美国道路线型设计规范AASHTO—Geometric Design of Highways and Streets
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CHAPTER 4
CROSS SECTION ELEMENTS
GENERAL
To assure consistency in this policy, the terms “roadway” and “traveled way” are defined by AASHTO as follows:
Roadway: The portion of a highway, including shoulders, for vehicular use. A divided highway has two or more roadways (see Exhibits 4-1 and 4-2).
Traveled way: The portion of the roadway for the movement of vehicles, exclusive of shoulders (see Exhibits 4-1 and 4-2).
PAVEMENT
Surface Type
The selection of pavement type is determined based on the traffic volume and composition, soil characteristics, weather, performance of pavements in the area, availability of materials, energy conservation, initial cost, and the overall annual maintenance and service-life cost. The structural design of pavements is not included in this policy, but is addressed in the AASHTO Guide for Design of Pavement Structures (1).
Important pavement characteristics that are related to geometric design are the effect on driver behavior and the ability of a surface to retain its shape and dimensions, to drain, and to retain adequate skid resistance. High-type pavements retain their shape and do not ravel at the edges if placed on a stable subgrade. Their smoothness and proper cross-slope design enable drivers to steer easily and keep their vehicles moving in the proper path. At the other extreme, low-type surfaces have a tendency toward raveling, which reduces their effective width and requires greater steering effort to maintain a correct path. Accordingly, low-type surfaces are used where traffic volume is light.
While the selection of design speed is dependent on many factors other than pavement surface type, high-type surfaces provide for higher operating speeds than do low-type surfaces. Therefore, the surface type provided should be consistent with the selected design speed for the highway.
Cross Slope
Undivided traveled ways on tangents, or on flat curves, have a crown or high point in the middle and a cross slope downward toward both edges. Unidirectional cross slopes across the
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Exhibit 4-1. Typical Cross Section, Normal Crown 310
Cross Section Elements
Exhibit 4-2. Typical Cross Section, Superelevated
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entire width of the traveled way may be utilized. The downward cross slope may be a plane or rounded section or a combination. With plane cross slopes, there is a cross slope break at the crown line and a uniform slope on each side. Rounded cross sections usually are parabolic, with a slightly rounded surface at the crown line and increasing cross slope toward the edge of the traveled way. Because the rate of cross slope is variable, the parabolic section is described by the crown height (i.e., the vertical drop from the center crown line to the edge of the traveled way). The rounded section is advantageous in that the cross slope steepens toward the edge of the traveled way, thereby facilitating drainage. Disadvantages are that rounded sections are more difficult to construct, the cross slope of the outer lanes may be excessive, and warping of pavement areas at intersections may be awkward or difficult to construct.
On divided highways each one-way traveled way may be crowned separately as on two-lane highways, or it may have a unidirectional cross slope across the entire width of the traveled way, which is almost always downward to the outer edge. A cross section with each roadway crowned separately, as shown in Exhibit 4-3A through Exhibit 4-3C, has an advantage in rapidly draining the pavement during rainstorms. In addition, the difference between high and low points in the cross section is minimal. Disadvantages are that more inlets and underground drainage lines are needed, and treatment of intersections is more difficult because of the number of high and low points on the cross section. Use of such sections should preferably be limited to regions of high rainfall or where snow and ice are major factors. Sections having no curbs and a wide depressed median are particularly well-suited for these conditions.
Exhibit 4-3. Roadway Sections for Divided Highway (Basic Cross Slope Arrangements) 312
Cross Section Elements Roadways with unidirectional cross slopes, as shown in Exhibit 4-3D through Exhibit 4-3G, tend to provide more comfort to drivers when they change lanes and may either drain away from or toward the median. Drainage away from the median may effect a savings in drainage structures, minimize drainage across the inner, higher-speed lanes, and simplify treatment of intersecting streets. Drainage toward the median is advantageous in that the outer lanes, which are used by most traffic, are more free of surface water. This surface runoff, however, should then be collected into a single conduit under the median. Where curbed medians exist, drainage is concentrated next to or on higher-speed lanes. When the median is narrow, this concentration results in splashing on the windshields of opposing traffic.
