Planning Glossary


The following is a listing of Safety terms developed by CCRPC. Acronyms are listed by letter, but are not in alphabetical order.

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Term Acronym Definition
511 511 National traveller information phone number designated by the FCC.
811 811 Underground damage prevention system (e.g. electric, natural gas, telephone, cable, etc.) information phone number designated by the Federal Communications Commission (FCC). In Vermont, this service is known as “Dig Safe”.
85th Percentile Speed   The maximum speed at which 85% of all vehicles are travelling.
Access Management AM Techniques of Transportation infrastructure management intended to; reduce congestion and accident rates, lessen need for highway widening, conserve energy, and reduce pollution. Examples include; limiting entrance and exit of traffic on highways, use of medians and turn lanes, placement and timing of signals, as well as implementation of supportive local ordinances.
Accessible Pedestrian Signal APS A device that communicates information about pedestrian timing in nonvisual format such as audible tones, verbal messages, and/or vibrating surfaces
Apron   See; “Truck Apron”
Chicane   A traffic-calming measure employing fixed objects (usually curbs, earth, fencing, etc.), which deliberately project into the travel lane or a road or shared-use path creating a curvature pattern in the line of travel.
Complete Streets   Coined in 2003 by bicycle and pedestrian advocates, “Complete Streets Design Techniques” are employed to enable safe access for all users. Pedestrians, bicyclists, motorists and transit riders of all ages and abilities must be able to safely move along and across a “Complete Street”.
Contraflow Lane   Otherwise known as a “reversible lane”, it is utilised for buses where the direction of travel is opposite to the flow of traffic in the other lanes. Contraflow lanes are also employed for maintenance purposes, or in cases of emergency evacuation where both sides of an interstate highway are used for outbound traffic.
Corner Sight Distance CSD The minimum distance a driver can see across an intersection corner in tandem with the length of time it takes the driver to safely traverse the intersection (past potential crossing vehicles) on to the roadway, and accelerating up to traffic flow speed.
Deflection   A horizontal change in the travel path of traffic due to a physical feature of a roadway. An example would be a “Splitter Island” of a roundabout. Here traffic is slowed by a geometric curvature (or deflection) from a straight trajectory before entering a channelized approach into the roundabout.
Detectable Warning   Standardized surface feature built into pedestrian or transit-related infrastructure, which is designed to warn visually impared pedestrians of changes in curbline, slope, crossings, etc. It is generally a tactile standardized feature, intended to function much like a stop sign. It alerts perceivers to the presence of a hazard in the line of travel, whereupon they would stop, and determine the nature and extent of the hazard, before proceeding further.
Distracted Driving   A specific type of inattention that occurs when drivers divert their attention away from the driving task to focus on another activity instead. These distractions can be from electronic distractions, such as navigation systems and cell phones, or more conventional distractions such as interacting with passengers and eating. These distracting tasks can affect drivers in different ways, and can be categorized into the following type; 1. Visual Distraction – Tasks that require the driver to look away from the roadway to visually obtain information; 2. Manual Distraction – Tasks that require the driver to take a hand off the steering wheel and manipulate a device; 3. Cognitive Distraction – Tasks that are defined as the mental workload associated with a task that involves thinking about something other than the driving task. The impact of distraction on driving is determined not just by the type of distraction, but also the frequency and duration of the task. That is to say, even if a task is less distracting, a driver who engages in it frequently or for long durations may increase the crash risk to a level comparable to that of much more difficult task performed less often. Because drivers often have a choice regarding when and how often to multitask when driving, their exposure to risk is typically within their control; however NHSTA research has shown that drivers underestimate the overall risk of various tasks.
Diverging Diamond Interchange DDI A variation on a “Diamond Interchange”, a DDI (also known as the “Double Crossover” intersection) increases safety by reducing the number of potential conflict points of traffic. The DDI accommodates left-turning movements at a signalized, grade-separated interchanges of arterials while eliminating the need for left-turn signal phasing. On an arterial (i.e. a high-volume road), traffic crosses over to the left side of the roadway between the nodes of the interchange. Two-phase traffic signals are installed at roadway crossovers. Once vehicles are on the left side of the arterial roadway, they may turn left onto limited-access ramps without stopping or conflicting with through traffic.
Double-Crossover Diamond Interchange DCD See “Diverging Diamond Interchange”. The DCD is distinguished from the conventional diamond interchange in that it combines left-turning traffic with through traffic. This is accomplished by having both left-turn and through vehicles cross over to the opposite sides of the roadway at the ramp terminals.
Dynamic Striping   A traffic calming technique using experimental systems of pavement markings, which is not yet mandated by the Manual on Uniform Traffic Control Devices (MUTCD).
