Have you ever found yourself wondering, "what are these road-oriented nerds talking about?" Have you found yourself washed away by a massive tidal wave of jargon that could only be the result of pretentious behavior? Or more likely, you have just found a word on this website whose definition is not clear to you? Well, unlike some members of the Roadgeek community, I actually want people to understand what it is I'm talking about (otherwise, what is the point of making a website at all?). That's why I have started developing the Roadgeek Glossary, and I aim to make this the most helpful source of explaining specific terms used by road enthusiasts.
Jargon is a requisite part of most specialized hobbies and fields of study, because people need words to describe items and processes that repeatedly appear. No one wants to stop and explain the entire context of these repeated items and processes every time they happen, so they create terms that may not be immediately recognized by the average person. However, because this is a website, I have the ability to create links wherever unusual terms may appear, and those links will take you to a definition in this glossary! Wherever you see the green links on road-related terms, you can click the link to instantly see a glossary definition for that term. And on a broader scale, if you see any obscure, technical, road-related terms anywhere on the Internet or elsewhere, I will endeavor to explain those terms in this handy glossary! Lots of terms that appear on the various pages of this website will appear as links to an entry in this glossary (just like how Wikipedia does things). A lot of the terms on this website pertain to roadway types, roadway configurations, roadway geometry, striping, and signage.
Access point- a point along a roadway meeting a driveway or another roadway. The linear density of access points along a roadway will be the most deterministic factor in speed limit reductions, as the amount of traffic slowing down to make turns will create a collective tendency to slow traffic as a whole. This is the whole reasoning behind a limited-access highway like an expressway or freeway. Generally speaking, reducing the frequency of access points will enable consistently higher speeds of travel.
Alignment- the carefully-chosen linear path to which a roadway or highway adheres. Some routes may occupy multiple alignments, which is to say, multiple paths. For example, a business alignment and abypass alignment for the same route designation may both exist in the same area. See also: bannered routes.
Clinching- to drive the entire stretch of a highway, or collect every incremental division of something. This idea appeals to those of us who like to have fun making our lives into travel-based collect-athons. =)
Corridor- a linear, narrow stretch of territory
Designation- an abstract label for a road or series of roads, because we need names for things in order to manage them or communicate about them. Often, these are called route designations or routes, but designations can also be a textual road name.
DOT- Department of Transportation. It's a government organization established to build and maintain all roadways within their jurisdiction. This includes the maintenance and operation of all public rights-of-way, the corridors of public property established to house the roadways. In the United States, each state has one. There is no national DOT or national entity that maintains roadways, but there is the Federal Highway Administration, that disburses funding to various major infrastructure improvements in all U.S. states. ("DOT" is pronounced as all three letters, as an initialism; it is not an acronym that would be pronounced like the word, "dot".)
Junction- the point in horizontal space where multiple routes cross, in any capacity. Interchanges and intersections are both examples of junctions.
Right-of-Way- this term has two different meanings in the context of roadways.
In terms of land use, "right-of-way" refers to the land owned by a government entity, for the purpose of providing a roadway on that land. Said government, then, is responsible for maintaining the roadway.
In the context of driving, "right-of-way" refers to a driver's permission to proceed, which can be granted by something like a green light. For another example, a "yield" sign's purpose is to tell drivers that they must yield the right-of-way to others.
Roadway- the portion of a corridor of right-of-way designated for vehicular travel.
Route- the path traversed by a traveler, or traversed by a designated label like a route number or name
Routesnapping- the act of snapping photos of route markers with the goal of completing a collection, consisting of photos of all the route markers in a designated category. This is a term I made up! It's not common practice to use it, but I hope it will be commonly accepted someday.
Street Lamp- a structure erected along a roadway to provide light, usually mounted on a pole or post
Street Light- a confusing term that either means "street lamp," a device providing illumination, or a "traffic signal," which communicates to drivers who has the right-of-way. Do not use this term! Say "street lamp", or say "traffic signal," as these two terms are specific to their respective subjects. (I'm looking at you, Journey.)
Traffic Signal- a series of installations established to communicate to drivers who has permission to proceed, using red, yellow, and green lights, usually placed at anat-grade intersection. They can also be erected in locations where conflicts of interest occur between multiple parties over the same geographical space (like a one-lane bridge or tunnel). See below for a whole bevy of terms pertaining to traffic signals.
Beltway- a highway constructed as a bypass around a focal point, usually a large city. A fully-realized beltway will form a complete circuit that fully encircles the city or focal point.
