Glossary of Engineering Terms

A | C | D | E | F | G | H | I | L | M | N | P | R | S | T | U | V | W

A

Acceleration Lane - A paved auxiliary lane, including tapered areas, allowing vehicles to accelerate when entering the through-traffic lane of the roadway.

Access - Permission, liberty, or ability to enter, approach, or make use of a roadway.

Access Control - The condition where public authority fully or partially controls the right of abutting owners access to the highway right of way. Full control of access is exercised to give preference to through traffic by providing access connections with selected public roads only and by prohibiting crossings at grade or direct private driveway connections. Partial control of access is exercised to give preference to through traffic to a degree than in addition to access connections with selected public roads, there may be some crossings at grade and some private driveway connections. Uncontrolled access over a highway, street or road does not limit the number of points of ingress or egress, except through the exercise of control over the placement and the geometrics of connections a necessity for the safety of the traveling public.

Access Point - An intersection, driveway, or opening on the right-hand side of the roadway.

Access Road - A road that gives direct access to the land and premises on one or both sides.

Accident Rate - The number of accidents that occurs per million vehicle miles of travel.

ADT - see Average Daily Traffic

Auxiliary Lane - An additional lane on a freeway to connect an on-ramp and an off-ramp.

Average Daily Traffic - The average number of vehicles that pass a specified point during a 24-hour period.

C

Centerline - A line that is equidistant from the sides of a roadway. The centerline is typically the horizontal alignment of a road.

Channelization - The separation or regulation of conflicting traffic movements into definite paths of travel by use of pavement markings, raised islands, or other means.

Clear Zone - The total roadside border area, starting at the edge of the traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, a non-recoverable slope and/or a clear run-out area. The desired width is dependent upon the traffic volumes and speeds and on the roadside geometry.

Clearance - Lateral distance from edge of traveled way to a roadside object or feature.

Corridor - In planning, a broad geographical band that follows a general directional flow or connects major sources of trips. It may contain a number of streets and highways, and transit lines and routes.

Cross Section - A representation of a road if cut off at a right angle to its centerline.

Crossroad - A road crossing a freeway, highway or other facility. It is typically the road carrying lesser traffic.

D

Deceleration Lane - A paved auxiliary lane, including tapered areas, allowing vehicles leaving the through-traffic lane of the roadway to decelerate.

Decision Sight Distance - Sight distance allowing for additional decision time in cases of complex conditions.

Design Concept Report (DCR) - A report produced to document criteria necessary to design improvements, identify available data, address alternatives, and recommend a solution. Once the DCR is approved, the development effort will proceed without further consideration of alternatives.

Design Criteria - Standards on which a judgement or decision about the roadway geometry may be based.

E

EA - See Environmental Assessment

Easement - A right acquired by one party to use or control property belonging to another party for a designated purpose, such as public utilities, streets or highways, transit lines.

Environmental Assessment (EA) - An environmental study used under the National Environmental Policy Act of 1969 to determine whether an environmental impact statement is required.

Environmental Impact Analysis - The analysis of environmental (including human) effects that would result from the implementation of a project's alternatives in conformance with the National Environmental Policy Act.

EO - Environmental Overview

Evaluation Criteria - Standards on which a judgement or decision on the various alternatives may be based.

Expressway - A divided arterial highway for through traffic. An expressway has full or partial control of access and generally has grade separations at major intersections.

F

Freeway - A divided highway for through traffic that has full access control and grade separations at all intersections; an expressway with full access control.

Frontage Road - A local street or road auxiliary to and located on the side of a freeway , expressway, or arterial highway for service to abutting property and adjacent areas and for control of access to the freeway, expressway, or arterial highway; a road contiguous to and generally parallel to an expressway , freeway, parkway, or through street. It is designed to intercept, collect and distribute traffic desiring to cross, enter, or leave such a highway and to furnish access to property that otherwise would be isolated as a result of the controlled-access feature. It is sometimes called a service road.

G

Grade - Rise in elevation within a specified distance. As an example, a 1-percent grade is a 1-foot rise in elevation in 100 feet of distance (measured horizontally).