The rate of cross-slope is an important element in cross-section design. Superelevation on curves is determined by the speed-curvature relationships given in Chapter 3, but cross slope or crown on tangents or on long-radius curves are complicated by two contradictory controls. On one hand, a reasonably steep lateral slope is desirable to minimize ponding of water on pavements with flat profile grades as a result of pavement imperfections or unequal settlement. A steep cross slope is also desirable on curbed pavements to confine water flow to a narrow width of pavement adjacent to the curb. On the other hand, steep cross slopes are undesirable on tangents because of the tendency of vehicles to drift toward the low edge of the traveled way. This drifting becomes a major concern in areas where snow and ice are common. Cross slopes up to and including 2 percent are barely perceptible in terms of vehicle steering. However, cross slopes steeper than 2 percent are noticeable and require a conscious effort in steering. Furthermore, steep cross slopes increase the susceptibility to lateral skidding when vehicles brake on icy or wet pavements or when stops are made on dry pavements under emergency conditions.
The prevalence of high winds may significantly alter the effect of cross slope on steering. In rolling or mountainous terrain with alternate cut-and-fill sections or in areas alternately forested and cleared, any substantial cross wind produces an intermittent impact on a vehicle moving along the highway and affects its steering. In areas where such conditions are likely, it is desirable to avoid high rates of cross slope.
On high-type two-lane roadways, crowned at the center, the accepted rate of cross slope ranges from 1.5 to 2 percent. When three or more lanes are inclined in the same direction on multilane highways, each successive pair of lanes or portion thereof outward from the first two lanes from the crown line may have an increased slope. The two lanes adjacent to the crown line should be pitched at the normal minimum slope, and on each successive pair of lanes or portion thereof outward, the rate may be increased by about 0.5 to 1 percent. As shown in Exhibit 4-3G, the left side has a continuous sloped pavement while the right has an increased slope on the outer lane.
Use of cross slopes steeper than 2 percent on high-type, high-speed highways with a central crown line is not desirable. In passing maneuvers, drivers cross and recross the crown line and negotiate a total rollover or cross-slope change of over 4 percent. The reverse curve path of travel of the passing vehicle causes a reversal in the direction of centrifugal force, which is further exaggerated by the effect of the reversing cross slopes. Trucks with high centers of gravity crossing over the crown line are caused to sway from side to side when traveling at high speed, at
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which time control may be difficult to maintain. Exhibits 4-3A through 4-3C are examples of roadway conditions where this situation would be encountered.
In areas of intense rainfall, a somewhat steeper cross slope may be needed to facilitate roadway drainage. In such cases, the cross slope on high-type pavements may be increased to 2.5 percent, with a corresponding crown line crossover of 5 percent. Where three or more lanes are provided in each direction, the maximum cross slope should be limited to 4 percent. Use of this increased cross slope should be limited to the condition described in the preceding discussion. For all other conditions, a maximum cross slope of 2 percent should be used for high-type pavements. In locations of intense rainfall and where the maximum cross slope is used, consideration should be given to the use of grooving or open-graded mixes.
The cross slope rates discussed above pertain largely to high-type surfaces. A greater cross slope should be utilized for low-type surfaces. Exhibit 4-4 shows a range of values applicable to each type of surface.
Surface type Range in cross-slope rate (%)
High 1.5–2
Low2–6
Exhibit 4-4. Normal Traveled-Way Cross Slope
Because of the nature of the surfacing materials used and surface irregularities, low-type surfaces such as earth, gravel, or crushed stone need an even greater cross slope on tangents to prevent the absorption of water into the surface. Therefore, cross slopes greater than 2 percent may be used on these types of surfaces.
