highway engineering(redirected from Geometrical road design)
highway engineering[′hī‚wā ‚en·jə′nir·iŋ]
A branch of civil engineering that includes planning, design, construction, operation, and maintenance of roads, bridges, and related infra-structure to ensure effective movement of people and goods. See Civil engineering
Highway planning involves the estimation of current and future traffic volumes on the road network. For purposes of design, traffic volumes are needed for a representative period of traffic flow. The capacity is the maximum theoretical traffic flow rate that a highway section is capable of accommodating under a given set of environmental, highway, and traffic conditions. The capacity of a highway depends on factors such as the number of lanes, lane width, effectiveness of traffic control systems, frequency and duration of traffic incidents, and efficiency of collection and dissemination of highway traffic information. Traffic conditions arising from the interplay of volume and capacity are perceived by road users in a way that is quantitatively termed level of service. See Traffic-control systems
Highway facilities often cause adverse effects on the environment, such as noise pollution, air pollution, water pollution, and ecological impacts. Tire/pavement interaction, vehicle exhausts, and engines cause traffic noise. Highway engineers strive to predict and mitigate all possible impacts of highway systems.
Through highway design, the most appropriate location, alignment, and shape of the highway are selected. Highway design involves the consideration of three major factors (human, vehicular, and roadway) and how these factors interact to provide a safe highway. Human factors include reaction time for braking and steering, visual acuity for traffic signs and signals, and car-following behavior. Vehicle considerations include vehicle size and dynamics that are essential for determining lane width and maximum slopes, and for the selection of design vehicles. Engineers design road geometry to ensure stability of vehicles when negotiating curves and grades and to provide adequate sight distances for undertaking passing maneuvers along curves on two-lane, two-way roads.
Location involves fitting the road efficiently onto the surrounding terrain and environment. Horizontal alignment is represented by an aerial view of the highway. It consists of straight lines and curves. Curves are fitted to provide a smooth transition between straight highway sections.
Intersections and interchanges occur where two or more highways cross each other at the same level. Since various vehicle maneuvers (turning, crossing, and through movements) all occur within a limited area as the volumes of these movements increase, there is increased likelihood of traffic conflicts and crashes. One way of reducing such danger is to use channelization to limit each stream to a unique path. In high traffic volume areas, movement of streams can be separated in time using multiphased traffic signals. The vertical alignment of a highway is represented by its longitudinal profile, which gives the elevation of all points along the length of the highway. The purpose of vertical alignment design is to determine the level of the highway at each point in order to ensure adequate safety and drainage.
Highway cross section refers to the profile of the road, perpendicular to the direction of travel and extending to the limits of the right of way within which the facility is constructed. Highway cross-section elements may include driving lanes, bicycle/pedestrian lanes, shoulders, medians, barriers, cross slope for drainage, and superelevation.
Pavement design is the process of selecting pavement layer types and thicknesses in order to withstand expected traffic loads in a cost-effective manner. Each pavement layer usually consists of mineral aggregates such as natural river or pit sand, natural gravel, and crushed rock. For rigid pavements, portland cement is mixed with water and aggregates to produce a viscous concrete mix that is poured into prepared forms and vibrated. See Cement
There are generally three types of pavements specified for pavement design. Gravel pavement is the simplest type of pavement and is often designed for lightly traveled roads. Flexible pavement is a multilayered structure that includes a subbase, a base, and an asphaltic wearing course. Rigid pavement consists of a plain or steel-reinforced portland cement concrete slab laid on a prepared crushed-stone base course. See Pavement, Precast concrete
Highway construction usually follows planning and design, and involves new or reconstructed facilities such as pavements, drainage structures, and traffic control devices. Road construction is often preceded by detailed stakeout surveys and preparation of the subgrade. See Construction engineering, Construction equipment
Traffic signals are the most important traffic control devices. The typical traffic signal for an intersection displays a sequence of green, amber, and red. One complete signal sequence is called a cycle. Traffic signals are either pretimed or demand-actuated. Flow-concentration controllers are capable of sensing detailed demand information and responding to it by revising the cycle length and phasing patterns of the signal.
The performance of highway infrastructure is measured in terms of pavement and bridge condition, level of service, and safety. Pavement condition is monitored over a period of time using a condition index or serviceability rating. Through the development and implementation of bridge management systems, many agencies have in place a decision support tool that supplies analyses and summaries of data, uses mathematical models to make predictions of bridge conditions, and provides the means by which alternative policies and programs may be efficiently evaluated. Congestion management is maintained by implementing measures to mitigate the magnitude and duration of traffic congestion. Safety management is a systematic process that has the goal of reducing the number and severity of traffic crashes by ensuring that all opportunities and identified, considered, implemented as appropriate, and evaluated in all phases of highway planning design, construction, maintenance, and operation. See Bridge