Fatality Analysis Reporting System FARS FARS is a federal census of crashes involving any motor vehicle on a trafficway, but only in fatal crashes. It is generally considered to be the most reliable national crash database. A large truck is defined in the FARS as a truck with a gross vehicle weight rating (GVWR) of more than 10,000 pounds. A bus is defined in the FARS as a large motor vehicle used to carry more than 10 passengers, including school buses, inter-city buses, and transit buses. FARS is maintained by the National Highway Traffic Safety Administration (NHTSA).
Field of View FOV Field of View
Fog Line FOGL Reflective white lines painted along the shoulder of a road demarking the legally extent a motor vehicle is allowed to operate.
Grade Crossing   Where a roadway intersects a rail line.
Grade Separation   The raising or lowering of a roadway to bridge over another roadway, thereby eliminating traffic conflict.
Green Book   Published by AASHTO, the “Green Book” is formally known as “A Policy on Geometric Design of Highways and Streets”. It covers the latest geometric design practices in standard use for highways, intersections, and interchanges.
Hazardous Material HAZMAT Classified by the U.S. Environmental Protection Agency (EPA), transport of HAZMAT is regulated by the U.S. Department of Transportation.
High Accident Location HAL A location on the federal-aid highway system that has experienced a minimum of five accidents over a five-year period and has an Actual Rate to Critical Rate Ratio (ARCR) of 1 or greater. The list of HALs is maintained by the Vermont Agency of Transportation – Highway Research Section.
High Risk Rural Roads Program HRRR A federal safety and funding provision addressing the high fatality and incapacitating injury rate, which occurs on rural roads (nationally, about 60% occur on Rural Major & Minor Collectors, as well as Rural Local Access roads).
High-Intensity Activated Crosswalk HAWK A pedestrian-activated beacon located on the roadside and on mast arms over major approaches to an intersection. The HAWK head consists of two red lenses over a single yellow lens. It displays a red indication to drivers when activated, which creates a gap for pedestrians to use to cross a major roadway.
Highway Safety Improvement Program HSIP Federal program assisting states to achieve a significant reduction in traffic fatalities and serious injuries on all public roads through the implementation of infrastructure-related highway safety improvements.
Inscribed Circle Diameter ICD Diameter of a Rotary, Roundabout, or Circulator intersection. Generally speaking, a smaller ICD of a modern roundabout keeps the traffic speed lower, and hence safer than the larger ICD’s commonly found with rotaries or traffic circles.
Institute of Transportation Engineers ITE Founded in 1930, the Institute of Transportation Engineers is an international educational and scientific association of transportation professionals who are responsible for meeting mobility and safety needs. ITE facilitates the application of technology and scientific principles to research, planning, functional design, implementation, operation, policy development and management for any mode of ground transportation. ITE further promotes professional development of its members, supports and encourages education, stimulates research, develops public awareness programs and serves as a conduit for the exchange of professional information.
Intersection Sight Distance ISD The AASHTO “Green Book” reference to the “Line of Sight” distance between a vehicle travelling on a roadway and a vehicle attempting to enter the roadway from an intersection or driveway.
Local Emergency Planning Committee LEPC A committee under the Chittenden County Regional Planning Commission (CCRPC), LEPC function is to provide resources and guidance to Chittenden County communities through education, coordination and assistance in hazmat planning, as well as to assure public health and safety.
Manual on Uniform Traffic Control Devices MUTCD Published by Federal Highway Administration (FHWA), this manual defines the standards used by road managers nationwide to install and maintain traffic control devices (signage, pavement markings, traffic signals, etc.) on all streets and highways.
Multi-Jurisdictional All-Hazards Mitigation Plan MJAHMP Hazard Mitigation is a sustained effort to permanently reduce or eliminate long-term risks to people and property from the effects of reasonably predictable hazards. The purposes of this plan are to: 1. Identify specific natural, technological and societal hazards that impact the communities of Chittenden County; 2. Prioritize hazards for mitigation planning; 3. Recommend regional level goals and strategies to reduce any losses from those hazards; and 4. Establish a coordinated process to implement the plan, taking advantage of a wide range of resources.
National Highway Traffic Safety Administration NHTSA NHTSA was established in 1970 by the Highway Safety Act of 1970. Its mission is to; “Save lives, prevent injuries and reduce economic costs due to road traffic crashes, through education, research, safety standards and enforcement activity.”
Perception, Identification, Emotion, and Volition PIEV According to the Institute of Transportation Engineers (ITE), PIEV times range from 2.5 sec. for general warning signs to 14.5 sec. for signs requiring a high degree of judgment from the driver. PIEV Distance may be expressed as: dp = 1.47 × V × t, where dp = Perception-reaction distance in feet, V = Speed in miles per hour (mph), t = Perception-reaction time in seconds, and 1.47 is a factor to convert speed from miles per hour (mph) to feet per second (fps).
