Chapter 6 CROSSSECTION ELEMENTSWord格式文档下载.docx

1、BACKGROUND The cross section of a road includes some or all of the following elements: Traveled way （the portion of the roadway provided for the movement of vehicles, exclusive of shoulders） Roadway （the portion of a highway, including shoulders, provided for vehicular use） Median area （the physical

2、 or painted separation provided on divided highways between two adjacent roadways） Bicycle and pedestrian facilities Utility and landscape areas Drainage channels and side slopes Clear zone width （i.e., the distance from the edge of the traveled way to either a fixed obstacle or nontraversable slope

3、） Considered as a single unit, all these crosssection elements define the highway rightofway. The rightofway can be described generally as the publicly owned parcel of land that encompasses all the various crosssection elements （see Figures 6.1 and 6.2）. Figure 6.1 Twolane rural highway crosssection

4、 design features and terms. Some decisions about cross section are made during project development, such as the capacity and number of lanes for the facility. Other decisions, such as functional classification, are made earlier in the process. Within these parameters, the Green Book guidelines recom

5、mend a range of values for the dimensions to use for crosssectional elements. Deciding which of the elements to include and selecting the appropriate dimensions within these ranges is the role of the designer. Figure 6.2 Urban highway crosssection design features and terms. In selecting the appropri

6、ate crosssection elements and dimensions, designers need to consider a number of factors, including the following: Volume and composition （percent trucks, buses, and recreational vehicles） of the vehicular traffic expected to use the facility The likelihood that bicyclists and pedestrians will use t

7、he route Climatic conditions （e.g., the need to provide storage space for plowed snow） The presence of natural or humanmade obstructions adjacent to the roadway （e.g., rock cliffs, large trees, wetlands, buildings, power lines） Type and intensity of development along the section of the highway facil

8、ity that is being designed Safety of the users The most appropriate design for a highway improvement is the one that balances the mobility needs of the people using the facility （motorists, pedestrians, or bicyclists） with the physical constraints of the corridor within which the facility is located

9、. ility. CROSSSECTION ELEMENTS Travel Lanes The number of lanes needed for a facility is usually determined during the concept stage of project development. It is usually the number of lanes necessary to accommodate the expected traffic volumes at a level of service determined to be appropriate for

10、the facility （see Chapter 4 for a discussion of level of service）. The number of lanes can only be added in integer units, i.e., a twolane highway can be widened to three or four lanes. Each additional lane represents an increase in the trafficcarrying capability of the facility. Knowing future proj

11、ected travel demands, the designer, using the analysis procedures in the Highway Capacity Manual, can provide input into the decisionmaking process during project development to determine the appropriate number of travel lanes for the level of service desired. Community input also plays a part in th

12、is decision. A community may decide through public involvement that a lower level of service is acceptable for the situation than the level of service normally provided for new construction projects. In urban and suburban areas, signalized intersections are usually the predominant factor controlling

13、 the capacity of the highway or street. There may be more latitude in determining the number of lanes for these types of facilities. For example, a twolane facility approaching an intersection can be expanded to four lanes （one left turn lane, two through lanes, one rightturn lane） at the intersecti

14、on itself and then returned to two lanes beyond the intersection. The need to distribute traffic safely will determine the need for any expansion of the approach roadway. The added lanes at the intersection can be in a variety of configurations to serve the travel desires of the traffic. Lane Width

15、The width of travel lanes is limited by the physical dimensions of automobiles and trucks to a range between 2.7 and 3.6 m （9 and 12 ft）. Generally, as the design speed of a highway increases, so must the lane width to allow for the lateral movement of vehicles within the lane. However, constricted

16、rightofway and other design restrictions can have an impact on this decision. Chapter IV of the Green Book recognizes the need for flexibility in these cases:Although lane widths of 3.6 m are desirable on both rural and urban facilities, there are circumstances that necessitate the use of lanes less

17、 than 3.6 m wide. In urban areas where rightofway and existing development become stringent controls, the use of 3.3 m lanes is acceptable. Lanes 3.0 m wide are acceptable on lowspeed facilities. Lanes 2.7 m wide are appropriate on lowvolume roads in rural and residential areas. A landscaped median.

