
• Test plans to see which conditions overwhelm and at
which point they are sluggish
• Outline a typical daily schedule of which plans are in
use at which times
• Look at how offset adjustments and cycle length
changes will be made. Have major changes occur at the
most congested intersection
The future of arterial systems
• Modern adaptive control and traffic responsive
control are not far apart
• Eventually, several adaptive control algorithms
will reside in a system and be available for use
when needed. This is similar to what happens
today with systems that switch between TOD &
TR
• Adaptive control algorithms will use normal
detector locations or use alternate detection
locations with video detection
Observations and lessons learned
• Hardware and technology can only go so far,
you still need quality people
• Early release is considered a “problem” for a
traffic responsive arterial system. For adaptive
control it’s considered a “feature”
• If you can understand it, it’s obsolete
ADAPTIVE…SHMADATIVE