Grade Crossing - A crossing or intersection of highways, railroad tracks, other guideways, or pedestrian walks, or combinations of these at the same level or grade.

Grade Separation - A vertical separation of intersecting facilities (road, rail, etc.) by the provision of crossing structures; the crossing of two highways, or a railroad and a highway, at different elevations.

H

Highway, Street or Road - General terms denoting a public way for purposes of vehicular travel, including the entire area within the right of way.

Horizontal Alignment - A combination of tangents, horizontal curves and spirals which defines the horizontal location of a roadway.

I

Incident - Any occurrence on a roadway that impedes the normal flow of traffic.

Interchange - The system of interconnecting ramps between two or more intersecting travel ways (highways, transit guideways, etc.) that are grade separated.

Intersection - The point at which two or more roads meet or cross.

L

Land Use - The purpose for which land or the structure on the land is being used, for example, residential, commercial, or light industry.

Level of Service - A qualitative rating of the effectiveness of a highway or highway facility in serving traffic, in terms of operating conditions (speed, travel time, comfort, convenience, traffic interruptions, freedom to maneuver). The Highway Capacity Manual identifies operating conditions ranging from A, for best operations (low volume, high speed) to F, for worst conditions.

Local Jurisdiction - The limits or territory within which the local government's authority may be exercised.

M

Mainline - The primary through roadway as distinct from ramps, auxiliary lanes, and collector-distributor roads.

Median - The portion of a divided highway separating traffic traveling in opposite directions.

N

No-Build Alternative - A planning option of leaving the situation as it already exists. Existing facilities and services are maintained, and existing transportation policies are continued.

P

Passing Sight Distance - The length of roadway ahead required to pass another vehicle without meeting an oncoming vehicle.

R

Recoverable Slope - A slope on which a motorist may, to a greater or lesser extent, retain or regain control of a vehicle. Slopes flatter than 1(vertical) to 4 (horizontal) are generally considered recoverable.

Right of Way - A general term denoting land, property, or interest therein, usually in a strip, acquired for or devoted to transportation purposes.

Road - see "Highway, Street or Road"

Rural Areas - Those areas outside the boundaries of urban areas.

Rural Freeway - A freeway of which the alignment and cross-sectional elements are more generous in design, which is commensurate with a higher design speed and generally with greater availability of right of way.

S

Safety - The condition of being safe from undergoing or causing hurt, injury or loss.

Service Interchange - An interchange between a freeway facility and a crossroad.

State Jurisdiction - The limits or territory within which the State's authority may be exercised.

State Route - A highway owned and maintained by the State.

Stopping Sight Distance - The distance that allows a driver traveling at the maximum speed to stop before hitting an observed object.

Street - see "Highway, Street or Road"

Superelevation - The slope (in ft/ft) across the roadway.

T

TAC (Technical Advisory Committee) - The TAC provides intellectual leadership from local agencies or organizations that provide independent advice and judgment on strategic issues.

Taper Area - An area characterized by a reduction or increase in pavement width to direct traffic.

Typical Section - A cross section that is representative of the roadway design throughout the project area.

U

Urban Areas - Places within boundaries set by the responsible State and local officials having a population of 5,000 or more.

Urban Freeway - A freeway, located in an urban area, which is capable of carrying high traffic volumes and generally characterized by lower design speeds and lesser availability of right of way.

Urban Fringe Areas - Areas that act as a "transitional" segment between urban and rural settings.

V

Vehicle Miles of Travel (VMT) - A measurement of the total miles traveled by all vehicles in the area for a specified time period. It is calculated by the number of vehicles times the miles traveled in a given area or on a given highway during the time period.

Vertical Alignment - A combination of tangent grades and vertical curves which defines the vertical location of a roadway.

Volume - In transportation, the number of units (passengers or vehicles) that pass a point on a transportation facility during a specified interval of time, usually 1 hour.

W

Warrants - The criteria by which the need for a safety treatment or improvement can be determined.

Weaving - The crossing of two or more traffic streams traveling in the same direction along a significant length of highway, without the aid of traffic control devices (except for guide signs).