18、 （I35E, St. Paul, MN） Medians An important consideration in the design of any multilane highway is whether to provide a median and, if one is provided, what the dimensions should be. The primary functions of highway medians are to: Separate opposing traffic flows Provide a recovery area for outofcon

19、trol vehicles Allow space for speed changes and leftturning and Uturning vehicles Minimize headlight glare Provide width for future lanes （particularly in suburban areas） Provide a space for landscape planting that is in keeping with safety needs and improves the aesthetics of the facility Provide a

20、 space for barriers. Depending on agency practice and specific location requirements, medians may be depressed, raised, or flush with the surface of the traveled way. Medians should have a dimension that is in balance with the other elements of the total highway cross section. The general range of m

21、edian widths is from 1.2 m （4 ft）, usually in urban areas, to 24 m （80 ft） or more, in rural areas. An offset of at least a 500 mm （1.5 ft） should be provided between any vertical element located within the median, such as a curb or barrier, and the edge of the adjacent traveled lane. The design and

22、 width of medians again require tradeoffs for designers. In locations where the total available rightofway is restricted, a wide median may not be desirable if it requires narrowing the areas adjacent to the outside edge of the traveled way. A reasonable border width is required to serve as a buffer

23、 between private development along the road and the edge of the traveled way, and space may be needed for sidewalks, highway signs, utilities, parking, drainage channels and structures, proper slopes and clear zones, and any retained native plant material. On the other hand, wider medians provide mo

24、re space for plant material, offer a refuge for pedestrians at intersections, and help soften the look of the roadway. Including and designing medians requires public input to find the design that meets the needs of the community. Twoway leftturn lanes improve safety and efficiency for vehicular tra

25、ffic but do not afford a safe refuge for pedestrians. The use of twoway leftturn lanes on urban streets in densely developed suburban commercial areas has increased as an alternative to raised medians with leftturn or Uturn bays. Although not as aesthetically pleasing as raised, planted medians, con

26、tinuous leftturn lanes can improve capacity. Twoway leftturn lanes generally are not recommended in residential areas because they do not afford a safe refuge for pedestrians. Also, the number of driveways can create unsafe vehicle maneuvers. Shoulders increase safety and highway capacity and provid

27、e a place for pedestrians and bicyclists when no sidewalks are provided. （Rt. 197, MD） Shoulders Although the physical dimensions of automobiles and trucks limit the basic width of travel lanes, the treatment of that portion of the highway to the right of the actual traveled way, that is, the roadwa

28、y edge, provides the designer with a greater degree of flexibility. This is true in both urban and rural areas, although different design elements are more appropriate in each location. Shoulder widths typically vary from as little as 0.6 m （2 ft） on minor rural roads, where there is no surfacing, t

29、o about 3.6 m （12 ft） on major highways, where the entire shoulder may be stabilized or paved. The treatment of shoulders is important from a number of perspectives, including safety, the capacity of the highway section, impact on the surrounding environment, and both the initial capital outlay and

30、ongoing maintenance and operating costs. The shoulder design should balance these factors. For example, a designer must consider the impact of the shoulder width and other roadside elements on the surrounding environment and, at the same time, how these dimensions will affect capacity. Even with a m

31、aximum lane width of 3.6 m （12 ft）, the absence of a shoulder or the presence of an obstruction at the edge of the travel lane can result in a reduction in capacity of as much as 30 percent, compared to an area where shoulder or clear zone exists that is a minimum 1.8 m （6 ft） wide. On the other han

32、d, significant environmental, scenic, or historic resources may be adversely affected by a widened shoulder. Another consideration is the accommodation of pedestrians and nonmotorized vehicles. In many parts of the country, highway shoulders provide a separate traveled way for pedestrians, bicyclists, and others （when no sidewalks are provided）. （a）（b