Bypass- a thru highway routed to sidestep around the outside of a densely-populated locale, constructed for the purpose of allowing traffic to continue moving at a higher speed, without the hassle of slowing down through a heavily populated area.
Expressway- a high-speed highway whose access is limited to a certain few points. Most are either freeways or tollways, whose access points are limited only to onrampsand offramps. (Rural expressways can have some grade crossings, though; see rural expressway.)
Freeway- a high-speed highwaywith at least two lanes in each direction, whose access points are limited only to onrampsand offramps. This is the definitive limited-access highway, as access to this type of highway is the most limited.
Frontage Road, also known as access road or service road- a regular surface road hugging right along side a larger limited-access highway like a freeway or expressway. The frontage road provides access points to the homes and businesses that can't be allowed along a limited-access highway; that's what makes it a limited-access highway! When a highway is upgraded to a freeway or expressway, the accesses to the establishments alongside that highway can't be directly connected to the upgraded highway. Frontage roads provide a solution to that problem, serving as the midpoint between the establishments and the highway. A frontage road doesn't have to flank a freeway or expressway, though; sometimes they will run alongside slower roadways because of high traffic volumes or changes in the roadway's elevation.
Highway- a roadway designed for fast, efficient travel over long distances. This term includes two-lane highways, four-lane highways, expressways, tollways, and freeways--as long as they are fast, efficient focal points for trips that can comprise long distances.
Interstate- a freeway (or tollway) comprising part of the nationwide Interstate highway system. Note that not all tollways and freeways in the United States are Interstates, but the rule is that all Interstates are freewaysor tollways.
Rural Expressway- a four-lane high-speed highway whose access is limited to interchanges at major junctions with other highways, but grade crossings with lightly-traveled crossroads and field entrances are allowed. US Route 36 in Missouri is a good example. State Route, or State Highway- a routedesignation belonging to a state's system thereof; each state operates a distinct system of highways apart fromUS Routes andInterstates. They are the most local level of highway maintained by a state's DOT (the US Routes and Interstates within a state's borders are also maintained by the state's DOT).
Surface Road- a term used to describe any road that is not an expressway or freeway.
Spur- a branch from a main route that doesn't double back to said main route. It sticks out from the main route and then ends elsewhere.
Tollway- a limited-accesshighway similar to a freeway, with the caveat that users are charged as they drive the highway. Tollways collect tolls at toll booths, which can be located along the mainline of the highway, or along an offramp or onramp.
Trunk Highway- another term for "main road" or "primary road", for people who like talking about trees.
US Route- a highway, part of the original nationwide system of highways devised in the 1920's. US Routes all originated as surface roads, but some US Routes have been supplanted by Interstates and other freeways. The term is interchangeable with "US Highway", "Federal Route", or "Federal Highway", in the United States.
Roadway Features
Barrier Median- a length of non-traveled space between lengths of pavement carrying opposing directions of traffic, delineated by raised concrete or curbs.
Channelizing Island- a non-traveled space, often triangular, separating turning traffic from the thru traffic at an intersection.
Carriageway- the stretch of pavement carrying traffic in one particular direction. Expressways are usually comprised of two carriageways, one for each direction of travel.
Collector/Distributor Lanes- a lane or set oflanes separated off to the side of the main thru lanes, allowing separate interaction with ramps or crossroads. This interaction can slow down traffic, especially when there is a high density of access points to or from ramps. That's why a separate lane or set of lanes will be created for ramp access--to prevent slowdowns impeding the thru traffic.
Crest Curve- avertical curve where the slope becomes more downward, or more negative, forming somewhat of a crest or hilltop. Crest curves are notorious for blocking a driver's ability to see beyond it. The following three scenarios all qualify as crest curves: a steep uphill slope becoming a more gradual uphill slope, an uphill slope becoming a downhill slope, and a gradual downhill slope becoming a steeper downhill slope.
Curb- a small upward protrusion lining the side of a roadway, comprised of concrete, usually used to channelize traffic and/or rainwater. Most curbs protrude upward from the edge of a roadway by six or twelve inches (in the United States).
Flyover Ramp- an interchange ramp that brings traffic upward over the main highways, to provide a more gradual horizontal curve allowing for faster, more efficient travel. Flyover ramps are usually for the left turning movements at theinterchange. Flyover ramps are usually a faster alternative constructed for situations where a slow-moving tight turn on a cloverleaf ramp won't facilitate enough speed or traffic volume.
Grade Crossing- a point where two travel corridors intersect at the same elevation; e.g. an intersection. The term can be used to describe two roadway grades meeting at an intersection, but it's usually used in reference to a point where a roadway crosses a railway at the same elevation or grade (i.e. not an underpass or overpass).
