Glossary

This page is meant to define and clarify the meanings of traffic control terms and lingo. Expand.......

Words and Meanings

Signal Phasing Terms

Leading / Lagging Arrow

When a dedicated turn arrow is offered at a signal-controlled intersection, its operation will often be defined as either 'Leading' or 'Lagging'. Leading arrows happen at the beginning of the through green cycle, and generally end with a yellow arrow before the through green is finished. Lagging arrows often happen towards the end of the through green cycle, and usually have their yellow arrows tied with the through yellow, or simply rely on the through yellow entirely.

Signals that work together at either end of a bridge or overpass often feature lagging arrows to help clear out the short span of road between the two intersections, to help prevent cars from getting stuck in line while trying to turn.

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Flashing Yellow Arrow

Many intersections contain doghouse signals to allow protected left turns. With leading left arrows, there's no harm. However, with lagging left arrows, the yellow trap becomes apparent. Pretend for a moment, you're approaching an intersection. There's 2 single face through signals, and a left turn doghouse over the left turn lane. There's already a car across from you waiting for a chance to make a left. All signals turn yellow, what do you think? You think the cycle is ending, so to prevent being stuck for longer than you need to, you try to beat the red and you make the left. Here's the problem: That yellow your side received isn't what the other side received. Your side got that yellow because the doghouses at that intersection are lagging, not leading. The car across from you waiting to turn triggered your side's cycle to end early, while the other side's through traffic still has green, right as you're cutting across the road. Bam! To fix this, engineers designed the Flashing Yellow Arrow, a signal configuration designed to prevent and eliminate yellow traps. Instead of leading drivers to make their own left turn judgments, it provides a flashing yellow arrow to signify left turns might not be safe, and the driver should yield when making left turns. A very useful webpage further explaining FYAs can be viewed here.

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Terms of Physical Signal Features

Doghouse Signal

When MUTCD started requiring green arrows to be terminated with yellow arrows (instead of just going dark) on shared-face signals, many jurisdictions didn't have the vertical clearance required for a signal with 5 sections (R-Y-G-YA-GA) to be hung in a stack over traffic. To overcome this issue, the doghouse configuration came to be. This consists of a red section centered above two 2-section stacks: one of yellow & green arrows for the turning traffic, the other of yellow & green balls for the thru traffic A Left Doghouse has the arrow stack on the left, a Right Doghouse has the arrow stack on the right. This creates a doghouse shape if one were to loosely trace the configuration.


Here are the most common terms for different size configurations of a typical doghouse signal:

"Doghouse"

All sections are 12"; red is centered

"Crooked Doghouse"

All sections are 12"; red is off center

" 8-inch Doghouse"

All sections are 8"

"Delaware Doghouse"

The Red is 12", all other sections are 8"

"Pinhead Doghouse"

The Red is 8", all other sections are 12"

"Cheap Doghouse"

The R, Y, & G sections are 8", the Arrows are 12"

"Franken-Doghouse"

The Red & Arrows are 12", the Green & Yellow are 8"

Before the "Yellow Trap" signal came around, the flashing yellow arrow concept was introduced by doghouse signals [?]. These are much less common than the "Yellow Trap" signals. Below are two versions of it.

"FYA Doghouse"

Functions the same as a "Yellow Trap" signal, but with an extra clearance yellow.

"Bimodal FYA Doghouse"

Functions the same as a normal doghouse, with an extra yellow arrow that flashes with the green ball whenever the green arrow isn't lit.

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WAIT-WALK Pedestrian Signal

[Insert info here]

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Franken-Signal

When a traffic signal (whether in public or not) is composed of sections or parts from multiple manufacturers or models from the same manufacturer that aren't designed to normally go together, this signal is unofficially called a Franken-Signal, in reference to Frankenstein's Monster. Due to newer signal models being composed of individual indication sections, contractors sometimes do this in the field to help use up old sections laying around and to save money. The term Franken-Signal generally only applies to the housings; lenses are often more difficult to spot as incorrect from ground level and often don't make enough of a difference to be discernible.

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Louver

A louver is a round metal frame with vertical or horizontal slats that is placed in the signal's visor(s) to limit visibility of the respective indication. Often thought of as the poor man's PV signal, it basically does the same thing, although louvers are limited to either vertical or horizontal view limiting at a given time, whereas a PV signal can limit specific spots and zones. Louvers are usually, but not always, placed in tunnel or full circle visors. Louvers are popular places for birds to create nests.

