The type of curb and its location affect driver behavior patterns, which, in turn, affect the safety and utility of a road or street. Curbs, or curbs and gutters, are used mainly in urban areas. They should be used with caution where design speeds exceed 40 mi/h (64 km/h). Following are various reasons for justifying the use of curbs, or curbs and gutters:

• Where required for drainage

• Where needed for channelization, delineation, control of access, or other means of improving traffic flow and safety

• To control parking where applicable

Types of Curb. There are two general categories of curbs: barrier curbs and mount­able curbs. Barrier curbs are relatively high [6 in (0.15 m) or more] and steep-faced. Mountable curbs are 6 in (0.15 m) or less in height and have flatter, sloping faces so that vehicles can cross them with varying degrees of ease. Figure 2.29 (Ref. 14) shows various curb designs that are commonly used on roadways. Types 1, 3, and 4 are examples of mountable curbs and are used for channelizing traffic, especially in islands and medians. Types 2 and 6 are barrier curbs used along pavement edges in urban areas and are designed to handle drainage more efficiently. Types 7 and 8 are tall barrier curbs designed to provide a more positive traffic barrier than the others. Type 7 is used as an alternate for guiderail in low-speed urban situations.

Position of Curb. Curbs are normally used at the edge of pavement on urban streets where the design speed is 40 mi/h (64 km/h) or less. Curbs at the edge of pavement have an effect on the lateral placement of moving vehicles. Drivers tend to shy away from them. Therefore, all curbs should be offset at least 1 ft (0.3 m) and preferably 2 ft (0.6 m) from the edge of the traffic lane. Where curb and gutter are used, the standard gutter width is 2 ft (0.6 m).

On roads where the design speed exceeds 40 mi/h (64 km/h), curbs should be used only in special cases. Special cases may include, but are not limited to, the use of curb to control surface drainage or to reduce right-of-way requirements in restricted areas.

Edge of groded shoulder Portion of volley In depressed aiedl<

‘-B

Treoted Shoulder

СгомоV#r povement Slope t as mo In По* profile.

SECTION Д-Д

Dimensions Applicable toVarying Median Demands M, ft D, in R-l, ft R-2, ft 84 17.25 24.87 55.20 60 11.50 16.31 35.24 50 9.13 12.73 26.93 40 6.75 9.16 18.61

0.04

0.04

Tv-Ospresslon

ДВДЕ

J

SECTION B-B

FIGURE 2.28 Design for U-turn median opening. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission)

‘Asphalt’concrete [ Base J
TYPE /
• Bull Joints shell be provided between combined cu/b-and-guller and new rigid pavements. with lit bars or hoot bolts provided at five foot Intervals. Combined curb-ond-gutter shell be tied to existing rigid pavements wllh expansion hoot bolls spaced at five foot Intorvols. if the combined curb-ond-gulfer adjoins a new rigid base or on existing rigid base or povemeni that Is to be surfoced with bituminous malerlol. a bull Jolnl shall be provided and tie bars.’hoot bolls or expansion hoot bolls shall be omitted.
TYPE 5
TYPE 2	TYPE 4

When it is necessary to use curbs on roads where the design speed is over 40 mi/h, they should not be closer to the traffic than 4 ft (1.2 m) or the edge of the treated shoulder, whichever is greater.

Curb/Guiderail Relationship. If curbs are used in conjunction with guiderail on roads having a design speed in excess of 40 mi/h (64 km/h), the face of curb should preferably be located either at or behind the face of guiderail. Under no conditions should the face of curb be located more than 9 in (0.23 m) in front of the face of rail. This restriction is necessary to prevent a vehicle from “vaulting” over the rail or strik­ing it too high to be contained. Although guiderail is not normally used on curbed roadways having design speeds of 40 mi/h (64 km/h) or less, the same criteria used for higher-speed roadways should apply. Where this is not feasible or practical, the curb may be placed in front of the rail. Regardless of the design speed of the roadway or the placement of the curb, the face of guiderail should not be located closer than 4 ft (1.2 m) to the roadway.

Curb Transitions. Curb and raised median beginnings and endings should be tapered from the curb height to 0 in (0 m) in 10 ft (3 m). When an urban-type section with curbs at the edge of pavement changes to a rural-type section without curbs, the curb should be transitioned laterally at a 4:1 (longitudinal:lateral) rate to the outside edge of the treated shoulder, or 3 ft (0.9 m), whichever is greater. When a curbed side road intersects a mainline that is not curbed, the curb should be terminated no closer to the mainline edge of pavement than 8 ft (2.4 m) or the edge of the treated shoulder of the mainline, whichever is greater.