Grade Separation- a point where two travel corridors meet the same point in horizontal space, but one passes over the other (i.e. an underpass or overpass). Sometimes, the term is used to describe the actual act of changing a grade crossing to a situation where one corridor passes over the other one.
Horizontal Curve- A curve traversed by a travel corridor through horizontal space; i.e., a turn traversed by a single railway or roadway. This is what most people think of when they hear the word, "curve", but vertical curves also exist as a very important consideration in roadway design! Horizontal curves are circular arcs by design (very rarely, a spiral curve might be used, especially on interchange ramps). The term "horizontal curve" is used to distinguish from vertical curves; see: vertical curve.
Interchange- a set of ramps providing access from a expressway to and from one or more other roadways. A complete interchange will have at least one offramp from and one onramp to each direction of travel on the expressway, connecting to a crossroad or surrounding district. Some interchanges, however, are incomplete (partial) by design.
Intersection- a point where two roadways meet. As traffic on multiple roadways meet at the same point, they will compete for the right to occupy the intersection, necessitating traffic control measures such as stop signs, yield signs, or traffic signals. Thus, intersections become points where traffic will wait to enter an intersection, due to the presence of one or more of those traffic control measures.
Jughandle- an offshoot of the main road meeting an intersecting road at a point separate from thejunction point of these two roads, meant to be used by traffic turning from the main road. Traffic intending to turn onto the intersecting road is routed on this offshoot to a separate intersection, where the traffic is allowed to turn. The jughandle is similar to an offramp, except that the two roads forming the junction are not necessarily grade-separated. The jughandle doesn't have an onramp to serve as its counterpart. Jughandles are used in situations where priority is meant to be given to thru traffic, over the turning traffic.
Lane- also known as Traveled Lane- a corridor of space defined for one vehicle to move through on a roadway. Every roadway needs to provide at least one lane in each direction of travel, which is to say, one direction plus the opposite direction. (Country roads with very low traffic volume will often obey this rule loosely.) Roads designed to carry more volume can have two, three, or even several lanes of travel in each direction (though this becomes fruitless in facilitating further volume if there are lots of places to turn). Lanes will have a consistent width wide enough to accommodate a standard-sized vehicle like a car or truck. A width of roadway can hold as many vehicles as there are lanes. (A "wheel lane" is something different; for that, see wheel lane.)
Median- a null space between the lanes or sets of lanes carrying opposite directions of traffic. Keeping one direction of traffic away from the oncoming traffic can help avoid head-on collisions an improve driver comfort. It can also be installed to prevent left turns from minor access points like side streets and driveways (while an inconvenience for those at the minor access points, this maintains motion for the thru movements, comprising the majority of traffic). Medians can be drawn out by yellow paint stripes on the pavement, carving out a null space in the middle of the road (see: striped median), or they can be green space lined by shoulders, curbs, or concrete walls (see: barrier median).
Merge- a point along a roadway where the pavement designated for one direction of traffic narrows, in a way where the number of vehicles that can move side-by-side decreases by one. (It should never decrease by more than one, as a decrease by just one is a dangerous enough situation already.) A merge can be a point where a lane ends, as a roadway designed for more traffic becomes one designed for less traffic; or, a merge can be a point where a ramp converges with another roadway moving in the same direction. Both are situations where multiple lanes converge, and drivers need to carefully position themselves such that they don't sideswipe a vehicle from an adjoining lane or ramp. In order to be safe, merges must be constructed with a very gradual taper into the narrower configuration, to allow drivers the time to react to the changing situation and shift themselves around. The reduction in capacity creates a bottleneck, the most classic and notorious source of traffic jams.
Offramp- an interchange ramp splitting gradually away from the main roadway, to allow access to another roadway. This can also be called an "exit ramp".
Onramp- an interchange ramp from a connecting roadway, gradually merging with the main roadway, providing an entrance to the main roadway. This can also be called an "entrance ramp".
Overpass- a bridge crossing carrying the roadwayof interest over something else, which might be a railway, a waterway, or another roadway.
Pavement Marking- a general term for markings applied to a roadway's pavement surface. Pavement markings are traditionally latex paint, but many pavement markings today can also be comprised of longer-lasting technological materials, such as thermoplastic. Most pavement markings are oriented longitudinally (parallel to the direction of vehicular movement), channelizing traffic by separating the pavement width into multiple lanes. There are also symbolic pavement markings, positioned in the middle of a lane, and transverse pavement markings that cut across the roadway, like stop bars, yield bars, and crosswalks.
