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Glossary of Road & Traffic Terms

This page last updated June 29, 2019


Below is a list of common highway-related words and terms, both the technical jargon as well as layman's terms, and their definitions, especially as they apply here in Texas and San Antonio.

acceleration lane - a short lane at the end of an entrance ramp that allows entering traffic to build-up speed before merging into the main travel lanes. In San Antonio, and in Texas in general, acceleration lanes are not as common as in other locales.

access road - the common term in San Antonio for a frontage road.

arterial - a main road or thoroughfare. Arterials are typically divided into "major arterials" and "minor arterials".

auxiliary lane - a lane that is added to a freeway at an entrance ramp and then is subsequently dropped at the next exit ramp. Compare with acceleration lane, deceleration lane, mainlanes, and collector/distributor.

average annual daily traffic (AADT) - a traffic count that counts the number of vehicles that pass a given point on a road during a 24-hour period, averaged over several counts taken during a 12 month period.

Barnes Dance - a colloquial term for a pedestrian scramble crossing. See pedestrian scramble.

bent - technical term for a bridge pier. Commonly referred to as a "bridge column".

Botts' Dots - colloquial term for raised pavement markers, also known as "traffic buttons" and "turtles".

braided ramp - a slip ramp configuration where an entrance ramp passes over an exit ramp, or vice-versa. There are numerous examples in San Antonio, such as the ramps along both sides of Loop 410 between Vance Jackson and Jackson-Keller. Braided ramps eliminate weaving in areas with limited room for conventional slip ramps. See also slip ramp.

cat's eyes - colloquial term for reflectorized raised pavement markers.

centerline mile - technical term for one mile of a roadway, regardless of the number of lanes. Compare with lane mile.

changeable message sign (CMS) - another term for a variable message sign.

clearway - the area adjacent to a roadway that, by design, is kept clear of immovable objects for safety reasons.

cloverleaf - an interchange that has four circular ramps that turn at 270 degrees to provide for left turn movements (which, when viewed from above, resembles a four-leaf clover) and four outer 45-degree ramps for right-turn movements. Many people refer to any freeway-to-freeway interchange as a "cloverleaf", but the term is reserved for the specific type of interchange with circular ramps. Compare with directional interchange.

collector - a street that provides connectivity between an arterial and local streets. The main street into and through subdivision is typically a collector.

collector/distributor - a roadway that runs parallel to but is separate from the mainlanes on a freeway that provides connectivity between the freeway mainlanes and another roadway, typically the ramps to another freeway. For instance, when you exit from I-10 to get onto Loop 1604 on the northwest side, you first use a collector/distributor road, then the ramp to 1604, then another collector/distributor road before you actually enter the Loop 1604 mainlanes. A similar arrangement exists on I-35 and Loop 1604 in Live Oak. Some people mistakenly conflate collector/distributors and access roads.

commuter rail - a type of public rail transportation that connects a central city with its outlying suburbs, satellite towns, or another city. Many people interchange the terms "commuter rail" and "light rail", but this is incorrect. Commuter rail differs from light rail in that is uses larger, heavier trains (such as subway or Amtrak style trains) that run entirely on exclusive rights-of-way. Commuter rail also specifically differs from light rail in that it connects a central city with its outlying areas and has far fewer stations and less-frequent schedules (headways) than a typical light rail system.

concurrency - when a single physical roadway shares two or more route numbers. An example in San Antonio is the freeway on the western edge of downtown, which is I-10, I-35, and US-87. Sometimes called "multiplexing".

connector - a ramp, often elevated, that connects two highways. These are commonly just referred to as a "ramp".

controlled-access - a type of roadway whereby traffic can only enter and exit at specific designated locations (typically entrance and exit ramps). Controlled access roads are generally referred to as freeways or expressways.

crossover - a paved area in a median of a divided highway where traffic is allowed to cross over to the other side of the highway or to an intersecting road. Compare with turnaround.

