Roads Flashcards
Acceleration lane
An auxiliary lane used by an entering vehicle to accelerate before entering the travelled way.
Access control
The condition whereby the road agency either partially or fully controls the right of abutting landowners to direct access to and from a public highway or road.
Access interchange
An interchange providing access to a freeway from the adjacent non-freeway road network.
Arterial
Highway designed to move relatively large volumes of traffic at high speeds over long distances. Typically, arterials offer little or no access to abutting properties.
Auxiliary lane
Short lane located immediately adjacent to the basic or through lane to accommodate some or other special circumstance such as a turning movement to right or to left, acceleration to or deceleration from the speeds prevailing on the travelled way or heavy vehicles reduced to crawl speeds on a steep upgrade.
Average Daily Traffic (ADT)
The number of vehicles per day passing a point on the highway during a defined period. If this period extends from 1 January to 31 December, reference is to Annual Average Daily Traffic (AADT)
Average running speed
The distance summation for all vehicles divided by the running time summation for all vehicles. Also referred to as space mean speed whereas time mean speed is simply the average of all recorded speeds.
Axis of rotation
The line about which the pavement is rotated to superelevate the roadway. This line normally maintains the highway profile
Barrier sight distance
The limiting sight distance below which overtaking is legally prohibited.
Boulevard
The area separating sidewalks from the through lanes.
Bridge
A structure erected with a deck for carrying traffic over or under an obstruction and with a clear span of six metres or more. Where the clear span is less than six metres, reference is to a culvert.
Broken-back curve
Two curves in the same direction with a tangent shorter than 500 metres long connecting them.
Camber
The slope from a high point (typically at the centre line of the highway) across the lanes of a highway. Negative camber refers to a central low point, usually with a view to drainage of a small urban street or alley.
Capacity
The maximum number of vehicles that can pass a point on a highway or in a designated lane in one hour without the density being so great as to cause unreasonable delay or restrict the driver’s freedom to manoeuvre under prevailing roadway and traffic conditions.
Carriageway
Roadway forming part of a divided highway and intended for movement in one direction only – hence dual carriageway as an alternative name for divided highway.
Catchwater drain
Located above a cut face to ensure that storm water does not flow down the cut face causing erosion and deposition of silt on the roadway.
Channel grading
Where side channels are designed to gradients that differ from those of the road centreline, typically on either side of the highest points on crest curves and the lowest points on sag curves where the centreline gradient is less than 0,5 per cent.
Channelisation
The use of pavement markings or islands to direct traffic through an intersection.
Clearance profile
Describes the space that is exclusively reserved for provision of the road or highway. It defines the minimum height of the soffit of any structure passing over the road and the closest approach of any lateral obstacle to the cross-section.
Cloverleaf interchange
An interchange with loop ramps in all quadrants to accommodate right turns and outer connectors for the left turns.
Collector
A road characterised by a roughly even distribution of its access and mobility functions.
Collector-Distributor road
A road used at an interchange to remove weaving from the through lanes and to reduce the number of entrances to and exits from the through lanes.
Compound curve
A combination of two or more curves in the same direction without intervening tangents between them.
Criterion
A yardstick according to which some or other quality of the road can be measured. Guideline values are specific numerical values of the criterion. For example, delay is a criterion of congestion.
Critical length of grade
The maximum length of a specific upgrade on which a loaded truck can operate without an unreasonable reduction in speed. Very often, a speed reduction of 15 km/h or more is considered “unreasonable”.
Cross fall
See camber. In the case of cross fall, the high point is at the roadway edge.
Cross-over crownline
The line across which an instantaneous change of camber takes place. In the case of a normally cambered road, the centreline is a special case of the cross-over crownline. The cross-over crownline can be located anywhere on the road surface and need not even be parallel to the road centreline.
Crosswalk
A demarcated area or lane designated for the use of pedestrians across a road or street.
Crown runoff
(Also referred to as tangent runout) The rotation of the outer lane of a twolane road from zero cross fall to normal camber (NC).
