The Commissioner — Spring 2002

The Language of Traffic

Steven J. Dush, AICP & Gregory P. Muhonen

The ability to understand the Language of Traffic is necessary for elected and appointed officials and others who wish to engage in rigorous, productive discussions of transportation impacts and policies. No, we are not talking about the use of verbal comments or non-verbal gestures that some people use to convey their frustration with an inconsiderate motorist, nor are we referring to the noise differences between compact cars and semi-trucks. For purposes of this article, the Language of Traffic is the terms and definitions that transportation professionals use to analyze, project and describe traffic. At times, this technical terminology can seem like a totally different language.

This article provides an explanation of the numerous technical terms and definitions commonly heard during public meetings when planning commission or council reviews a development proposal. We will help unveil the mystery of the terminology so the next time you hear a technical description of a "really bad intersection" by a transportation professional, you won't think you are hearing language from another planet.

Armed with this understanding, decision makers are better equipped to:

  • Link land-use planning and other community issues to transportation planning efforts.
  • Use transportation terminology to effectively communicate with the transportation professionals.
  • Communicate concepts often masked behind cryptic transportation terms and acronyms to lay persons who may be attending their first public hearing and who may be passionate about the traffic and traffic safety issues related to the proposal.
  • Relate the concepts to their daily driving habits and understand how steps can be taken to ensure a desired LOS (level of service) is maintained.
Definitions and Their Application

The first step is to understand the terms and acronyms. The following is not by any means a complete list, but it identifies and defines the more common ones.

ADT: Average daily traffic. The term used to describe the number of vehicles on a roadway segment during a non-holiday week day.

Bike Lane: A lane devoted to non-motorized bicycles.

DOT: Department of Transportation. Most state departments of transportation place one or two letters before the DOT in their name. For instance, Colorado's DOT is CDOT and Missouri's is MODOT.

Geometric Improvements: Improvements to roads such as widening, adding signals to intersections, or adding turning lanes. These are required to mitigate traffic impacts and maintain a required level of service (LOS).

HOV: High Occupant Vehicle. Any vehicle carrying two or more passengers. Many larger communities have HOV lanes on major highways, that permit only HOV's to use them.

ITE: Institute of Transportation Engineers. Organization for professional transportation engineers. ITE publishes the Trip Generation Manual, which provides information on trip generation for land uses and building types. For instance, if an individual needs to know the number of trip ends (see definition below) produced by an industrial park, the report provides a trip rate based upon the size of the building. The report also divides the trip rate into peak hour rates, weekday rates, etc.

ISTEA: Intermodal Surface Transportation Efficiency Act of 1991. This Congressional act requires states to develop a Statewide Transportation Plan and a Statewide Transportation Improvements Program (STIP) that identifies short-term project needs and priorities. It has also been a major source of funding for transportation planning and encourages the linking of transportation and community planning. (See also TEA-21 below).

Intersection LOS: Level of Service. This is a measure of the average delay experienced by each vehicle passing through an intersection. It can be measured for the vehicles making each directional turning movement, using each approach leg, or as a composite average value for all vehicles using the intersection. Similar to roadway level of service, it is reported with a letter grade designation ranging from A to F. An LOS A represents insignificant delay (less than 10 seconds per vehicle); LOS F represents significant waiting .This means more than 50 seconds per vehicle for intersections with non-existent or inadequate signals or more than 80 seconds per vehicle for intersections with signals.

Roadway LOS: Roadway Level of Service. This is a measure of roadway congestion ranging from LOS A — least congested — to LOS F — most congested. LOS is one of the most common terms used to describe how "good" or how "bad" traffic is projected to be. LOS serves as a benchmark to determine whether new development will comply with an existing LOS or if it will exceed the preferred or adopted LOS. As part of planning for new projects or developments, transportation professionals conduct a Traffic Impact Study (TIS). The TIS determines how specific streets and intersections will function with increased traffic volumes either with or without improvements.

There are six levels of service letter grades typically recognized by transportation planners and engineers. They are as follows:

Level of Service A
Level of Service A describes a condition of free flow, with low volumes and high speeds.

Level of Service B
Level of Service B is the zone of stable flow, with operating speeds beginning to be restricted
somewhat by traffic conditions. Drivers still have reasonable freedom to select their speed and lane of operation.

Level of Service C
Level of Service C is the zone of mostly stable flow, but speeds and maneuverability are more closely constricted by the higher volumes.

