Traffic Terms

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Definitions are categorized as follows:

1. Geometric Cross Section
2. Highway Structures
3. Highway Types
4. Interchanges and Intersections at Grade
5. Traffic
6. Sight Distance

The definitions are presented below:

1. Geometric Cross Section

    (1) Lane.

      (a) Auxiliary Lane--The portion of the roadway for weaving, truck climbing, speed change, or for other purposes supplementary to through traffic movement.

      (b) Lane Numbering--On a multilane roadway, the traffic lanes available for through traffic traveling in the same direction are numbered from left to right when facing in the direction of traffic flow.

      (c) Multiple Lanes--Freeways and conventional highways are sometimes defined by the number of through traffic lanes in both directions. Thus an 8-lane freeway has 4 through traffic lanes in each direction. Likewise, a 4-lane conventional highway has 2 through traffic lanes in each direction.

      (d) Median Lane--A speed change lane within the median to accommodate left turning vehicles.

      (e) Separate Turning Lane--An auxiliary lane for traffic in one direction which has been physically separated from the intersection area by a traffic island.

      (f) Speed Change Lane--An auxiliary lane, including tapered areas, primarily for the acceleration or deceleration of vehicles entering or leaving the through traffic lanes.

      (g) Traffic Lane--The portion of the traveled way for the movement of a single line of vehicles.

    (2) Median. The portion of a divided highway separating the traveled ways for traffic in opposite directions.

    (3) Outer Separation. The portion of an arterial highway between the traveled ways of a roadway for through traffic and a frontage street or road.

    (4) Roadbed. That portion of the roadway extending from curb line to curb line or shoulder line to shoulder line. Divided highways are considered to have two roadbeds.

    (5) Roadside. A general term denoting the area adjoining the outer edge of the roadbed. Extensive areas between the roadbeds of a divided highway may also be considered roadside.

    (6) Roadway. That portion of the highway included between the outside lines of the sidewalks, or curbs and gutters, or side ditches including also the appertaining structures, and all slopes, ditches, channels, waterways, and other features necessary for proper drainage and protection.

    (7) Shoulder. The portion of the roadway contiguous with the traveled way for accommodations of stopped vehicles, for emergency use, and for lateral support of base and surface courses.

    (8) Traveled Way. The portion of the roadway for the movement of vehicles, exclusive of shoulders.

2. Highway Structures

    (1) Bridges. Structures that span more than 6.1 m over a waterway, measured under the copings along the centerline of the road and multiple span structures where the individual spans are in excess of 3 m, measured from center to center of supports along the centerline of the road.

3. Highway Types

    (1) Freeway. A freeway, as defined by statute, is a highway in respect to which the owners of abutting lands have no right or easement of access to or from their abutting lands or in respect to which such owners have only limited or restricted right or easement of access. This statutory definition also includes expressways.

    The engineering definitions are:

      (a) Freeway--A divided arterial highway with full control of access and with grade separations at intersections.

      (b) Expressway--An arterial highway with at least partial control of access, which may or may not be divided or have grade separations at intersections.

    (2) Controlled Access Highway. Freeways and expressways are controlled access highways.

    (3) Conventional Highway. A highway without control of access which may or may not be divided. Grade separations at intersections or access control may be used when justified at spot locations.

    (4) Highway.

      (a) Arterial Highway--A general term denoting a highway primarily for through traffic usually on a continuous route.

      (b) Bypass--An arterial highway that permits traffic to avoid part or all of an urban area.

      (c) Divided Highway--A highway with separated roadbeds for traffic in opposing directions.

      (d) Major Street or Major Highway--An arterial highway with intersections at grade and direct access to abutting property and on which geometric design and traffic control measures are used to expedite the safe movement of through traffic.

      (e) Through Street or Through Highway--Every highway or portion thereof at the entrance to which vehicular traffic from intersecting highways is regulated by stop signs or traffic control signals or is controlled when entering on a separate right-turn roadway by a yield-right-of-way sign.

    (5) Street or Road.

      (a) Cul-de-Sac Street--A local street open at one end only, with special provisions for turning around.

      (b) Dead End Street--A local street open at one end only, without special provisions for turning around.