Porkchop- a colloquial term for the triangular islands used at entrances designed to permit only right turns in or out of a driveway or other access road. Highway corridors allowing too many turning movements per unit length cannot operate at high speeds; as left turns especially bring the mobility down for thru traffic, road officials will sometimes propose these "right-in, right-out" connections to driveways and side roads.
Rotary- a high-speed circuitous one-way roadway with various inlets and outlets used to manage and facilitate movement of traffic. Compared to roundabouts, rotaries tend to move at higher speeds and take up a larger area, since higher speeds require more gradual curves. Rotaries tend to be the choice for highways and interchange ramps, whereas roundabouts tend to be found on low-speed streets and surface roads.
Roundabout- an invention created to manage and facilitate the movement of traffic at intersections between multiple roads, consisting of an added small circuitous one-wayalignment of roadway centered at the point where the roads would intersect. Roundabouts tend to be slower than rotaries, used on surface roads and low-speed streets.
Rumble Strips- a closely-packed sequence of shallow grooves carved into the roadway, created to generate a warning noise when vehicle tires come in contact with it. There are two typical applications of rumble strips, as follows. First, a short set of rumble strips spanning an entire lane or approach can be used to warn drivers of an upcoming hazard or stop sign. Second, an extensive narrow stretch of rumble strips can be installed along longitudinal pavement markings delineated a roadway's lanes, as a method of warning drivers when their vehicle is drifting out of its intended lane.
Sag Curve- a vertical curve where the slope of the roadways becomes more positive, or more upward. Sag curves limit visibility at night, as headlight beams will just hit a couple spots on the pavement ahead, rather than continuing forward. The following three scenarios all qualify as sag curves: a steep downward slope becoming a more gradual downhill slope; a downhill slope becoming an uphill slope; and a slight upward slope becoming a steeper uphill slope.
Shoulder- a null space provided outside of the designated travel lanes along a roadway. Such a null space is provided in case people need to pull their car to the side of the road, so the car exits completely or at least partially out of the traveled lanes. Shoulders can be fully paved, partially paved with gravel, or entirely gravel. A shoulder can be mud or dirt, too, though, which can get your wheels stuck, so be careful of the surface you pull your car onto!
Superelevation- the act of banking or twisting the surface of the roadway, so it slopes downward toward the inside of a horizontal curve. Superelevations are constructed to promote safety, providing somewhat of a counter against the inertial tendency for a vehicle to want to continue out the outside of the horizontal curve, off of the roadway. This is sometimes called simply a "super" in the industry. Superelevations help to provide rider comfort along horizontal curves as well, preventing the jerking of passengers' bodies toward the outside of the curve. (The first time I encountered this concept was in the Rollercoaster Tycoon computer game, where you have the option of banking the rollercoaster tracks along the horizontal curves, to reduce the G-forces jerking riders toward the outside of the curves.)
Switchback, also known as Hairpin Turn- a sharp horizontal curve in a roadway with a deflection angle nearing or eclipsing 180 degrees, usually included in the pathing of a roadway's climb or descent on a steeper slope. These are much more common in mountainous regions with heavy geographical constraints on where one can construct a road.
Underpass- a bridge crossing carrying the roadway of interest under something else, which might be a railway, another roadway, or an important point of interest on the surface.
Vertical Curve- the general term for the designed changes in grade (vertical slope) that occur along a roadway. While horizontal curves turn a road to the left or the right, vertical curves will change the road to a more uphill slope or a more downhill slope. Vertical curves are designed as parabolas, meaning they are modeled by quadratic functions (and you thought you'd never see that math lesson used in real life!). Vertical curves have two different types. A vertical curve changing to a more uphill, or positive, slope is known as a sag curve. The other type of vertical curve changes the road to a more downhill, or negative, slope, and it's called a crest curve. Not every vertical curve transitions the road from a negative slope to a positive one, or from a positive one to a negative one. For example, a vertical curve where a slight positive slope transitions to a steeper, more positive slope, is still a sag curve. For more, see: crest curveand sag curve.
Wheel Lane- a narrow width of pavement situated at the right lateral position to get continually run over by the overwhelming majority of vehicle wheels. Each traveled lane, channelizing a row of cars, contains two wheel lanes, which each channelize the movement of a row of vehicle wheels. The wheel lanes are often a main focus of pavement engineering, as they mark the long-narrow sites of the most wear and tear on the roadway.
Wye, Wye-Junction, or Wye Split- a three-way junction, where each approach curves toward the other two via a triangle of three connecting two-way roads. (Example)
Bannered Route- a subsidiary route of another route with the same number, using a word as a sort of prefix. The labeling of the route in this way communicates that it is like an alternate version of the main route. Common banners include "truck", "alternate" (often abbreviated to "ALT"), "business", and "bypass".