Level of Service D
Level of Service D is a zone that approaches unstable flow, with tolerable operating speeds, however driving speed is considerably affected by changes in operating conditions.

Level of Service E
Level of Service E is a zone that cannot be described by speed alone. Operating speeds are lower than in Level D, with volume at or near the capacity of the highway.

Level of Service F
Level of Service F is a zone in which the operating speeds are controlled by stop-and-go mechanisms, such as traffic lights. This is called forced flow operation. The stoppages disrupt the traffic flow so that the volume carried by the roadway falls below its capacity; without the stoppages, the volume of traffic on the roadway would be higher, or in other words, it would reach capacity.
It should be noted that LOS is a measure of a roadway segment's (zone's) efficiency at moving automobiles through the zone. By definition, it places a high emphasis on the free-flowing speeds of autos and does not give consideration to the comfort or safety other roadway users such bicyclists or pedestrians.

Link Volumes: The number of vehicles using a specific street segment. It is typically expressed as average daily traffic (ADT) or vehicle per peak hour (VPH).

Linked Trip/Trip Chain: The sequence of grouping stops between the origin and ultimate destination. The intermediate stops made while enroute to the ultimate destination are referred to as passby trips. The term is used in the evaluation of the operation of the accesses or driveways serving the uses at the intermediate stops.

Median: A physical divider separating lanes of traffic that typically are traveling in opposite directions. A median is often installed to prohibit unsafe turning movements. It can also be used to beautify a streetscape.

MPO: Metropolitan Planning Organization. The agency which administers the federally required transportation planning processes in a metropolitan area. An MPO must be in place in every urbanized area with a population over 50,000, and is responsible for the 20-year long-range plan and the Transportation Improvement Program (TIP). The MPO is the coordinating agency for grants, billings and policy-making for transportation.

Multimodal: More than one mode of transportation in the same geographic area.

NHS: National Highway System.

Peak Hour: The one hour period during which the roadway carries the greatest number of vehicles. Traffic impacts are typically evaluated during the morning and afternoon peak hours when the greatest number of motorists are traveling to and from work.

Pedestrian LOS: Level of service for pedestrians can also be studied as part of a transportation or traffic analysis. This is less common. It is typically only an issue in larger urban areas. Exhibit 1 illustrates the congestion of a proposed pedestrian walkway LOS.

Platoon: A grouping of vehicles traveling in the same direction at the same approximate speed.

RTP: Regional Transportation Plan. The RTP is created by the Metropolitan Planning Organization (MPO) or the regional planning commission (see above).

Reverse Commute: The travel from the city center to suburban locations, moving counter to the primary or major volume of traffic flow.

Stacking: The process of vehicles forming a line or queue. If the stacking extends into the through-lanes, delays and unsafe conditions become prevalent.

SOV: Single Occupant Vehicle or one person per vehicle.

Street Cross-Section: A term used to describe the total number of lanes on a street. For instance, a street that has two lanes of north bound traffic, two lanes of southbound traffic, and a refuge lane is commonly referred to as a five-lane cross-section.

Traffic Calming: The process of designing streets or adding design elements to tame fast traffic and address unsafe traffic conditions. Design elements include, for example, speed humps, narrowed streets, added traffic circle. Good initial design and street layout can prevent the need to install traffic calming measures after the street is built.

Traffic Impact Study (TIS): A study conducted by a transportation professional using transportation modeling and analysis software to predict the volumes and associated impacts from traffic generated by a proposed land use or development project. The study analyzes the impacts to roads and intersections and include recommendations for roadway improvements that may be needed to mitigate unsafe situations and comply with the regulations of the reviewing jurisdiction.

TAZ: Transportation Analysis Zone. A geographic area that identifies land uses and associated trips that is used for making land use projections and performing traffic modeling.

TEA21: Transportation Equity Act of the 21st Century. TEA 21 was enacted June 9, 1998 as Public Law 105-178. TEA-21 authorizes and funds the Federal surface transportation programs for highways, highway safety, and transit for the 6-year period 1998-2003. The TEA 21 Restoration Act, enacted July 22, 1998, provided technical corrections to the original law. (See also ISTEA above).

Trip End: The term used to describe trips in terms of their common origins or destination.

Turn Lane: A lane devoted to vehicles making a turning movement to go in a different direction. Turn lanes are necessary to ensure the free-flow of traffic in the through lanes by providing a separate area/lane for turning traffic to slow down and complete the turning maneuver without impeding the through traffic.