deceleration lane - a short lane just prior to an exit ramp that allows exiting traffic to reduce speed before leaving the main travel lanes. In San Antonio, and in Texas in general, deceleration lanes are not as common as in other locales.

diamond interchange - an interchange between a freeway and surface street where the entrance and exit ramps form a diamond shape as viewed from the air. In most places, the ramps connect directly from the freeway to the surface street, and so the diamond pattern is obvious. In Texas, however, the ramps usually connect to a frontage road, so the diamond may be "elongated". In San Antonio, an example of a typical diamond interchange without frontage roads is the US 281 exit to Hildebrand or the I-37 exit to Hot Wells. A couple of examples of a diamond interchange with frontage roads are I-35 at O'Connor and Loop 410 at Ray Ellison. In congested areas with frontage roads, diamond interchanges are oftentimes replaced with X-interchanges to reduce weaving on the mainlanes. Compare with X-interchange.

directional interchange - an interchange, usually a freeway-to-freeway interchange, where connectors, typically elevated ("flyovers"), provide direct connections to the intersecting roadway by turning in the direction of travel. In other words, the connector for the left turn movement actually bends to the left rather than making a 270-degree right turn as it does in a cloverleaf interchange. Most fully-directional interchanges are known colloquially as "stacks". Compare with cloverleaf.

divided highway - a roadway that has a median or some other physical barrier separating the two opposing sides of traffic. Some people automatically assume that any divided highway is a freeway or expressway, but this is an incorrect assumption. While nearly all freeways are divided highways, not all divided highways are freeways. In San Antonio, Bandera Rd. outside Loop 410 and US 181 are examples of standard surface-level divided highways that are not an expressway or freeway.

dynamic message sign (DMS) - another term for a variable message sign.

easement - the use of a part of private property for a public purpose. Typically, easements are used for utility lines. Owners of property with an easement are required to provide access to the easement and are typically not compensated for the use. Compare with right-of-way.

expressway - a road that has controlled-access. In most parts of the country, including San Antonio, the term "expressway" and "freeway" are essentially interchangeable. However, the technical term "expressway" differs from "freeway" in that expressways can have at-grade intersections, whereas freeways cannot. In San Antonio, an example of a true expressway is Spur 422 near Palo Alto College. Compare with freeway, highway, and parkway.

Farm-to-Market (FM) road - the network of rural roads in Texas that connect farming and ranching areas with nearby towns. The FM system also contains Ranch-to-Market (RM) roads. In metropolitan areas, many FM roads no longer serve that purpose and instead are simply state-maintained arterials.

feeder road - the common term in Houston and southeast Texas for a frontage road.

flyover - a general term for an overpass, especially an overpass that is an interchange ramp or connector.

freeway - a road designed for high-speed traffic with controlled-access and grade-separated intersections. Contrary to popular belief, a freeway is not a road free from tolls, but rather a road that is free from signals and intersections; a "free-flowing" road. Indeed, when one uses the term freeway, it conjures images of a certain type of road based on its functional capabilities, and, in fact, most tollways are functionally-classified as freeways. Compare to expressway, highway, and parkway.

frontage road - a road that runs parallel to a freeway or expressway for the purpose of providing access to adjacent properties and intersecting surface streets. Typically, there will be a frontage road on each side of a freeway. In urban areas, frontage roads are one-way, while they are usually two-way in rural areas. While referred to colloquially as access roads in San Antonio, feeder roads in Houston, gateways in El Paso, and service roads in Dallas, "frontage road" is the official term and is also the common term in the Austin area.

gateway - the common term in El Paso for a frontage road.

grade-separated - a intersection of two roads where one roadway passes over the other. In other words, the roads do not intersect at-grade. Every intersection along a freeway is grade-separated. See also interchange.

High Occupancy Vehicle (HOV) lane - a dedicated lane for vehicles that have a certain number of occupants, typically 2 or more. HOV lanes are used extensively in the Houston area and are also found in the Dallas area and soon will also be in the San Antonio area.

highway - technically, a highway is any public roadway. Typically, the term highway is generally used to refer to a major roadway, usually maintained by the state. In many places, the term is often used colloquially to refer to a freeway.