Perception-Reaction Time PRT PRT outlines four distinct processes a driver must perform for roadway navigation; Detection, Identification, Decision, & Response. Where PRT varies widely among drivers, AASHTO suggests it to be 2.5 seconds (where 90% of drivers will have a PRT as fast as, or faster than 2.5 seconds). ITE suggests it is 1.0 seconds (where 85% of drivers would have as an aspect of reacting to signal timing, but it would be considerably higher for a braking response upon a highway). Other factors that influence a PRT are; age, fatigue, complexity of reaction, and alcohol.
Professional Traffic Operations Engineer PTOE The Professional Traffic Operations Engineers (PTOE) certification is accredited by the National Commission for Certifying Agencies (NCCA). This certification requires that the holder be a licensed professional engineer if he or she practices in the United States, Canada or any other country that provides governmental licensing of engineers.
Property Damage Only PDO A reportable crash, which does not result in any fatalities or noteworthy injuries.
Public Nuisance   Something that endangers the health or safety of the public or is offensive to the senses of an ordinary person.
Public Safety Answering Pont PSAP A facility that receives emergency calls (such as E-911) and dispatches public safety services.
Retroreflective   A type of material on signage or pavement markings used to optimize brightness for the nighttime driver. Retroreflective material make signage legible at further distances by reflecting most all of the light striking it from a light source directly back towards it. This reflection is contained in a narrow cone that spreads out wide enough to include the driver (almost directly behind a vehicle’s headlamps). Hence, this special type of “back to the source” reflection is called “retroreflection”. A sign may be reflective, but without this engineered cone of retroreflection, it would be considered “diffuse reflection”. When the viewer is not near a light source, many diffuse-reflecting materials will usually be brighter than retroreflective materials.
Road Diet   A term used to convey phenomena of increased safety and reduced traffic congestion via a reduction in the number of travel lanes on a roadway, usually (but not always) from four lanes to three.
Roadway Safety Audit Review RSAR A formal safety performance examination of a road or intersection by an multi-disciplinary team. An RSAR reports on potential safety concerns and investigates strategies to improve safety at the specified location. Issues that should be addressed in the report ought to include; 1. Aspects of a safety concern in or around the roadway, and 2. Opportunities to mitigate or eliminate identified safety concerns.
Rotary   A large, circular, one-way, multi-lane, and often higher-speed (above 30 MPH) intersection that commonly serves as an access and exit point to interstates, freeways, limited-access highways, or major arterials. Rotaries are NOT roundabouts. Most commonly found in the northeastern U.S., rotaries large size (> 600′ width) and low deflection around their center island allow for high speeds (in some cases as high as 45 MPH). Because of this, and Right-of-Way often being given to entering vehicles, rotaries have much higher crash and injury rates than do the more modern Roundabouts.
Roundabout   Non-signalized circular intersection with specific design and traffic control features to ensure low travel speeds and efficient traffic movement.
Rubbernecking   In traffic parlance, “Rubbernecking” is driver reduction of otherwise normal traffic speed for a roadway, in order for to observe the scene of an accident (having significant safety and environmental impacts).
Rumble Strips   Rumble strips are raised or grooved patterns constructed on, or in travel lane and shoulder pavements. The texture of rumble strips is different from the road surface. Vehicle tires passing over them produce a sudden rumbling sound and cause the vehicle to vibrate. Road agencies use rumble strips to warn motorists of an upcoming change that may require them to act. For example, the need to slow down for a toll plaza ahead, change lanes for a work zone around the curve, stop for a traffic signal, or steer back onto the roadway. Rumble strips in travel lanes often precede intersections, especially dangerous ones. They are used primarily on expressways, interstate highways, and parkways, although some States install them on 2-lane rural roads that have high numbers of single-vehicle crashes.
Safe Routes To School SRTS Mandated by federal law (SAFETEA-LU), “Safe Routes To School” is intended to benefit children in primary and middle schools (K-8) by encouraging them to walk and bike to school regularly, routinely, and safely. SR2S integrates elements of transportation, economics, health, physical activity, environmental awareness and safety.
Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users SAFETEA-LU On 10 August 2005, the new Federal surface transportation act (SAFETEA-LU) was signed into law, authorizing a $286.4 billion transportation program for a 5-year period; FFY 2004-2009. This act covers all surface transportation programs, such as highways, highway safety, transit, freight, and transportation research.
Sharrow   Otherwise known as a “Shared-Lane Arrow”, sharrows are employed on roadways too narrow for striped bicycle lanes and help to provide guidance to bicyclists and warn motorists about the presence of bicyclists. The sharrow is a printed image of a bike symbol underneath two chevrons upon the roadway.
