Signal Phasing

What sort of information are you looking for?

FHWA has a Signalized Intersections Informational Guide at
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: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.

^ That's perfect thanks so much. Well I mostly just wanted to get a better feel for actuated versus pretimed because I use HCS a lot and the way other engineers code in the phases with actuated and pretime gets me confused sometimes. And with Synchro, coding in the signal phases there is sometimes more complicated (to me).

Actually that website doesn't talk much about Actuated signals. Do you know of anywhere that explains that? Thanks.

Are you asking for information on actuated signals in general, or on how to model actuated signals?

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.

Hi--I do a significant amount of signal timing and phasing in PA.
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---

If you ask a specific question regarding phasing, I can probably help you. I've been in the signal phasing, signal timing, signal coordination business for 12 years.

I'm a traffic engineer in Rochester who over the past few years has gotten into signal timing/phasing myself. First of all, I would say thoroughly read through Chapter 11 of the Highway Design Manual and try to understand it. Secondly, try to get your hands on some recent plans with signals on them, look them over and try to understand what is going on. There is a LOT that goes into signals beyond phasing, such as wiring, detectors, switch packs, etc. Also section 680 of the Std. Specs is surprisingly helpful with wiring and switchpacks.

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.

A signal phase is the logic in a signal controller that controls a load switch. The load switch is what turns a group of traffic signal heads that serve a specific movement off and on. It's as simple as that.

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

Thanks for all your responses so far guys!

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

The five section cluster is less confusing for color-blind drivers.

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:



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