Culvert
A structure, usually for conveying water under a roadway but can also be used as a pedestrian or stock crossing, with a clear span of less than six metres.
Cut
Section of highway or road below natural ground level. Sometimes referred to in other documents as a cutting or excavation.
Cycle lane
A portion of the roadway which has been designated by road markings, striping and signing as being exclusively for the use of cyclists.
Cycle path
Also known as a bike way. A path physically separated from motorised traffic by an open space or barrier and located either within the road reserve or an independent reserve.
Decision sight distance
Sometimes referred to as anticipatory sight distance, allows for circumstances where complex decisions are required or unusual manoeuvres have to be carried out. As such, it is significantly longer than Stopping Sight Distance.
Density
The number of vehicles occupying a given length of road. Usually averaged over time and expressed as vehicles per kilometre.
Depressed median
A median lower in elevation than the travelled way and so designed to carry portion of the storm water falling on the road.
Design domain
The range of values of a design criterion that are applicable to a given design, e.g. lane widths of more than 3,3 metres.
Design hour
The hour in which the condition being designed for, typically the anticipated flow, is expected to occur. This is often the thirtieth highest hour of flow in the design year.
Design speed
The speed selected as the basis for establishing appropriate geometric elements for a section of road.
Design vehicle
A compilation of the 85th percentile values of the various parameters of the vehicle type being designed for, e.g. length, width, wheelbase, overhang, height, ground clearance, etc.
Design year
The last year of the design life of the road or any other facility, often taken as twenty years although, for costly structures such as major bridges, a longer period is usually adopted.
Directional distribution (split)
The percentages of the total flow moving in opposing directions, e.g. 50:50, 70:30, with the direction of interest being quoted first.
Divided highway
A highway with separate carriageways for traffic moving in opposite directions.
Driveway
A road providing access from a public road to a street or road usually located on an abutting property.
Eighty-fifth percentile speed
The speed below which 85 per cent of the vehicles travel on a given road or highway.
Footway
The rural equivalent of the urban sidewalk.
Freeway
Highest level of arterial characterised by full control of access and high design speeds.
Frontage road
A road adjacent and parallel to but separated from the highway for service to abutting properties and for control of access. Sometimes also referred to as a service road.
Gap
The elapsed time between the back of one vehicle passing a point on the road or highway and the nose of the following vehicle passing the same point. A lag is the unexpired portion of a gap, i.e. the elapsed time between the arrival of a vehicle on the minor leg of an intersection and the nose of the next vehicle on the major road crossing the path of the entering vehicle.
Gore area
The paved triangular area between the through lanes and the exit or entrance ramps at interchanges plus the graded areas immediately beyond the nose (off-ramp) or merging end (on-ramp).
Grade line
The line describing the vertical alignment of the road or highway.
Grade
The straight portion of the grade line between two successive vertical curves.
Grade separation
A crossing of two highways or roads, or a road and a railway, at different levels.
Gradient
The slope of the grade between two adjacent Vertical Points of Intersection (VPI), typically expressed in percentage form as the vertical rise or fall in metres/100 metres. In the direction of increasing stake value, upgrades are taken as positive and downgrades as negative.
Guideline
A design value establishing an approximate threshold, which should be met if considered practical. It is a recommended value whereas a standard is a prescriptive value allowing for no exceptions.
High occupancy vehicle ( HOV) lane
A lane designated for the exclusive use of buses and other vehicles carrying more than two passengers.
High-speed
Typically where speeds of 80 km/h or faster are being considered.
Horizontal sight distance
The sight distance determined by lateral obstructions alongside the road and measured at the centre of the inside lane.
Interchange
A system of interconnecting roads (referred to as ramps) in conjunction with one or more grade separations providing for the movement of traffic between two or more roadways which are at different levels at their crossing point.
Intersection sight distance
The sight distance required within the quadrants of an intersection to safely allow turning and crossing movements.