Where roadways are designed with outer curbs, the lower values in the ranges of cross slopes in Exhibit 4-4 are not recommended because of the increased likelihood of there being a sheet of water over a substantial part of the traveled way adjacent to the curb. For any rate of rainfall, the width of traveled way that is inundated with water varies with the rate of cross slope, roughness of gutter, frequency of discharge points, and longitudinal grade. A cross slope greater than 1 percent is desirable, and in some cases, a cross slope of more than 1.5 percent is needed to limit inundation to about half of the outer traffic lane. A cross slope of 1.5 percent is suggested as a practical minimum for curbed high-type pavement. Curbs with steeper adjacent gutter sections may permit the use of lesser rates of cross slope. A preferred cross-section treatment is the use of a straight shoulder slope and the avoidance of curbs, whenever practical.
Skid Resistance
Skidding crashes are a major concern in highway safety. It is not sufficient to attribute skidding crashes merely to “driver error” or “driving too fast for existing conditions.” The 314
Cross Section Elements roadway should provide a level of skid resistance that will accommodate the braking and steering maneuvers that can reasonably be expected for the particular site.
Research has demonstrated that highway geometrics affect skidding (2). Therefore, skid resistance should be a consideration in the design of all new construction and major reconstruction projects. Vertical and horizontal alignments can be designed in such a way that the potential for skidding is reduced. Also, improvements to the vertical and horizontal alignments should be considered as a part of any reconstruction project.
Pavement types and textures also affect a roadway’s skid resistance. The four main causes of poor skid resistance on wet pavements are rutting, polishing, bleeding, and dirty pavements. Rutting causes water accumulation in the wheel tracks. Polishing reduces the pavement surface microtexture and bleeding can cover it. In both cases, the harsh surface features needed for penetrating the thin water film are diminished. Pavement surfaces will lose their skid resistance when contaminated by oil drippings, layers of dust, or organic matter. Measures taken to correct or improve skid resistance should result in the following characteristics: high initial skid resistance durability, the ability to retain skid resistance with time and traffic, and minimum decrease in skid resistance with increasing speed.
Tining during placement leaves indentations in the pavement surface and has proved to be effective in reducing the potential for hydroplaning on roadways with portland cement concrete surfaces. The use of surface courses or overlays constructed with polish-resistant coarse aggregate is the most widespread method for improving the surface texture of bituminous pavements. Overlays of open-graded asphalt friction courses are quite effective because of their frictional and hydraulic properties. For further discussion, refer to the AASHTO Guidelines for Skid Resistant Pavement Design (3).
LANE WIDTHS
The lane width of a roadway greatly influences the safety and comfort of driving. Lane widths of 2.7 to 3.6 m [9 to 12 ft] are generally used, with a 3.6-m [12-ft] lane predominant on most high-type highways. The extra cost of providing a 3.6-m [12-ft] lane width, over the cost of providing a 3.0-m [10-ft] lane width is offset to some extent by a reduction in cost of shoulder maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the pavement edges. The wider 3.6-m [12-ft] lane provides desirable clearances between large commercial vehicles traveling in opposite directions on two-lane, two-way rural highways when high traffic volumes and particularly high percentages of commercial vehicles are expected.
Lane widths also affect highway level of service. Narrow lanes force drivers to operate their vehicles closer to each other laterally than they would normally desire. Restricted clearances have much the same effect. In a capacity sense the effective width of traveled way is reduced when adjacent obstructions such as retaining walls, bridge trusses or headwalls, and parked cars restrict the lateral clearance. Further information on the effect of lane width on capacity and level of service is presented in the Highway Capacity Manual (HCM) (4). In addition to the capacity effect, the resultant erratic operation has an undesirable effect on driver comfort and crash rates.
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316Where unequal-width lanes are used, locating the wider lane on the outside (right) provides
more space for large vehicles that usually occupy that lane, provides more space for bicycles, and allows drivers to keep their vehicles at a greater distance from the right edge. Where a curb is used adjacent to only one edge, the wider lane should be placed adjacent to that curb. The basic design decision is the total roadway width, while the placement of stripes actually determines the lane widths.
