- 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
- a long word describing a route
that was deleted, meaning that the relevant DOT
no longer maintains the road.
- another word for the concurrency
of two routes.
- 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 DOT
s in the United States, a change from Interstate 15 to State Route 15 will not result in a jurisdictional transfer.
- 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.
- 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
- 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 Interstate designation
. 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.
- 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.
- a route marker
with an arrow, pointing travelers to where they need to be if they want to access that route
- 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
- 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
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.
- another term for a striped median
indicated by repeated diagonal yellow lines. The term, "banjos" typically refers to the painted diagonal yellow lines themselves.
- 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.
- 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
- 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
- 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 app
lications 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 an intersection
- 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.
- 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
- 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
- the border protruding out of the edges of the signal head
from behind it, placed to help a driver's eyes to distinguish the signal indication
from other ambient light effects. Backplates are especially helpful in assisting the visibility of signal indications
around sunrise and sunset.
- the big bulky metal box housing the traffic signal controls and the computer system making the decisions. Wiring from all the handholes
will end up converging at this box.
- the summation of all sequenced phases
for a traffic signal that are executed before starting the sequence over again.
- 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.
- a colloquial term for a five-section signal 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-section head
consisting entirely of arrow indications
is the hint that this is the case. Some intersections have three-section heads
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.
- 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 single intersection
approach, and send the signals along another wire to the central control cabinet
- 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.
- 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.
- The vertical pole, to which the mast arm
- A horizontal pole attached to a vertical pole, erected for the purpose of suspending signal heads or signage above a roadway
approach. Most states west of the Eastern Time Zone use mast arms to suspend signal heads
- 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.
- 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.
- 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-section signal heads
designed for this are doghouses
- 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.
- 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.
- 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.
- 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
- 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.
--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.
- 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
- 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
. 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.
Traffic Analysis Terms
average daily traffic, a measure of the traffic volume
on a road, taken by counting the vehicles using a segment
over the course of a single calendar day. This can be further extrapolated to AADT, which is the average annual daily traffic.
the amount of traffic a segment
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.
- 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.
- the number of vehicles occupying a roadway
, measured in terms of distance. The units are usually vehicles per mile.
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).
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.
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
at a particular point in time will equal the amount of traffic demand
on that segment, at that time. An analysis of the traffic demand will seek to identify the number of trips using all the different segments
on the roadway network. This act of seeking to understand the type and number of trips using different roadways is the study of trip generation
the behavior and phenomena
driving 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.).
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