Business Route- an alternate alignment of the main route that passes through the business district, or downtown, of a city or town. Usually, the provision of an alternate business alignment occurs after a fast-moving bypass is constructed around the city or town, but the original route through town is preserved for the sake of locals, whose origin or destination is in that city or town. Some business routes are business spurs, which provide access from the main route to the business district in only one direction, instead of in two.
Child Route- a route operating as a branch or subsidiary of a more prominent route, whose number is derived from the number of said prominent route. For example, in the numbering schemes of both US Route and Interstate Routes, routes with 3-digit numbers are branches of those with 2-digit numbers, and the final two digits match those of the 2-digit number (e.g., Interstate 485 is a branch of Interstate 85). See also:parent route.
Decommissioned- a long word describing a route that was deleted, meaning that the relevant DOT no longer maintains the road.
Duplex- another word for the concurrency of two routes.
Jurisdictional Transfer- a legal agreement to transfer the ownership, governing authority, and maintenance responsibilities of a length of road from one agency (DOT) to another. For example, this might be a municipal road becoming a county road, or a township road becoming a municipal road. Not all changes in the route designation for a certain roadway involve a transfer to another jurisdiction; for example, since state routes, US Routes, and Interstates are all maintained by state DOTs in the United States, a change from Interstate 15 to State Route 15 will not result in a jurisdictional transfer.
Multiplex- another word for concurrency, where multiple route designations occupy the same section of roadway. When that number of routes is two, the word "duplex" can be used. Some use the word "triplex" when the number is three.
Parent Route- the main route after which a subsidiary branch route is numbered. These subsidiary branch routes are analogously called child routes. Agencies will name subsidiary routes after a parent route by adding a digit to the front or back of the parent route's number, adding a letter suffix to the parent route's number (see: suffixed route) adding a word prefix on a banner sign above the route to indicate an alternate alignment (see: bannered route).
State Route- a route designation given by a state's government. Every state in the Union operates a network of state highways that aren't covered by a US Route or Interstatedesignation. While some argue that different states have different technical terms for what they call the roads on their state-level highway network, this term is a blanket term that includes the numbered state designations in any state. State highway is a functionally equivalent term.
Suffixed Route- a route number with a letter at the end, indicating a route that's an alternate or offshoot of the main course. Example: Massachusetts Route 3A is a suffixed route, serving as an alternate to Massachusetts Route 3.
Assembly- a word used to indicate the whole of a signage display, including posts and their attached panels.
Banner- a sign panel with an additional word adorning the top of it, commonly used above route markers. Banners can indicate direction, the start or end of a zone or designation, an alternate version of a route, or something that's happening with a route (see: bannered route).
Gantry- a truss-like structure erected above the span of a width of roadway, to which signs and potentially other communicative devices are attached for the purpose of suspending them above the roadway. This allows drivers to see signs and other communicative devices in front of them, without them having to twist their necks to the left or right.
Guide Marker- a route marker with an arrow, pointing travelers to where they need to be if they want to access that route.
Junction Marker- a sign assembly announcing the upcoming appearance of a route, usually in the form of a route marker with a "JCT" symbol.
Reassurance Marker- A route marker, usually with a directional banner, reassuring travelers by telling them which route they are using, and potentially in which direction. Also known as a trailblazer.
Route Marker- a sign with a number in some kind of symbol, with the number indicating a route number, and the enclosing symbol representing the system containing that route.
Trailblazer- A route marker, usually with a directional banner, affirming to travelers which route they are using, and potentially in which direction. Also known as a reassurance marker.
Unisign- when a complex sign assembly, that could consist of multiple standard sign panels, is combined into a single large sign panel with many different symbols. The term is not for just any sign panel with multiple elements mentioning multiple pieces of information; the term reserved specifically for sign panels that combine multiple sign panels into one. The sign assemblies transformed into unisigns are almost always those containing route markers.
Striping Terms
Banjos- another term for a striped median indicated by repeated diagonal yellow lines. The term, "banjos" typically refers to the painted diagonal yellow lines themselves.
Centerline- a linear stretch of paint dividing the sides of the roadway intended for one direction of travel from each other. In the United States, the centerline must be painted with yellow paint. It is either a pair of close, parallel, solid yellow lines, a solid yellow line alongside a yellow dashed line, or a single yellow dashed line. The parallel pair of solid yellow lines indicates that neither direction of travel can cross the centerline to pass; a solid yellow line alongside a line of yellow dashes shows that passing is allowed in one direction only. A lone dashed yellow line does not restrict passing.