Kerb
Concrete, often precast, element adjacent to the travelled way and used for drainage control, delineation of the pavement edge or protection of the edge of surfacing. Usually applied only in urban areas.
Kerb ramp
The treatment at intersections for gradually lowering the elevation of sidewalks to the elevation of the street surface.
K-value
The distance over which a one per cent change in gradient takes place.
Level of Service (LOS)
A qualitative concept, from LOS A to LOS F, which characterises acceptable degrees of congestion as perceived by drivers. Capacity is defined as being at LOS E.
Low speed
Typically where speeds of 70 km/h or slower are being considered.
Median
The portion of a divided highway separating the two travelled ways for traffic in opposite directions. The median thus includes the inner shoulders.
Median opening
An at-grade opening in the median to allow vehicles to cross from a roadway to the adjacent roadway on a divided road.
Modal transfer station
The public facility at which passengers change from one mode of transport to another, e.g. rail to bus, passenger car to rail.
Mountainous terrain
Longitudinal and transverse natural slopes are severe and changes in elevation abrupt. Many trucks operate at crawl speeds over substantial distances.
Normal crown (NC)
The typical cross-section on a tangent section of a two-lane road or fourlane undivided road.
Overpass
A grade separation where a minor highway passes over the major highway
Outer separator
Similar to the median but located between the travelled way of the major road and the travelled way of parallel lanes serving a local function if these lanes are contained within the reserve of the major road. If they fall outside this reserve, reference is to a frontage road.
Partial Cloverleaf (Par-Clo) Interchange
An interchange with loop ramps in one, two or three (but usually only two) quadrants. A Par-Clo A Interchange has the loops in advance of the structure and Par-Clo B Interchange has the loops beyond the structure. A Par-Clo AB Interchange has its loops on the same side of the crossing road.
Passenger car equivalents (units) (PCE or PCU)
A measure of the impedance offered by a vehicle to the passenger cars in the traffic stream. Usually quoted as the number of passenger cars required to offer a similar level of impedance to the other cars in the stream. Passing sight distance. The total length visibility, measured from an eye height of 1,05 metres to an object height of 1,3 metres, necessary for a passenger car to overtake a slower moving vehicle. It is measured from the point at which the initial acceleration commences to the point where the overtaking vehicle is once again back in its own lane.
PC (Point of curvature)
Beginning of horizontal curve, often referred to as the BC.
PI (Point of intersection)
Point of intersection of two tangents.
PRC (Point of reverse curvature)
Point where a curve in one direction is immediately followed by a curve in the opposite direction. Typically applied only to kerb lines.
PT (Point of tangency)
End of horizontal curve, often referred to as EC.
PVC (Point of vertical curvature)
The point at which a grade ends and the vertical curve begins, often also referred to as BVC.
PVI (Point of vertical intersection)
The point where the extension of two grades intersect. The initials are sometimes reversed to VPI.
PVT (Point of vertical tangency)
The point at which the vertical curve ends and the grade begins. Also referred to as EVC.
Quarter link
An interchange with at-grade intersections on both highways or roads and two ramps (which could be a two-lane two-way road) located in one quadrant. Because of its appearance, also known as a Jug Handle Interchange.
Ramp
A one-way, often single-lane, road providing a link between two roads that cross each other at different levels.
Relative gradient
The slope of the edge of the travelled way relative to the gradeline.
Reverse Camber (RC)
A superelevated section of roadway sloped across the entire travelled way at a rate equal to the normal camber.
Reverse curve
A combination of two curves in opposite directions with a short intervening tangent
Road safety audit
A structured and multidisciplinary process leading to a report on the crash potential and safety performance of a length of road or highway, which report may or may not include suggested remedial measures.
Roadside
A general term denoting the area beyond the shoulder breakpoints.
Road bed
The extent of the road between shoulder breakpoints.
Road prism
The lateral extent of the earthworks.
Road reserve
Also referred to as Right-of-way. The strip of land acquired by the road authority for provision of a road or highway.
