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Geometric design is the design of the visible dimensions of a highway with the objective of forming or shaping the facility to the characteristics and behavior of drivers， vehicles and traffic.Therefore， geometric design deals with features of location， alignment， profile， cross section，intersection and highway types.
Highway classification refers to a process by which roadways are classified into a set of subsystems， described below， based on the way each roadway is used. Central to this process is an understanding that travel rarely involves movement along a single roadway. Rather each trip or sub-trip initiates at a land use， proceeds through a sequence of streets， roads and highways， and terminates at a second land use.
The highway classification process is required by federal law. Each state must assign roadways into different classes in accordance with standards and procedures established by the Federal Highway Administration. Separate standards and procedures have been established for rural and urban areas. For a further description of the classification process， see Highway Functional Classification： Concepts， Criteria and Procedures， FHWA ① ， revised March 1989.
The location and geometric design of highways are affected by numerous factors and controlling features. These may be considered in two broad categories as follows：
（1）Primary Controls
a. Highway Classification
b. Topography and Physical Features
c. Traffic
（2）Secondary Controls
a. Design Speed
b. Design Vehicle
c. Capacity
Separate design standards are appropriate for different classes of roads， since the classes serve different types of trips and operate under different conditions of both speed and traffic volume. The design of streets and highways on the State highway system should conform to the guidelines as indicated in specifications for highway. In special cases of restrictive or unusual conditions， it may not be practical to meet these guide values.
The location and the geometric features of a highway are influenced to a large degree by the topography， physical features， and land use of the area traversed. The character of the terrain has a pronounced effect upon the longitudinal features of the highway， and frequently upon the cross sectional features as well. Geological conditions may also affect the location and the geometrics of the highway.Climatic， soil and drainage conditions may affect the profile of a road relative to existing ground.
Man-made features and land use may also have considerable effect upon the location and the design of the highway. Industrial， commercial， and residential areas will each dictate different geometric requirements.
The traffic characteristics， volume， composition and speed， indicate the service for which the highway improvement is being made and directly affects the geometric features of design.
The traffic volume affects the capacity， and thus the number of lanes required. For planning and design purposes， the demand of traffic is generally expressed in terms of the design-hourly volume （ DHV）， predicated on the design year. The design year for new construction and reconstruction is to be 20 years beyond the anticipated date of Plans， Specifications and Estimate（PS ＆ E）， and 10 years beyond the anticipated date of PS ＆ E for resurfacing， restoration and rehabilitation projects.
The composition of traffic， i.e.， proportion of trucks and buses， is another characteristic which affects the location and geometrics of highways. Types， sizes and loadpower characteristics are some of the aspects taken into account.
“Design Speed” is a selected speed used to determine the various design features of the roadway.The assumed design speed should be a logical one with respect to topography， anticipated operating speed， the adjacent land use， and the functional classification of the highway. Except for local streets where speed controls are frequently included intentionally， every effort should be made to use as high a design speed as practicable to attain a desired degree of safety， mobility and efficiency within the constraints of environmental quality， economics， aesthetics and social or political impacts. Once the design speed is selected， all of the pertinent features of the highway should be related to it to obtain a balanced design. Above minimum design values should be used，where practical. Some design features， such as curvature， superelevation， and sight distance are directly related to and vary appreciably with， design speed. Other features， such as widths of lanes and shoulders， and clearances to walls and rails， are not directly related to design speed， but they affect vehicle speeds. Therefore， wider lanes， shoulders， and clearances should be considered for higher design speeds. Thus， when a changes is made in design speed， many elements of the highway design will change accordingly.
Since design speed is predicated on the favorable conditions of climate and little or no traffic on the highway， it is influenced principally by：
Character of the terrain； Extent of manmade features； Economic considerations （ as related to construction and right-of-way costs）.
These three factors apply only to the selection of a specific design speed within a logical range pertinent to a particular system or classification of which the facility is a part.
The physical characteristics of vehicles and the proportions of the various size vehicles using the highways are positive controls in geometric design. A design vehicle is a selected motor vehicle， the weight， dimensions and operating characteristics of which are used to establish highway design controls to accommodate vehicles of a designated type.
The term“ capacity ” is used to express the maximum number of vehicles which have a reasonable expectation of passing over a section of a lane or a roadway during a given time period under prevailing roadway and traffic conditions. However， in a broad sense， capacity encompasses the relationship between highway characteristics and conditions， traffic composition and flow patterns， and the relative degree of congestion at various traffic volumes throughout the range from light volumes to those equaling the capacity of the facility as defined above.
Highway capacity information serves three general purposes：
1）For transportation planning studies to assess the adequacy or sufficiency of existing highway networks to current traffic demand， and to estimate when， in time， projected traffic demand， may exceed the capacity of the existing highway network or may cause undesirable congestion on the highway system.
2）For identifying and analyzing bottleneck locations （both existing and potential）， and for the evaluation of traffic operational improvement projects on the highway network.
3）For highway design purposes.
The level of service concept places various traffic flow conditions into 6 levels of service. These levels of service， designated A through F， from best to worst， cover the entire range of traffic operations that may occur.
The factors that may be considered in evaluating level of service include the following：
（1）Speed and travel time；
（2）Traffic interruptions or restrictions；
（3）Freedom to maneuver；
（4）Safety；
（5）Driving comfort and convenience；
（6）Economy.
geometric design 线形（几何）设计
alignment n. ＆ v. 线形（尤其指道路中线的位置与方向）；定线
profile n. 纵断面，剖面
cross section 横断面
intersection n. 交叉，交点，（道路）平面交叉口
topography n. 地形，地形学
design vehicle 设计车辆
terrain n. 地形，地势
location n. 定线，定位
capacity n. 通行能力
level of service 服务水平
①FHWA： Federal Highway Administration 联邦公路管理局