Crossbuck- a white symbol containing a large X with a small capital "R" on its left and right sides, painted on the pavement to provide advance notice of a railroad grade crossing.
Dashed Line- a line type with significant dashes broken up with spaces, used to define lane extents and communicate the permission to make a controlled, short-lived shift beyond the line into the lane beyond it. In other words, you are allowed to cross a dashed line. The dashed white line used to separate adjacent lanes of traffic moving in the same direction, and the dashed yellow line indicates permission to pass using the oncoming lane (when safe). The dashes are usually on the order of ten feet in length, with spacings a few times longer than the dashes. Exact distances for dash lengths and spacings are not provided in the MUTCD, and different road agencies will provide different specifications for these distances. However, dashed lines will always be at least somewhat similar in style from agency to agency, as every agency will still have to distinguish them from dotted lines, which are different (see: dotted line).
Dotted Line- a line type with short and frequent breaks in it; the segments of paint and the broken segments will both be shorter than those of a dashed line. The painted segments on a dotted line will only be a couple to several feet in length, which look very short to fast-moving drivers; the breaks in the line will be a couple to a few times longer than the painted segments (as an example, Illinois's DOT defines a dotted line as having 2-foot painted segments and 6-foot breaks). Dotted lines are used at the births and deaths of lanes that diverge from or merge into a persisting lane, and at other incidental applications where the line is to play a minor role. Applications include the taper into a new turn lane, the merge at the tail end of an onramp, and the delineation of a turn movement through anintersection.
Edge Line- a solid white line defining the outer edge of the outermost lane on a roadway. The null space outside this line is defined as the shoulder.
MUTCD- The Manual on Uniform Traffic Control Devices. This guidebook standardizes striping and signage practices across the entire United States.
Solid Line- a consistently painted line with no breaks. Solid lines usually communicate a restriction against crossing it, unless a driver is turning to or from a driveway or other connecting alignment.
Stop Bar- a 24-inch thick transverse line painted across the traveled lanes to indicate the position of where a vehicle is to wait for its chance to proceed, at a stop sign or traffic signal.
Striped Median- a null space between opposing lanes of travel, defined by lane lines on the pavement rather than anything structural (see also: barrier median). A striped median will be outlined by yellow lines, and the center will be marked as a null space by repeated diagonal yellow stripes (as long as this null space is wide enough) (see: banjos).
Suicide lane- a center lane between the opposing lanes of travel, which allows vehicles from either direction to occupy it as said vehicles wait for a chance to turn left.
Traffic Signal Terms
Backplate- the border protruding out of the edges of the signal head from behind it, placed to help a driver's eyes to distinguish the signalindication from other ambient light effects. Backplates are especially helpful in assisting the visibility of signalindications around sunrise and sunset.
Cabinet- the big bulky metal box housing the traffic signal controls and the computer system making the decisions. Wiring from all the handholeswill end up converging at this box.
Cycle- the summation of all sequenced phases for a traffic signal that are executed before starting the sequence over again.
Detector Loop- a circuit of coiled wire laid on or under the pavement, usually about six feet in diameter, at the approach to a signalized intersection, installed to recognize traffic that has arrived at that leg of the intersection. The coils of wire use electromagnetic induction to send a signal to the central controls (in the cabinet), so the system can recognize that it needs to accommodate the traffic triggering the signal.
Doghouse- a colloquial term for a five-sectionsignal head where the indications of a certain color are situated next to each other, at the same height. The bottom row has a green arrow next to a green ball, the middle row has a yellow arrow next to a yellow ball, and the top row is just a single red ball in the top center. The overall shape of the head looks like a square with a top that tapers upward toward the center, similar to the shape of a doghouse.
Flashing Yellow Arrow- a newly-popular way of signalizing protected/permitted turns, where a flashing yellow arrow indicates that turning is permitted, but not protected. Protection may be provided as well, in a separate phase when a green arrow is displayed; a four-sectionhead consisting entirely of arrow indications is the hint that this is the case. Some intersections have three-sectionheads for a turning movement, showing a red arrow, a steady yellow arrow, and a flashing yellow arrow. These signal heads announce when turning is permitted, but provide no phase when turning is protected.
Handhole- a square manhole in the ground that's an intermediate focal point for sub-surface traffic signal wiring. Handholes will often collect the wiring underneath a singleintersection approach, and send the signals along another wire to the central control cabinet.