Intelligent Transportation System (ITS) - a system that uses technology to monitor and manage traffic on a roadway or network of roadways. The first ITS systems in Texas were TransGuide in San Antonio and TransStar in Houston.

interchange - a grade-separated junction of two roadways with full or partial access between them. Technically, the term "interchange" can refer to a junction between a freeway and a surface street, but typically the term is reserved for freeway-to-freeway intersections.

interchange sequence series sign - a road sign that lists the next three (sometimes two) exits and the distance to each. Interchange sequence series signs are used sporadically and inconsistently in the San Antonio area.

interconnect - a system whereby traffic signals communicate with each other and/or a central controller, usually as part of a coordinated progression system. Most traffic signals along San Antonio's major arterials are interconnected. In the past, this was done by means of a cable or radio, but nowadays is done mostly via celluar modems. See also progression.

Interstate - a freeway that is part of the federally-designated Interstate Highway System. This term is often used colloquially (although technically incorrectly) to refer to any freeway.

Jersey barrier - the ubiquitous concrete barrier used to separate opposing traffic flows on freeways as well as protective barriers in construction zones and as bridge rails. The term "Jersey barrier" technically refers to a specific type of barrier, but is commonly used to refer to all sloped concrete barriers. Newer concrete barriers that are not technically "Jersey barriers" are generally "constant slope" barriers. Jersey barriers are known as "K-rails" in some areas.

lane control signal (LCS) - a signal above a lane that shows a red, green, or yellow arrow or "X" symbol to denote whether the lane is open or closed. In San Antonio, LCSs were previously used on TransGuide-equipped freeways, but those have since been turned off and many have been removed due to maintenance costs. LCSs are still found on the reversible lanes around the AT&T Center.

lane drop - a place on a freeway where a mainlane terminates by either becoming an an exit-only lane or by way of simply ending and merging into the adjacent lane.

lane mile - one mile of one lane of road. One mile of a roadway with four lanes is four lane miles. Contrast with centerline mile.

level of service (LOS) - a mostly subjective measurement of the performance of a roadway or intersection based on the traffic conditions. LOS is graded on a scale of A-F (including E), where LOS-A is free-flowing traffic and LOS-F is stop-and-go congestion.

light rail - a type of public rail transportation that utilizes streetcar-like vehicles on streets with shared right-of-way or on exclusive right-of-way. The term "light tail" is oftentimes incorrectly used interchangeably with "commuter rail". Compare with commuter rail for differences between the two.

limited-access - another term for controlled access.

loop - in Texas, a loop is a state highway that connects two or more other state highways. Usually, a loop is circular in nature, but frequently is not. For example, in San Antonio, while Loops 410 and 1604 ring the city, several other state highways are also designated as Loops: Fredericksburg Rd. (Loop 345) and Austin Hwy. (Loop 368) are two prime examples. Perhaps the best-known non-circular "loop" in Texas is Loop 1, or MoPac, in Austin.

mainlanes - the main or primary travel lanes on a freeway or other highway, as opposed to the frontage road lanes. See also acceleration lane, deceleration lane, auxiliary lane, and collector/distributor.

major thoroughfare plan - a plan by a municipality, county, or other transportation planning agency that classifies existing arterial roadways and designates corridors for possible future arterials. The primary purpose of such a plan is twofold-- to designate roadways for funding purposes and to lay out a comprehensive road network so as to preserve the locations for possible future roads.

managed lane - a lane or set of lanes on a road, typically on a freeway, that is separated from the general-purpose mainlanes by barriers, to which access is regulated by some means or criteria (typically tolls) to maintain a certain level of service.

Manual of Uniform Traffic Control Devices (MUTCD) - the standard guide that defines and regulates all traffic signs, signals, and markings in the United States. Many states have their own MUTCD as well.

median - an unpaved area that separates two parallel roadways. Sometimes mispronounced or misspelled as "medium".