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Stuff to sort out

Interval

TBD

Phase

TBD

Face

TBD

Aspect

TBD

n-Way Signal

A single signal assembly that has lights on n sides. These may also be referred to as 4-way signals with (4-n) blank sides. Other common trade names for them are square and box signals.

Sectional

Loosely termed as a way to describe a signal in which the various lense sections (IE Red, Yellow, Green) are physically seperate and not cast as a solid piece of metal. Both modern signals and the rodded style signals fall under this definition.

Section

Like Sectional, section refers to one housing, lens, visor, etc. assembly of a signal. A stoplight or caution flasher can be a signal of only one section.

Nonadjustable

Describes any signal, such as a 4-way, in which the body or signal assembly has fixed angles, typically 90deg, between the various viewing faces. Compare with adjustable.

Adjustable

Any signal, or correctly a signal cluster, in which the various faces can be set at differing angles from one another. Compare with nonadjustable.

Cluster

Often mistakenly called 4-ways, a cluster is a pole or spanwire group of signals that are mounted together with common hardware fittings. By definition, clusters are adjustable signals.

Visor

The hood, flange, or shield that is used to provide a tunnel effect around a lens. Common styles are the western Full Circle, exactly as its name suggests; the Tunnel (or Combination), a circle with no bottom, and the Cutaway or Cap, which is a tunnel with the sides also removed. These are typically from 7-10" long with various downward angles. Many special purpose variations exist. Angled visors are extended length full circle visors with an angled, instead of flush cut at the end to restrict the viewing angle and are produced in directional specific (right, left) variants. Tunnaways are a nickname given to older Marbelite signal visors due to their unique shape - while they were scalloped like a cutaway visor, the cut was made much closer to the front of the signal than a typical cutaway giving ti a tunnel visor appearance. A rarer version of the Tunnaway was made by Crouse-Hinds, where one side of the visor was a typical CH cap pattern, but the other side had a full tunnel shape to it. Louvered visors are circles or tunnels (albeit inadvertently converted to a circle) with an arrangement of horizontal or vertical bars installed in it. These bars are adjusted to affect the viewing angle of the signal on the street.

Cast Visor

Term referring to a signal that has the visor assembled as a nonremovable component of the body, housing, or door. Much more popular in the early days and porthole signals such as Darley and Crouse-Hinds through the Deco era.

Porthole

General descriptive term referring to several early styles of signals. These signals did not have a conventional rectangular/square door that opened to allow bulb and interior access; instead, the door was a minimial ring containing the lens and visor resembling the porthole window style on ships. In the case of Tokheim, Harrington-Seaberg, Autoflow and a few others these portholes were hinged to the body while signals such as Crouse-Hinds and W. S. Darley had hingeless doors that were removed from the signal for servicing.

Command Lens

A signal lens with a message embossed on the glass; typically STOP for the red lens, CAUTION for the yellow, and GO for the green lens. One common example of a special-purpose command lens is THANK YOU on a green tollbooth signal.

Bar Lens

A signal lens with a smooth glass "bar" through the center. Akin to some railroad signals, the bar is horizontal for red, diagonal for yellow, and vertical for green. (Adler reference?)

Chinese Arrow

A non-standard arrow indication. The cutout is much thicker, and the stem is tapered along with the arrowhead.

Programmed Visibility

A signal that is designed to project an indication to a specific lane of traffic only. This is accomplished by using a special diffractive lens that filters the light waves in one direction. Most programmed visibility signals in the USA are made by the 3M Company. Such signals are employed in intersections with potentially ambiguous geometry, or for left-turn only signal faces that are likely to be misread by straight-thru traffic. They may also be used to prevent confusion when one signal quickly succeeds another.

Four Way Independent Amber

An early signal color pattern in which the amber lamp illuminates during change from green to red (as in modern lights) as well as in red to green. Popular in the 1920's and 30's.

Overlapping Amber

(see Split Amber)

Split Amber

An early signal color pattern in which the amber lamp illuminates during the end of the green lamp cycle following which both lamps turn off and the red lamp illuminates. Popular in the 1920's and 30's.