Although lane widths of 3.6 m [12 ft] are desirable on both rural and urban facilities, there are circumstances where lanes less than 3.6 m [12 ft] wide should be used. In urban areas where pedestrian crossings, right-of-way, or existing development become stringent controls, the use of 3.3-m [11-ft] lanes is acceptable. Lanes 3.0 m [10 ft] wide are acceptable on low-speed facilities, and lanes 2.7 m [9 ft] wide are appropriate on low-volume roads in rural and residential areas. For further information, see NCHRP Report 362, Roadway Widths for Low-Traffic Volume Roads (5). In some instances, on multilane facilities in urban areas, narrower inside lanes may be utilized to permit wider outside lanes for bicycle use. In this situation, 3.0-m to 3.3-m [10- to 11-ft] lanes are common on inside lanes with 3.6-m to 3.9-m [12- to 13-ft] lanes utilized on outside lanes.
Auxiliary lanes at intersections and interchanges often help to facilitate traffic movements. Such added lanes should be as wide as the through-traffic lanes but not less than 3.0 m [10 ft]. Where continuous two-way left-turn lanes are provided, a lane width of 3.0 m to 4.8 m [10 to 16 ft] provides the optimum design.
It may not be cost-effective to design the lane and shoulder widths of local and collector roads and streets that carry less than 400 vehicles per day using the same criteria applicable to higher volume roads or to make extensive operational and safety improvements to such very low-volume roads. AASHTO is currently evaluating alternative design criteria for local and collector roads and streets that carry less than 400 vehicles per day based on a safety risk assessment.
SHOULDERS
General Characteristics
A shoulder is the portion of the roadway contiguous with the traveled way that accommodates stopped vehicles, emergency use, and lateral support of subbase, base, and surface courses. In some cases, the shoulder can accommodate bicyclists. It varies in width from only 0.6 m [2 ft] on minor rural roads where there is no surfacing, or the surfacing is applied over the entire roadbed, to approximately 3.6 m [12 ft] on major roads where the entire shoulder may be stabilized or paved.
The term “shoulder” is variously used with a modifying adjective to describe certain functional or physical characteristics. The following meanings apply to the terms used here:
Cross Section Elements •The “graded” width of shoulder is that measured from the edge of the traveled way to the intersection of the shoulder slope and the foreslope planes, as shown in
Exhibit 4-5A.
•The “usable” width of shoulder is the actual width that can be used when a driver makes an emergency or parking stop. Where the sideslope is 1V:4H or flatter, the “usable”
width is the same as the “graded” width since the usual rounding 1.2 to 1.8 m [4 to 6 ft]
wide at the shoulder break will not lessen its useful width appreciably. Exhibits 4-5B
and 4-5C illustrate the usable shoulder width.
Exhibit 4-5. Graded and Usable Shoulders
Shoulders may be surfaced either full or partial width to provide a better all-weather load support than that afforded by native soils. Materials used to surface shoulders include gravel, shell, crushed rock, mineral or chemical additives, bituminous surface treatments, and various forms of asphaltic or concrete pavements.
The shoulder on minor rural roads with low traffic volume serves essentially as structural lateral support for the surfacing and as an additional width for the traveled way. This permits drivers meeting or passing other vehicles to drive on the edge of the roadway without leaving the surfacing, thus making use of the shoulder itself. Roads with a narrow traveled way, narrow shoulders, and an appreciable traffic volume tend to provide poor service, have a relatively higher crash rate, and need frequent and costly maintenance.
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318Well-designed and properly maintained shoulders are needed on rural highways with an
appreciable volume of traffic, on freeways, and on some types of urban highways. Their advantages include:
•Space is provided away from the traveled way for vehicles to stop because of mechanical difficulties, flat tires, or other emergencies.
•Space is provided for motorists to stop occasionally to consult road maps or for other reasons.
•Space is provided for evasive maneuvers to avoid potential crashes or reduce their severity.
•The sense of openness created by shoulders of adequate width contributes to driving ease and reduced stress.