Head- a signal head is the piece of hardware housing one set of several lights, arranged in a short row or column. Heads are usually mounted on a post, a mast, a mast arm, or by suspension on overhead cables.
Indication- the symbol made visible by whichever light is activated on a signal. Usually the symbol is either a full, glowing circle of light (called a "ball" indication), or an arrow. Signal indications for vehicular traffic are always red, yellow, or green; for pedestrian signals, indications are orange or white. Typically, each signal section holds one light bulb (or conglomerate of LEDs) intended for one indication.
Mast- The vertical pole, to which the mast arm is attached.
Mast Arm- A horizontal pole attached to a vertical pole, erected for the purpose of suspending signal heads or signage above a roadway orintersection approach. Most states west of the Eastern Time Zone use mast arms to suspendsignal heads above intersection approaches.
Phase- a designated period of time occupied by a signal's displaying of a certain indication. A basic signal pointing toward one approach of an intersection will have a green phase, followed by a short yellow phase, and an even shorter all-red phase. Then the other road at the intersection will have a green phase, followed by its short yellow phase, and another all-red phase. When we loop back to the beginning, we complete what we call a signal's cycle. Signals will also sometimes have turning phases, showing left or right arrows corresponding to permissions or restrictions given specifically to the drivers seeking to make a turn.
Protected-Only Turn- a signal approach where turning is forbidden outside of the green arrow phase for those seeking to turn in a certain direction from that particular approach. In situations like this, you will likely either see a red arrow indication distinguishing from the green arrow, or a sign that says "Left turn on green arrow only." At the very least, the turning traffic will have a signal head provided exclusively for the turning movement.
Protected/Permitted Turn- a signal approach where turning is protected by a green arrow provided just for those making the turn, but turning is also allowed when a green ball indication is pointing their way. This situation is announced by the green arrow indication coming from the same signal head as a green ball indication (the five-sectionsignal heads designed for this are doghouses and towers).
Section- a front-facing part of the signal head home to one light bulb or indication. The classic and most basic signal head has three sections: a red ball section, a yellow ball section, and a green ball. (We say "ball" to specify that we're not talking about an arrow or something.) Other common signal heads can have four sections or five sections.
Visor- a piece of plastic installed around a signal section, protruding forward, to prevent sun glare from intervening with a driver's ability to see the lights.
Cloverleaf- the classic fully-directional interchange design, named for the four ramps making 270-degree turns at the interior of the interchange, which together form the shape of a four-leaf clover if viewed from above. These four "clover leaves" form the left turns from one highway to the other; the outer ramps draw a diamond just on the outer periphery of the "clover leaves", and they host the right turning movements. An interchange cannot be considered a full cloverleaf interchange unless it includes the four inner "cloverleaf" ramps and the four outer ramps for the right turns. (Interchanges with a small number of these ramps missing will be considered as certain varieties of partial cloverleaves.) Click here to see a diagram and explanation on the Geometry Page.
Diamond Interchange- the simplest and most common interchange, linking anexpressway to a crossroad (that isn't another expressway). Each direction along the expressway has an offramp and an onramp, making a total of four ramps. The onramp from one direction along the expressway and theofframp to that same direction meet the crossroad at a four-way intersection, and an analogous setup for the opposite direction of theexpressway sits on the other side of the expressway. The ramps appear to make a diamond shape, when viewed from above, giving the interchange its name. Click here to see a diagram and explanation on the Geometry Page.
Directional Interchange- an interchange where all ramps orient travelers in a particular direction on the destination road. Interchanges between multiple freeways must be fully directional, for example. Some interchanges have some directional ramps and some non-directional ramps (see: partial cloverleaf). A fully-directional interchange has strictly directional ramps.
Directional Ramp- an individual ramp that orients travelers in a particular direction on the destination road. The ramp will reach the destination road as an onramp, at a merge.
Diverging Diamond Interchange (DDI)- a new type of interchange configuration, not implemented until the last decade or so, having four ramps with the same locations and orientations as in a traditional diamond interchange, but with traffic on the crossroad driving on the left side of the road along the stretch between the two access points connecting to the interchange ramps. This takes at least a diagram, and probably a whole educational video, to adequately explain. The advantage of this type of interchange relates to signal phasing, as only two signal phases are required to service all traffic. The intersections with ramps on a traditional diamond interchange require signals having three phases each; by reducing the number of phases when traffic is waiting to proceed, more mobility is enabled. Also, diverging diamonds don't occupy a larger footprint than that of traditional diamonds, making it easier to retrofit existing interchanges with this new design. Click here to see a diagram and explanation on the Geometry Page.