VMT: Vehicle Miles Traveled. Increases in VMT from existing residents are occurring every year, contributing to added congestion on roadways.

VPH: Vehicle per peak hour. This relates to Link Volumes (see above).

Volume-to-Capacity Ratio: Expressed as v/c, this is a measure of traffic demand on a facility (expressed as volume) compared to its traffic-carrying capacity. A v/c ratio of 0.7, for example, indicates that a traffic facility is operating at 70 percent of its capacity. In evaluating the performance of a roadway, v/c ratios should be considered together with the letter grade system, which is more of a qualitative assessment based heavily on speeds and travel time. With traffic moving at an acceptable rate of speed, roadways will perform at favorable Level of Service grades. However, even with an acceptable LOS grade, a v/c ratio may indicate that the same facility is operating at or near full capacity (e.g., 0.95 to 0.99). Conversely, road segments operating at deficient levels of service (e.g., peak-hour LOS E and F) may have an acceptable v/c ratio in cases where the adjoining intersections are not operating efficiently (e.g., cycle lengths on the traffic signals are long or the signal progressions are poor). Consequently, a high v/c ratio does not always imply that a facility has more volume than it can handle nor does a deficient LOS grade necessarily indicate that there is insufficient roadway capacity available.

Weaving: The process of exiting a site and merging across multiple lanes "with traffic" to reach an intersection and go in a different direction.

Enhancing Communication

You will need to understand and communicate these definitions and terms to lay persons in order to have a productive discussion regarding transportation issues. Because traffic is often a controversial topic at public meetings when a new land use is proposed, it becomes imperative for the decision makers (planning commissioners and councilors/supervisors) to explain the traffic impacts to lay audiences. Equally important, officials should ask proper questions of an applicant's transportation professional. This helps insure that the concerns of both the decision makers and their constituents are addressed. The following are some tips when discussing traffic issues.

Tip One: Be Precise
When asking a question of a transportation professional, be very precise and try to link the technical results with everyday results. If a TIS indicates that an intersection will operate at a LOS C after a proposed development is built, but operates at a LOS A currently, ask the applicant's traffic consultant or your professional staff what that means in common terms. For instance, will it mean that a motorist will have to wait behind a queue of a few cars or through one or more cycles of the traffic signal? Most individuals are not particularly concerned with what letter rating a street or intersection has, but wants to know precisely how a street will be affected by development

Tip Two: Ask Questions
Be sure to fully understand how a proposed development will affect traffic patterns, volumes, and travel times to ensure compliance with adopted LOS standards or community standards. It is important to clarify which component of the traffic stream is actually impacted. The public often expresses fear about "more traffic" before they understand that perhaps only the left turning vehicles, or some other subset of the traffic stream, will be impacted. Ambiguity in the analysis or the explanations of the results leaves the listeners' mind free to apply their own interpretations of what the forecasted impacts will be. Ask questions as needed to gain an accurate picture.

Tip Three: Rely on your Professional Staff
Prior to preparing a TIS, responsible transportation professionals will consult with a jurisdiction's professional staff to discuss requirements and the all-important assumptions to be used within a TIS. This is done prior to its preparation. Encourage your in-house professional staff to insist on such a "pre-application" conference. It will help insure that the time, money, and effort invested in the TIS is properly and comprehensively directed to the relevant traffic issues. Furthermore, in the public meetings., utilize the training and expertise of your professional staff to respond to citizen concerns, respond to technical questions regarding traffic impacts, and confirm the validity of the perspectives offered by the applicant's traffic professional.

Tip Four: Avoid Talking Like a Technocrat
Once you understand the terms and definitions and can apply them, don't begin speaking to lay persons using the technical jargon without fully explaining a term or concept.

Tip Five: Share Your Language
Practice your new language, and share it with friends and neighbors. Discussing your passby trips to the coffee shop and gas station on your way to work and the platoon you were in on the freeway which was operating at a LOS-C during the AM peak hour might generate an odd look, but the more we use the terms and definitions, the more common the terms will become, which makes communication easier and fosters informed decision making regarding transportation impacts and policies.

Steven J. Dush, AICP, is a Principal Planner for Clark County, Nevada. Gregory P. Muhonen, P.E., is the Principal of Land Development Solutions in Loveland, Colorado.