Lane Widths and Transitions. When considering the physical characteristics of cross sections, the values selected will depend on location (rural or urban), speed, traffic volumes, functional classification, and, in urban areas, the type of adjacent development. Tables 2.21, 2.22, and 2.23 provide values currently used in Ohio. Lane width is dependent on design speed, especially in rural areas. Widths may be as narrow as 9 ft (2.74 m) for a local, low-volume road. In urban areas, lane widths can be as narrow as 10 ft (3.05 m), if the road is primarily a residential street. The maximum lane width is generally accepted to be 12 ft (3.66 m) in all locales.
In some cases it may be necessary to widen the pavement on sharp curves to accommodate off-tracking of larger vehicles. Table 2.24 provides a chart of recommended pavement widening based on degree of curvature and design speed. These values are based on a WB-50 design vehicle. The widened portion of the pavement is normally placed on the inside of the curve. Where curves are introduced with spiral transitions, the widening occurs over the length of the spiral. On alignments without spirals, the widening is developed over the same distance that the superelevation transition occurs. The centerline pavement marking and the center joint (if applicable) should be placed equidistant from the pavement edges. See Fig. 2.12 for illustrations of curve widening.
Whenever the driver’s lane is being shifted—for example, when lanes are being added or eliminated—the shifting rate should be controlled using the following equations:
L = WS for design speeds over 40 mi/h (2.5)
S 2
L = W for design speeds up to 40 mi/h (2.6)
where L = approach taper length, ft W = offset width, ft S = design speed, mi/h
Where lanes are being added but the driver is not being “forced” to follow the actual transition (such as in adding right turn lanes), the transition can occur in 50 ft (15 m) on most roadways or 100 ft (30 m) on freeway designs.
Pavement Cross Slopes. Roadways on tangent or relatively straight alignments where no superelevation is required are normally crowned (peaked) in the middle. Cross slopes are usually in the range of 0.015 to 0.020 ft/ft (m/m). Urban areas with curbed pavements are more likely to have a slope near the upper limit, while rural roadways tend to have a little flatter cross slope. The following guidelines are applicable to the location of the crown point:
• Crowns should be located at or near lane lines.
• For pavements with three or four lanes, no more than two should slope in the same direction.
• Undivided pavement sections should be crowned in the middle when the number of lanes is even, and at the edge of the center lane when the number is odd.
• Narrow raised median sections should be crowned in the middle, so that the majority of the pavement will drain to the outside.
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TABLE 2.21 Guide for Selecting Lane Width for Rural Areas(A)
Conversions: 1 mi/h = 1.609 km/h, 1 ft = 0.305 m. Notes: |
(A) There may be some rural locations that are urban in character. An example would be a village where adjacent development and other conditions resemble an urban area. In such cases, urban design criteria may be used.
(B) The number of lanes should be determined by a capacity analysis.
(C) May be 11 ft on nonfederal projects if design year ADT includes less than 25 (B) and (C) truck units.
(D) An 11-ft lane width may be retained on reconstructed highways if the alignment and safety records are satisfactory.
Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with
permission.
Shoulders. A shoulder is the area adjacent to the roadway that (1) when properly designed, can provide lateral support to the pavement, (2) is available to the motorist in emergency situations, and (3) can be used to maintain traffic during construction. Graded shoulder width is the width of the shoulder measured from the edge of the pavement to the intersection of the shoulder slope and the foreslope. Treated shoulder width is that portion of the graded shoulder that has been improved to at least stabilized aggregate or better. Figure 2.13 illustrates these definitions.
Four basic types of shoulders are used: (1) paved, (2) bituminous surface treated,
(3) stabilized aggregate, and (4) turf. Paved shoulders may be rigid (concrete) or flexible (asphalt). Turf shoulders are usually used on low-volume, uncurbed, local roads. Tables 2.22 and 2.23 provide recommended shoulder widths and types based on functional classification and traffic volumes or locale.
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(E) If 6 or more lanes, use 17 ft. If the truck traffic is less than 250 DDHV use 15 ft.
(F) If 6 or more lanes, use 12 ft. If truck traffic is less than 250 DDHV, 10 ft treated width may be used.
(G) Use 10 ft if truck traffic is less than 250 DDHV. If 10 ft treated width is used, graded width may be reduced by 2 ft.
(H) Guardrail offset is treated width plus 2 ft.
(I) A fully paved shoulder is preferred, but may not be economically feasible. Therefore, a minimum 2 ft of the treated shoulder should be paved. The remainder of the treated shoulder may be either stabilized aggregate or bituminous surface-treated material according to the criteria stipulated in Notes (K) and (L).
(J) Use bituminous surface treated if design year ADT includes between 250 and 1000 (B) and (C) truck units.
(K) The median shoulder width criteria for interstates, other freeways and expressways shall apply to the medians of divided arterials and divided collectors.
(L) Stabilized aggregate may be used on state-maintained roads if the design year ADT includes less than 250 (B) and (C) truck units. Paved shoulders are recommended if the design year ADT includes over 1000 (B) and (C) truck units.
(M) Use 6 ft if design year ADT includes less than 501 (B) and (C) truck units. If 6 ft treated width is used, graded width may be reduced to 10 ft and minimum barrier offset will be 8 ft.
(N) Whenever a design exception is approved for graded shoulder width, the guardrail offset may be reduced but shall not be less than 4 ft.
(O) A 6-ft turf shoulder may be used with a 4:1 or flatter foreslope.
(P) See AASHTO’S Guidelines for Geometric Design for Very Low-Volume Local Roads for values.
(Q) An 8-ft graded shoulder may be used with a 4:1 or flatter foreslope.
Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission.
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TABLE 2.23 Guide for Selecting Lane Width and Shoulders for Urban Areasa
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Conversions: 1 mi/h = 1.609 km/h, 1 ft = 0.305 m.
aUse rural criteria (Tables 2.21 and 2.22) for uncurbed shoulders. Rural functional classification should be determined after checking the urban route extension into a rural area.
‘See Sections 305.3.2 and 305.3.3 for use of curbs and notes on curb/guardrail relationships.
cUse minimum lane width if, in the foreseeable future, the parking lane will be used for through traffic during peak hours or continuously.
dUse 10 ft median shoulder on facilities with 6 or more lanes. Use 12 ft median shoulder on facilities with 6 or more lanes and when truck traffic exceeds 250 DDHV.
eMay be reduced to 10 ft if the truck traffic is less than 250 DDHV.
fMay be reduced to 8 ft if DHV is less than 250.
gThe median shoulder width for divided arterials shall follow the median criteria for Interstates, other Freeways and Expressways.
hLane width may be reduced to 11 ft where right-of-way is limited and current truck ADT is less than 250; however, on all Federal Aid Primary (FAP) roadways at least one 12-ft lane in each direction is required. FAP listings may be obtained from Office of Technical Services’s Roadway Inventory reports.
‘Lane width may be 9 ft where right-of-way is limited and current ADT is less than 250.
Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission.
Whenever practical, shoulders should be designed to be wide enough and strong enough to accommodate temporary traffic, especially on high-volume roadways. Figures 2.14, 2.15, and 2.16 provide information on recommended cross slopes [ft/ft (m/m)] and allowable grade breaks depending on the type of shoulder chosen.
