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 characte
ristics, 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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AASHTO—Geometric Design of Highways and Streets
unequalExhibit 4-1. Typical Cross Section, Normal Crown 310
Cross Section Elements
Exhibit 4-2. Typical Cross Section, Superelevated
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AASHTO—Geometric Design of Highways and Streets
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 cu
rb. 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 norma
l 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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