Signal Phasing
Signal Phasing
(OP)
Hi guys, I'm new to this forum. I'm a relatively new traffic engineer in Buffalo, NY with only a couple years under my belt.
My question has to do with signal phasing...does anyone know where I can find out information on phasing? I'm looking for anything that has do with how many phases a signal has, what the phases are, perm, perm+protected, splits, overlaps, etc.
I learned on the job all about phases, but I'm not totally confident in my ability because everytime i think I've got it figured out it turns out that I've missed something. I've checked the ITE Traffic Engineering Manual, MUTCD, NYSDOT, CALTRANS, HCM, and a number of other sources, but there doesn't seem to be anything concrete on paper somewhere for me to refer to.
Thanks for any help you guys can offer.
My question has to do with signal phasing...does anyone know where I can find out information on phasing? I'm looking for anything that has do with how many phases a signal has, what the phases are, perm, perm+protected, splits, overlaps, etc.
I learned on the job all about phases, but I'm not totally confident in my ability because everytime i think I've got it figured out it turns out that I've missed something. I've checked the ITE Traffic Engineering Manual, MUTCD, NYSDOT, CALTRANS, HCM, and a number of other sources, but there doesn't seem to be anything concrete on paper somewhere for me to refer to.
Thanks for any help you guys can offer.





RE: Signal Phasing
FHWA has a Signalized Intersections Informational Guide at
http://www.tfhrc.gov/safety/pubs/04091/. It is a well written, comprehensive reference for traffic signals.
I would also recommend Jonathan Reid's "Unconventional Intersection Design, Management and Operations Strategies". It is on the Parsons Brinkerhoff Quade & Douglas website at:http://www.pbworld.com/library/fellowship/reid/
It has an excellent discussion of the capacity constraints that come with signal operation.
I don't like the Traffic Signal Phasing section of FHWA's Traffic Control Systems Handbook because it implies that the movement numbering convention for analyzing signalized intersections (odd number lefts, even numbered thrus) has a one to one correspondence with signal controller phase numbers. That only occurs in a quad-left dual-ring configuration.
RE: Signal Phasing
RE: Signal Phasing
RE: Signal Phasing
NCHRP Report 233 Selecting Traffic Signal Control at Individual Intersection discusses the advantages of actuated signals over pre-timed, but it predates the current HCS methodologies.
One of the problems with HCS is it only models single-ring, sequential traffic signal phasing. When using HCS the person doing the analysis can only approximate the phase sequence of a multi-phase, dual-ring operation. SYNCHRO allows you to define the ring structure which may provide a more "accurate" analysis for dual-ring operation.
There are so many parameters that make up the analyses that calling one procedure more "accurate" than the other is probably misleading. Either method should provide a reasonable timing plan for the intersection under study.
At best, the analysis is only an approximation of the expected operation based on a statistical model of the intersection.
I generally prefer to use SYNCHRO for analyses because it will model multi-ring operation. I used to prefer PASSER over HCS for the same reason.
One caveat regarding signalized intersection analysis. Every model has some quirks. The printout should never be accepted as gospel. Computer based analysis allow for multiple iterations with little additional effort, but do not ensure the result is reasonable. Every analysis should be reviewed with some skepticism.
RE: Signal Phasing
There are three types of signal timing plans:
Pretimed, Fully Actuated, and Semi-Actuated
Actuation occurs when there are detectors present near a signalized interesection.
PreTimed means that the traffic signal bounces back and forth between the phases and with information that is "programmed or pretimed" into the controller cabinet. Usually the timings are determined through turning movement counts for different time periods (AM,PM and Mid-Day)
Semi Actuated means not all phases have detectors on the approaches; For under 35mph, the signal will most likely give green to the Major approach, until presence is detected on the Minor approach.
Fully actuated means there are detectors present on all approaches to an intersection and the controller oscillates between the major and minor approach based on the volume demand. (There is a whole lot of stuff behind this timing plan that I won't go into)
Detectors can be video, microwave, and loop detectors (there are others although in a nutshell this covers most design aspects.)
Detectors can be installed at an intersection (which indicates presence) or it be utilized prior to the intersection for volume/density monitoring.)
Grab a hold of a State DOT manual/publication and most have information detailing how to set up a permit plan timing plan.
Hope this helps---
RE: Signal Phasing
RE: Signal Phasing
If you are doing DOT work it is almost certain that your signal will be fully actuated - everything I've done for them has been. Also, when working with the DOT, they are going to go over your signal pretty thoroughly and have a lot of input into the phasing.
RE: Signal Phasing
A typical intersection has 8 phases because it has 8 movements, each of which needs its own signal heads. Therefore, the controller cabinet needs 8 load switches to turn those heads on and off, and the load switches are controlled by 8 phase outputs from the controller. The settings that control each phase can be related to the physical approach (e.g. minimum green and extension are based on detector placement and approach speed, yellow and red clearance are based on approach speed and grade), operational constraints on actuated operation (e.g. maximum green and volume-density settings), or coordination (e.g. force-offs and permissive periods).
A good source of info for phasing is the National Highway Institute course called Traffic Signal Design and Operation. That course has been taught in New York, but so far only for NYSDOT folks. Contact your local FHWA guy if you want it again with more focus on local agencies.
Rick Denney
RE: Signal Phasing
I was wondering about two seperate types of signal heads in particular, and when each one is supposed to be used and why one is preferred over the other in certain circumstances. Any help would be greatly appreciated.
The two signal heads are 1) four bulb vertical head, and 2) "dog house" five bulb head.
Thanks-
bjfan82
RE: Signal Phasing
The four section head is lighter, but since the yellow and green arrows are in the same place, you lose the spatial cue that helps you tell which arrow is lit by where it is.
If I recall correctly, it's more of a problem for the relatively rare people with total or blue/yellow color blindness than those with red/green color blindness.
Check here for more info on colorblindness, including a simulator that will show what a picture file or website looks like to people with the condition:
www.vischeck.com
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928
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