Roadway
The lanes and shoulders excluding the allowance (typically 0,5 metres) for rounding of the shoulders.
Rolling terrain
The natural slopes consistently rise above and fall below the highway grade with, occasionally, steep slopes presenting some restrictions on highway alignment. In general, rolling terrain generates steeper gradients, causing truck speeds to be lower than those of passenger cars.
Rural road or highway
Characterised by low volume high-speed flows over extended distances. Usually without significant daily peaking but could display heavy seasonal peak flows.
Shoulder
Usable area immediately adjacent to the travelled way provided for emergency stopping, recovery of errant vehicles and lateral support of the roadway structure.
Shoulder breakpoint
The hypothetical point at which the slope of the shoulder intersects the line of the fill slope. Sometimes referred to as the hinge point.
Side friction (f)
The resistance to centrifugal force keeping a vehicle in a circular path. The designated maximum side friction (fmax) represents a threshold of driver discomfort and not the point of an impending skid.
Sidewalk
The portion of the cross-section reserved for the use of pedestrians.
Sight triangle
The area in the quadrants of an intersection that must be kept clear to ensure adequate sight distance between the opposing legs of the intersection.
Simple curve
A curve of constant radius without entering or exiting transitions.
Single point urban interchange
A diamond interchange where all the legs of the interchange meet at a common point on the crossing road.
Speed profile
The graphical representation of the 85th percentile speed achieved along the length of the highway segment by the design vehicle.
Standard
A design value that may not be transgressed, e.g. an irreducible minimum or an absolute maximum. In the sense of geometric design, not to be construed as an indicator of quality, i.e. an ideal to be strived for.
Stopping sight distance
The sum of the distance travelled during a driver’s perception/reaction time and the distance travelled thereafter while braking to a stop.
Superelevation
The amount of cross-slope provided on a curve to help counterbalance, in combination with side friction, the centrifugal force acting on a vehicle traversing the curve.
Superelevation runoff
(Also referred to as superelevation development) The process of rotating the outside lane from zero crossfall to reverse camber (RC), thereafter rotating both lanes to the full superelevation selected for the curve.
Systems interchange
Interchange connecting two freeways, i.e. a node in the freeway system.
Tangent
The straight portion of a highway between two horizontal curves.
Tangent runoff
See crown runoff
Traffic composition
The percentage of vehicles other than passenger cars in the traffic stream, e.g. 10 per cent trucks, 5 per cent articulated vehicles (semi-trailers) etc.
Transition curve
A spiral located between a tangent and a circular curve.
Travelled way
The lanes of the cross-section. The travelled way excludes the shoulders.
Trumpet interchange
A three-legged interchange containing a loop ramp and a directional ramp, creating between them the appearance of the bell of a trumpet.
Turning roadway
Channelised turn lane at an at-grade intersection.
Turning template
A graphic representation of a design vehicle’s turning path for various angles of turn. If the template includes the paths of the outer front and inner rear points of the vehicle, reference is to the swept path of the vehicle.
Underpass
A grade separation where the subject highway passes under an intersecting highway.
Urban road or highway
Characterised by high traffic volumes moving at relatively low speeds and pronounced peak or tidal flows. Usually within an urban area but may also be a link traversing an unbuilt up area between two adjacent urban areas, hence displaying urban operational characteristics.
Value engineering
A management technique in which intensive study of a project seeks to achieve the best functional balance between cost, reliability and performance.
Verge
The area between the edge of the road prism and the reserve boundary
Warrant
A guideline value indicating whether or not a facility should be provided. For example, a warrant for signalisation of an intersection would include the traffic volumes that should be exceeded before signalisation is considered as a traffic control option. Note that, once the warranting threshold has been met, this is an indication that the design treatment should be considered and evaluated and not that the design treatment is automatically required.
Yellow line break point
A point where a sharp change of direction of the yellow edge line demarcating the travelled way edge takes place. Usually employed to highlight the presence of the start of a taper from the through lane at an interchange.