Speed Bumps   A Speed Bump is also a raised pavement area across a roadway. Speed bumps are typically found on private roadways and parking lots and do not tend to exhibit consistent design parameters from one installation to another. Speed bumps generally have a height of 3 to 6 inches (76 to 152 mm) with a travel length of 1 to 3 feet (0.3 to 1 m). From an operational standpoint, speed bump impacts within typical residential operational speed ranges slow vehicles to about 5 mph or less at each bump. Speed bumps of varying design have been routinely installed on private roadways and parking lots without the benefit of proper engineering study regarding their design and placement.
Speed Humps   A Speed Hump is a raised area in the roadway pavement surface extending transversely across the travel way. Speed humps are sometimes referred to as “pavement undulations” or “sleeping policemen”. Most agencies implement speed humps with a height of 3 to 3.5 inches (76 to 90 mm) and a travel length of 12 to 14 feet (3.7 to 4.3 m). Speed humps are generally used on residential local streets. From an operational standpoint, speed humps impact within typical residential operational speed ranges, slow vehicles to about 20 mph (32 km/h) on streets with properly spaced speed humps. Speed humps have evolved from extensive research and testing and have been designed to achieve a specific result on vehicle operations without imposing unreasonable or unacceptable safety risks.
Speed Tables   Speed Tables are essentially flat-topped speed humps, and may have a textured material on the flat section with asphalt or concrete for the approaches. Speed tables are sometimes referred to as “trapezoidal humps” or “speed platforms”. If marked as a pedestrian crossing, speed tables may also be referred to as “raised crosswalks” or “raised crossings”. Most agencies implement speed tables with a height of 3 to 3.5 inches (76 to 90 mm) and a travel length of 22 feet (6.7 m). Speed tables generally consist of 10 foot (3.1 m) plateau with 6 foot (1.8 m) approaches on either side that can be straight, parabolic or sinusoidal in profile. The longer lengths of speed tables provide a gentler ride than speed humps and generally result in vehicle operating speeds ranging from 25 to 30 mph (40 to 48 km/h) on streets depending on the spacing between speed tables. Speed tables are generally used on residential collectors, emergency routes or transit routes.
Splitter Islands   A dual-purpose component of a roundabout that serves to deflect traffic speed of entering and exiting vehicles, as well as provide a temporary refuge to pedestrians where crossing is permitted along roadway approaches.
Stopping Sight Distance SSD The minimum distance a driver can see ahead in tandem with how long it takes for her/him to stop. According to the AASHTO “Green Book”, this distance is comprised of two components; 1. Perception-Reaction Time, which covers the distance a vehicle travels from the moment the driver sees an object necessitating a stop, to the instant the brakes are applied, and — 2. Braking Distance, which is the distance a vehicle travels during the braking maneuver.
Strategic Highway Safety Plan SHSP Required by federal law (SAFETEA-LU, 2005), An SHSP identifies a State’s key safety needs, whilst guiding investment decisions to achieve significant reductions in highway fatalities and injuries on all public roads. The SHSP facilitates all State highway safety programs to work in concert to align all its resources to collectively address safety challenges on all its public roads.
Technical Assistance TA The CCMPO technical assistance program provides technical support relating to transportation issues for member municipalities and agencies. Services typically provided through this program include: Traffic Counts, Speed Limit Recommendations, Traffic Signal Optimization, Traffic Control Warrant Analysis, Roadway Surface Management Studies, Small Area Transportation Studies, and Safety Studies.
Traffic Calming   The use of education, enforcement, and engineering to change the behavior of drivers and increase safety.
Traffic Circle   Traffic circles (or rotaries) are intersections designed for high speed entry and multi-lane maneuvering. Historically, many have given driver Right-of-Way to entering vehicles, causing gridlock and traffic congestion. Because traffic circles suffered high crash rates and operational problems, they fell out of favor in the US during the 1950s and 1960s. Traffic circles are NOT Roundabouts.
Traffic Impact Study TIS Also known as Traffic Impact Analysis (TIA), these are studies which collect and analyze information to determine need, impact, and impact mitigation for major roadway improvements. Some activities of a TIS include; Traffic counts, Crash evaluation, Capacity & Level Of Service calculation, Signal Warrant analysis, Development of condition diagrams, Forecasting future traffic volumes with new development and/or alternatives.
Truck Apron   Component of a roundabout central island, which accommodates the “Offtracking” of large trucks or buses, but is not intended to be driven by automobiles. Truck aprons generally are a raised and widened concrete or brick pad constructed along the perimeter of the central island.
Yield-to-Left Y2L Principle where circulating vehicles within a roundabout, traffic circle, or rotary have the Right-of-Way over entering vehicles. Such practice eliminates traffic gridlock within the intersection. However this policy may vary by state, region, or other jurisdiction.

Other Transportation Glossaries