Metropolitan Planning Organization (MPO) - a state-designated agency that is required under federal law to coordinate federal and state transportation funding among all agencies that receive such funding in a metropolitan area.

multiplex - another term for concurrency.

parkway - a roadway that typically runs through a park-like setting. In the Northeast, parkways are generally freeways where commercial vehicles are prohibited. In San Antonio, the term "parkway" has started to acquire a slightly different meaning, that being a roadway that is essentially a minor expressway or "super-arterial", such as the Wurzbach Parkway or planned Kelly Parkway.  However, the term has also been used arbitrarily on roads that typically would otherwise be referred to as a boulevard or avenue, such as Stone Oak Parkway and AT&T Center Parkway.

pedestrian scramble - an intersection where signals stop all motor traffic to allow pedestrians to cross in all directions simultaneously. In San Antonio, pedestrian scramble systems have been installed at several downtown intersections. Also known as a "Barnes Dance."

pre-emption - a system whereby specific vehicles can interrupt the normal traffic signal cycle and obtain a green signal for their direction of travel. In San Antonio and many cities, signal pre-emption is used by emergency vehicles. VIA's Primo buses also have pre-emption capabilties along most of their routes.

progression - a system whereby traffic signals are coordinated along a given corridor to provide for all green signals as a group of vehicles (known as a "platoon") makes its way through the corridor at a set speed. Known colloquially as "signal synchronization". See also interconnect.

raised pavement marker - a device, usually made of ceramic or plastic, that is used as a lane marker. These are known colloquially as "Botts' Dots", "turtles", or "traffic buttons". Oftentimes, raised pavement markers will be reflectorized; these are sometimes known as "cat's eyes".

ramp - a short road that provides a connection between two other roads. See also slip-ramp and connector.

ramp metering - a system that uses traffic signals on a freeway entrance ramp to regulate (meter) the number of vehicles that can enter the freeway, typically during peak periods. Ramp metering was fairly widespread on I-35 and I-10 in downtown San Antonio in the '70s and early '80s, but was removed during the double-decking of those roads in the mid and late '80s. Ramp metering was reintroduced in Houston in the mid '90s and are known as "flow signals."

Ranch Road - another term for a Ranch-to-Market road, although there is one official "Ranch Road" (RR1) that serves the LBJ Ranch. See Ranch-to-Market road.

Ranch-to-Market road - another name for a Farm-to-Market road, typically used west of US 281. See Farm-to-Market road.

right-of-way - 1. a strip of land owned by a public entity for use by transportation, utility, and/or communication purposes. Compare with easement. 2. the legal authority of one vehicle to have precedence over another at an intersection.

riprap - pavement or other treatment along a slope to prevent erosion.

roundabout - a roadway intersection that utilizes a circular roadway to connect the intersecting roads. Roundabouts, sometimes incorrectly referred to as "traffic circles", are a form of traffic calming.

service road - the common term in Dallas-Ft. Worth for a frontage road.

shoulder - a paved area adjacent to a travel lane, typically on the right-hand edge of the road, that is not intended for use as a main travel lane, but rather as an area for vehicles to slow or stop in an emergency or to turn right. In Texas, an improved shoulder (one that is paved and of similar width to a normal travel lane) can also be used to pass someone who is slowing or stopped to turn left on the main travel lane or can be driven on by slow vehicles to allow faster traffic to pass.

signal pre-emption - See pre-emption.

Single Point Urban Interchange (SPUI) - a type of diamond interchange whereby the left turn movements are curved in such as way as to provide a single intersection for them centered under or above the freeway instead of two distinct intersections on each side of the freeway. The first SPUI in Texas was at US 75 and Parker Rd. in Plano.

slip ramp - a short ramp between two adjacent, parallel roads, such as the entrance and exit ramps between a freeway's mainlanes and its frontage road.

spur - a short state highway that branches-off from another highway.

stack - a multi-level, fully-directional interchange between two freeways, so-called because the roadways and flyovers are "stacked" one atop the other. In San Antonio, older stacks are located downtown at I-10/I-37 and I-35/US 90, and at I-37/Loop 410, with newer stacks at I-10/Loop 410 (NW), US 281/Loop 410, and US 281/Loop 1604.