Folded Diamond- A modified version of the diamond interchange, with the same four ramps serving the same four purposes, but all four ramps are located on one side of the crossroad. Two ramps form side-by-side horseshoes, like a rotated number "3", and the other two ramps hug the outside of the horseshoes until turning inside out to a point. The overall shape is like this: "3}". This interchange design is handy for situations where building on one side of the crossroad is difficult or impossible, like when the surface road runs alongside a river or railroad track. Click here to see a diagram and explanation on the Geometry Page.
Fountain Interchange--my term for a folded diamond interchange. This is a term I made up! It's not an established term; admittedly, I'm just trying to make it a thing.
Partial cloverleaf- an interchange with some directional ramps, but also with at least one ramp that meets the crossroad at an at-grade intersection. There are many different interchange configurations that qualify as partial cloverleaves, and most are some kind of hybrid between a diamond and a cloverleaf interchange.
Single-point urban interchange (SPUI)- an interchange to a surface road (non-expressway) similar to a diamond interchange, but all four ramps converge at a single point, forming one intersection at the surface road, either above or below the expressway. (Since not all SPUI's occur in urban areas, I have been going rogue in various places on my website, calling them simply, "single-point interchanges".) Click here to see a diagram and explanation on the Geometry Page.
Trumpet- the classic, fully-directional terminal interchange, named for its shape resembling that of the valve on a trumpet. This is the choice interchange for situations where some roadway terminates at an expressway, freeway or tollway. The right turns are direct directional ramps curving to the right, unobstructed. The left turns are continuations of the terminating road across the thru expressway, with one of them wrapping into a 270-degree turn; the other follows along the outside of that 270-degree loop, for about 135 degrees of that turn, before bending the opposite way and joining the highway. The loop and the adjoining ramp can wrap to the left or the right; either configuration is acceptable. Click here to see a diagram and explanation on the Geometry Page.
ADT- average daily traffic, a measure of the traffic volume on a road, taken by counting the vehicles using a segment of roadway over the course of a single calendar day. This can be further extrapolated to AADT, which is the average annual daily traffic.
Capacity- the amount of traffic a segment of roadway is capable of supporting. Capacity is similar to the volumetric flow of water through a pipe, in that a higher speed traveled by the traffic or water will increase the capacity, as it means more transportation of volume through the system.
Demand- the number of vehicles seeking to use a particular segment of roadway. Demand is typically measured in real-time, by simply counting the cars actually using the roadway on a given day (a boring and tedious process). When demand exceeds capacity, traffic jams will occur.
Density- the number of vehicles occupying a roadway, measured in terms of distance. The units are usually vehicles per mile.
Headway- the spacing that road users will allow ahead of their vehicle, between their vehicle and the next vehicle ahead of them. The capacity of a roadway is figured based on dividing the total area of a roadway by the area occupied by individual cars; that area occupied by an individual car has to also include its headway, to provide an accurate figure of the maximum number of cars potentially using the roadway at a specific point in time. Drivers moving at higher speeds will create more space between themselves and the vehicle ahead of them; thus, vehicles occupy more space overall when their speed is greater. Headway can be measured as space headway (linear feet between vehicles) or time headway (seconds of time between consecutive vehicles reaching the same point).
Segment- a piece of roadway between two designated access points. Because vehicles will turn onto and off of a roadway at its various access points, the traffic count on one segment of roadway can and will be different from the count on an adjacent segment. The traffic demand on a roadway will always vary between Point A along that roadway and Point B on the same roadway, as long as there is at least one access point between A and B. That's why we have to analyze down to the detail of each segment, not just each roadway. Chopping up a road network into a series of defined pieces is the "network segmentation," and it's an important preliminary step in traffic analysis. You already know what a segment was, but I put this definition here so I could talk about this part of the analysis.
Trip- exactly what it sounds like, a trip is a vehicle's path from origin to destination. The number of trips that utilize a particular segment of roadway at a particular point in time will equal the amount of traffic demandon that segment, at that time. An analysis of the traffic demand will seek to identify the number of trips using all the different segments of roadway on the roadway network. This act of seeking to understand the type and number of trips using different roadways is the study oftrip generation.
Trip Generation- the behavior and phenomenadriving road users' decisions to embark on a trip and to select whichever combination of roadway segments that determines their route. The study of trip generation is paramount in learning the traffic demand on the various segments comprising a roadway network (this website is not sponsored by Paramount Pictures, Inc.).
Volume- the number of vehicles using a segment of roadway during a certain period of time. It usually is measured in vehicles per hour. Adding together the volumes observed over a 24-hour period will yield the ADT.