      (c) Frontage Street or Road--A local street or road auxiliary to and located on the side of an arterial highway for service to abutting property and adjacent areas and for control of access.

      (d) Local Street or Local Road--A street or road primarily for access to residence, business, or other abutting property.

4. Interchanges and Intersections at Grade

    (1) Channelization. The separation or regulation of conflicting traffic movements into definite paths of travel by the use of pavement markings, raised islands, or other suitable means to facilitate the safe and orderly movement of both vehicles and pedestrians.

    (2) Geometric Design. Geometric design is the arrangement of the visible elements of a road, such as alignment, grades, sight distances, widths, slopes, etc.

    (3) Gore. The area immediately beyond the divergence of two roadbeds bounded by the edges of those roadbeds.

    (4) Grade Separation. A crossing of two highways or a highway and a railroad at different levels.

    (5) Interchange. A system of interconnecting roadways in conjunction with one or more grade separations providing for the interchange of traffic between two or more roadways on different levels.

5. Traffic

    (1) Annual Average Daily Traffic. The average 24 hour volume, being the total number during a stated period divided by the number of days in that period. The term is commonly abbreviated as ADT or AADT.

    (2) Delay. The time lost while traffic is impeded by some element over which the driver has no control.

    (3) Density. The number of vehicles per mile on the traveled way at a given instant.

    (4) Design Vehicles. A truck or large vehicle for which the road has been designed to accommodate.

    (5) Design Volume. A volume determined for use in design, representing traffic expected to use the highway. Typically it is an hourly volume.

    (6) Diverging. The dividing of a single stream of traffic into separate streams.

    (7) Headway. The time in seconds between consecutive vehicles moving past a point in a given lane, measured front to front.

    (8) Level of Service. A rating using qualitative measures that characterize operational conditions within a traffic stream and their perception by motorists and passengers.

    (9) Merging. The converging of separate streams of traffic into a single stream.

    (10) Running Time. The time the vehicle is in motion.

    (11) Spacing. The distance between consecutive vehicles in a given lane, measured front to front.

    (12) Speed.

      (a) Design Speed--A speed selected to establish specific minimum geometric design elements for a particular section of highway.

      (b) Running Speed--The speed over a specified section of highway, being the distance divided by running time. The average for all traffic, or component thereof, is the summation of distances divided by the summation of running times.

    (13) Traffic Control Devices.

      (a) Traffic Markings--All lines, words, or symbols, except signs, officially placed within the roadway to regulate, warn, or guide traffic.

      (b) Traffic Sign--A device mounted on a fixed or portable support, conveying a message or symbol to regulate, warn, or guide traffic.

      (c) Traffic Signal--A power operated traffic control device except signs, barricade warning lights, and steady burning electric lamps, by which traffic is regulated, warned, or alternately directed to take specific actions.

    (14) Volume. The number of vehicles passing a given point during a specified period of time.

    (15) Weaving. The crossing of traffic streams moving in the same general direction accomplished by merging and diverging.

6. Sight Distance

    (1) Sight Distance. Sight distance is the continuous length of highway ahead visible to the driver.

    (2) Stopping Sight Distance. The minimum stopping sight distance is the distance required by the driver of a vehicle, traveling at a given speed, to bring his vehicle to a stop after an object on the road becomes visible. Stopping sight distance is measured from the driver's eyes, which are assumed to be 3.5 feet above the pavement surface, to an object 6 inchs high on the road.

    (3) Passing Sight Distance. Passing sight distance is the minimum sight distance required for the driver of one vehicle to pass another vehicle safely and comfortably. Passing must be accomplished without reducing the speed of an oncoming vehicle traveling at the design speed should it come into view after the overtaking maneuver is started. The sight distance available for passing at any place is the longest distance at which a driver whose eyes are 3.5 feet above the pavement surface can see the top of an object 4.25 feet high on the road.

    (4) Corner Sight Distance. Corner sight distance is the minimum sight distance required for the driver on an uncontrolled side street to see on-coming traffic. It is 7.5 seconds of travel time for a driver whose eyes are 3.5 feet above the pavement surface and looking at a 4.25 feet high on-coming vehicle.

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