State highway - 1. any roadway built and maintained by the state. 2. a specifically numbered roadway that is part of the state highway system. State highways are prefixed with "SH". In San Antonio, two examples are SH 16 and SH 151.

super-street - a roadway with one or more intersections where the straight-through and left-turn movements of the cross street have been eliminated and replaced by right-turns coupled with downstream turnarounds. The first super-street in Texas will be built on US 281 north of Loop 1604 in San Antonio. Super-streets are sometimes incorrectly called "Michigan Lefts", which are similar but have a different traffic pattern.

surface street - a standard road that runs at ground level. This term is generally used to differentiate a particular road from a freeway.

tollway - a road where motorists are charged to use the road. Because of the inherent need to control access to a tollway for the purposes of toll collection, nearly all tollways are also freeways. Sometimes, tollways are built as managed lanes on an otherwise toll-free freeway. See also freeway.

traffic button - colloquial term for raised pavement markers, also known as "Bott's dots" and "turtles".

traffic calming - techniques used to slow traffic. Such techniques include roundabouts, speed humps, chicanes, and bottlenecking. See also roundabout.

TransGuide - the Intelligent Transportation System in San Antonio. TransGuide monitors area freeways using cameras and speed sensors, then notifies motorists of incidents or congestion using variable message signs and lane control signals. TransGuide is also known as a type of advanced "traffic management system".

turnaround - a ramp that allows traffic to make a U-turn across a divided highway. In Texas, turnarounds are used predominately to allow traffic to go from a one-way frontage road on one side of a freeway to the opposing one-way frontage road on the other side of the freeway without having to traverse the intersections for the cross street. In most places, turnarounds are marked with a simple U-turn sign, but in San Antonio, signs for turnarounds actually use the term "turnaround".

turtle - colloquial term for raised pavement markers, also known as "traffic buttons" and "Botts' Dots".

US highway - a highway that is part of the nationally-coordinated system of highways that bear the same name. Contrary to popular belief, US highways are not under any federal jurisdiction, but rather are state highways that use a coordinated national numbering system to provide for easier interstate travel. The numbering system is administered by the American Association of State highway and Transportation Officials (AASHTO).

variable message sign (VMS) - electronic signs along or above freeways and other highways that provide dynamic messages to alert the motoring public of incidents, congestion, construction, or other information. In San Antonio, permanent VMSs are operated by TransGuide along most area freeways and include mainlane and entrance ramp VMSs. VMSs are also known-as changeable message signs and dynamic message signs.

vehicle miles traveled (VMT) - the total number of miles driven by all vehicles on a specific road, class of roads, or in a geographical area during a specific time period. For instance, the total number of miles driven by all vehicles in Texas during 2009.

X-interchange - an interchange between a freeway and a surface street where the entrance and exit ramps connect to a frontage road and form an "X" shape relative to the cross street as viewed from the air. In a standard frontage-road "diamond" interchange, the entrance ramp from one cross street typically precedes the exit ramp for the next cross street. However, in an X-ramp configuration, those ramps are reversed. This reduces weaving on the mainlanes. The process of converting a diamond interchange to an X-interchange is known as "ramp reversal". In San Antonio, a couple of examples of X-interchanges are I-10 at DeZavala and I-35 at FM 3009. Compare with diamond interchange

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This page and all its contents are Copyright 2019 by Brian Purcell

The information provided on this website is provided on an "as-is" basis without warranties of any kind either express or implied.  The author and his agents make no warranties or representations of any kind concerning any information contained in this website.  This website is provided only as general information.  The author expressly disclaims all liability with respect to actions taken or not taken based upon the information contained herein or with respect to any errors or omissions in such information.  All opinions expressed are strictly those of the author.  This site is not affiliated in any way with any official agency.