•Sight distance is improved in cut sections, thereby potentially improving safety.
•Some types of shoulders enhance highway aesthetics.
•Highway capacity is improved because uniform speed is encouraged.
•Space is provided for maintenance operations such as snow removal and storage.
•Lateral clearance is provided for signs and guardrails.
•Storm water can be discharged farther from the traveled way, and seepage adjacent to the traveled way can be minimized. This may directly reduce pavement breakup.
•Structural support is given to the pavement.
•Space is provided for pedestrian and bicycle use, for bus stops, for occasional encroachment of vehicles, for mail delivery vehicles, and for the detouring of traffic
during construction.
For further information on other uses of shoulders, refer to NCHRP Report 254, Shoulder Geometrics and Use Guidelines (6).
Urban highways generally have curbs along the outer lanes. A stalled vehicle, during peak hours, disturbs traffic flow in all lanes in that direction when the outer lane serves through-traffic. Where on-street parking is permitted, the parking lane provides some of the same services listed above for shoulders. Parking lanes are discussed later in this chapter in the section on “On-Street Parking.”
Width of Shoulders
Desirably, a vehicle stopped on the shoulder should clear the edge of the traveled way by at least 0.3 m [1 ft], and preferably by 0.6 m [2 ft]. This preference has led to the adoption of 3.0 m [10 ft] as the normal shoulder width that should be provided along high-type facilities. In difficult terrain and on low-volume highways, shoulders of this width may not be practical. A minimum shoulder width of 0.6 m [2 ft] should be considered for the lowest-type highway, and a 1.8- to 2.4-m [6- to 8-ft] shoulder width is preferable. Heavily traveled, high-speed highways and highways carrying large numbers of trucks should have usable shoulders at least 3.0 m [10 ft] wide and preferably 3.6 m [12 ft] wide; however, widths greater than 3.0 m [10 ft] may encourage unauthorized use of the shoulder as a travel lane. Where bicyclists and pedestrians are to be accommodated on the shoulders, a minimum usable shoulder width (i.e., clear of rumble strips) of
1.2 m [4 ft] should be used. For additional information on shoulder widths to accommodate bicycles, see the AASHTO Guide for the Development of Bicycle Facilities (7). Shoulder widths for specific classes of highways are discussed in Chapters 5 through 8.
Where roadside barriers, walls, or other vertical elements are present, it is desirable to provide a wide enough graded shoulder that the vertical elements will be offset a minimum of 0.6 m [2 ft] from the outer edge of the usable shoulder. To provide lateral support for guardrail posts and/or clear space for lateral dynamic deflection of the particular barrier in use, it may be appropriate to provide a graded shoulder that is wider than the shoulder where no vertical elements are present. On low-volume roads, roadside barriers may be placed at the outer edge of the shoulder; however, a minimum clearance of 1.2 m [4 ft] should be provided from the traveled way to the barrier.
Although it is desirable that a shoulder be wide enough for a vehicle to be driven completely off the traveled way, narrower shoulders are better than none at all. For example, when a vehicle making an emergency stop can pull over onto a narrow shoulder such that it occupies only 0.3 to 1.2 m [1 to 4 ft] of the traveled way, the remaining traveled way width can be used by passing vehicles. Partial shoulders are sometimes used where full shoulders are unduly costly, such as on long (over 60 m [200 ft]) bridges or in mountainous terrain.
Regardless of the width, a shoulder should be continuous. The full benefits of a shoulder are not realized unless it provides a driver with refuge at any point along the traveled way. A continuous shoulder provides a sense of security such that almost all drivers making emergency stops will leave the traveled way. With intermittent sections of shoulder, however, some drivers will find it necessary to stop on the traveled way, creating an undesirable situation. A continuous paved shoulder provides an area for bicyclists to operate without obstructing faster moving motor vehicle traffic. Although continuous shoulders are preferable, narrow shoulders and intermittent shoulders are superior to no shoulders. Intermittent shoulders are briefly discussed below in the section on “Turnouts.”
Shoulders on structures should normally have the same width as usable shoulders on the approach roadways. As previously discussed, the narrowing or loss of shoulders, especially on structures, may cause serious operational and safety problems. Long, high-cost structures usually warrant detailed special studies to determine practical dimensions. Reduced shoulder widths may be considered in rare cases. A discussion of these conditions is provided in Chapters 7 and 10.
Shoulder Cross Sections
Important elements in the lateral drainage systems, shoulders should be flush with the roadway surface and abut the edge of the traveled way. All shoulders should be sloped to drain away from the traveled way on divided highways with a depressed median. With a raised narrow median, the median shoulders may slope in the same direction as the traveled way. However, in regions with snowfall, median shoulders should be sloped to drain away from the traveled way to avoid melting snow draining across travel lanes and refreezing. All shoulders should be sloped sufficiently to rapidly drain surface water, but not to the extent that vehicular use would be
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restricted. Because the type of shoulder construction has a bearing on the cross slope, the two should be determined jointly. Bituminous and concrete-surfaced shoulders should be sloped from 2 to 6 percent, gravel or crushed-rock shoulders from 4 to 6 percent, and turf shoulders from 6 to 8 percent. Where curbs are used on the outside of shoulders, the cross slope should be appropriately designed with the drainage system to prevent ponding on the traveled way.
It should be noted that rigid adherence to the slope rates outlined in this chapter may present minor traffic operational problems if they are applied without regard to the cross section of the paved surface. On tangent or long-radius curved alignment with normal crown and turf shoulders, the maximum algebraic difference in the traveled way and shoulder grades should be from 6 to 7 percent. Although this maximum algebraic difference in slopes is not desirable, it is tolerable due to the benefits gained in pavement stability by avoiding storm water detention at the pavement edge.
Shoulder slopes that drain away from the paved surface on the outside of well-superelevated sections should be designed to avoid too great a cross-slope break. For example, use of a 4 percent shoulder cross slope in a section with a traveled way superelevation of 8 percent results in a 12 percent algebraic difference in the traveled way and shoulder grades at the high edge-of-traveled way. Grade breaks of this order are not desirable and should not be permitted (Exhibit 4-2A). It is desirable that all or part of the shoulder should be sloped upward at about the same rate or at a lesser rate than the superelevated traveled way (see the dashed line labeled Alternate in Exhibit 4-2A). Where this is not desirable because of storm water or melting snow and ice draining over the paved surface, a compromise might be used in which the grade break at the edge of the paved surface is limited to approximately 8 percent by flattening the shoulder on the outside of the curve (Exhibit 4-2B).
One means of avoiding too severe of a grade break is the use of a continuously rounded shoulder cross section on the outside of the superelevated traveled way (Exhibit 4-2C). The shoulder in this case is a convex section continuing from the superelevation slope instead of a sharp grade break at the intersection of the shoulder and traveled way slopes. In this method, some surface water will drain upon the traveled way; however, this disadvantage is offset by the benefit of a smoother transition for vehicles that may accidentally or purposely drive upon the shoulder. It should also be noted that convex shoulders present more difficulties in construction than do planar sections. An alternate method to the convex shoulder consists of a planar shoulder section with multiple breaks in the cross slope. Shoulder cross slopes on the high side of a superelevated section that are substantially less than those discussed above are generally not detrimental to shoulder stability. There is no discharge of storm water from the traveled way to the shoulder and, therefore, little likelihood of shoulder erosion damage.
In some areas, shoulders are designed with a curb or gutter at the outer edge to confine runoff to the paved shoulder area. Drainage for the entire roadway is handled by these curbs, with the runoff directed to selected outlets. The outer portion of the paved shoulder serves as the longitudinal gutter. Cross slopes should be the same as for shoulders without a curb or gutter, except that the slope may be increased somewhat on the outer portion of the shoulder. This type of shoulder is advantageous in that the curb on the outside of the shoulder does not deter motorists from driving off the traveled way, and the shoulder serves as a gutter in keeping storm 320。