Cross-section information pertaining to interchange elements, such as ramps and directional roadways, is given in Fig. 2.26. This information includes pavement and shoulder dimensions for acceleration-deceleration lanes, one – and two-lane directional roadways, and medians between adjacent ramps. Notice that for a single-lane ramp, the shoulder and guiderail offset distances are greater on the driver’s right-hand side than on the left. This is to provide more width for drivers to pull over in emergencies and to allow people a better opportunity to go around disabled vehicles.

2.3.2 Medians

A median is a desirable element on all streets or roads with four or more lanes. The principal functions of a median are to prevent interference of opposing traffic, to provide a recovery area for out-of-control vehicles, to provide areas for emergency stopping and left turn lanes, to minimize headlight glare, and to provide width for future lanes. A median should be highly visible both day and night and in definite contrast to the roadway.

Width. The width of a median is the distance between the inside edges of the pave­ment. See Fig. 2.27 for examples of various medians. The width depends upon the type of facility, topography, and available right-of-way. In rural areas with flat or rolling terrain, the desirable median width for freeways is 60 to 84 ft (18 to 26 m). Although the minimum median width is normally 40 ft (12 m), narrower medians may be used in rugged terrain. A constant-width median is not necessary, and in fact,

Lateral clearances, ft
Vertical clearance over surfaced roadway, ftc
On bridgea
Traffic Design year ADT
Rural	Under bridge^7
Functional class	Urban, minimum
Preferred
Minimum
Minimum	Preferred	Minimum	Preferred
Interstates, All Right, 12а’ Right, 14^g Freeways and Left, ¥■• Left, 6 expressways Arterial s > 4000 W 12 2001-4000 ¥ 10 1001-2000 &> 8 400-1000 &’ 8 < 400 4 6 Collectors > 4000 8"’ 10 2001-4000 4 m 8 1001-2000 4?я, я 6 400-1000 4»г, я 4 < 400 4 m, o 4 Locals > 4000 8"’ 10 2001-4000 3 8 1001-2000 3 6 400-1000 3 4 < 400 2 4
16.5й	17.0
.Й л £ п
16.5й
17.0
•9 H л <3 43 3 о
00 О К, 8
14.5	15.0
-с
14.5	15.0
и

Conversion: 1 ft = 0.305 m.

"Distance measured from edge of the traveled lane to face of curb or railing if no curb is provided.

^Distance measured from edge of traveled lane to face of walls or abutments and piers.

cTo minimize structure cost, design tolerances for clearances are plus 4 in, minus 0 in. Sign supports and pedestrian structures have a 1 – ft additional clearance. Clearances shown are over paved shoulder as well as pavement width.

dli bridge is considered to be a major structure having a length of 200 ft or more, the width may be reduced, subject to economic studies, but to no less than 4 ft.

eWhere the truck DDHV is 250 or less, may be reduced 2 ft.

/Where the truck DDHV is 250 or less, the right shoulder width may be reduced 2 ft.

AVhere concrete barrier is used on the approach slabs or in advance of the bridge, the preferred shoulder width will equal the minimum shoulder width.

hA 16.5-ft minimum vertical clearance applies to all rural sections and the single designated route in urban areas. On other urban routes, not on the single designated route, the vertical clearance should not be less than 15.5 ft.

‘If 6 or more lanes, provide 12 ft width. Where truck DDHV is 250 or less, the left shoulder bridge width may be reduced by 2 ft.

;If 6 or more lanes, provide 14 ft width. Where the truck DDHV is 250 or less, the left width may be reduced 2 ft.

*In locations with restricted right-of-way, may be reduced to a clearance of 8.0 ft right side, 4.5 ft median side, plus barrier clearance, except where footnote l applies.

гМау be reduced to a clearance of 2 ft plus barrier clearance on urban streets with restricted right-of-way and a design speed less than 50 mi/h (80 km/h).

mMay be З-ft width if bridge length exceeds 100 ft.

ftMay be З-ft width if turf shoulder is used.

°May be 2-ft width if turf shoulder is used.

?Clear zone width is defined in Art. 6.2.

Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission.

(A)

Use 18′ when inside pavement edge radius is less than 200′.

(B) May be reduced Г if the face of the mainline barrier is 2′ from the outside edge of fhe graded shoulder.

(C) Or match mainline dimension if lesser.

(D) Rounding is 4′ when barrier is used. No rounding is required when foreslope is 6:1 or flatter.

TWO-WAY RAMP MEDIAN

Roundi ng

MINIMUM TWO-WAY RAMP MEDIAN

FIGURE 2.26 Cross-section information for interchange elements—pavement, shoulders, and medians. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission) variable-width medians and independent profiles may be used for the two roadways. Narrow medians with a barrier (barrier medians) are normally used in urban areas. Under normal design, the median width will vary depending on the width of the barrier and the shoulder width required (Table 2.23).

Types. Medians are divided into types depending upon width and treatment of the median area and drainage arrangement. In general, raised or barrier medians are applic­able to urban areas, while wide, depressed medians apply to rural areas. Figure 2.27 shows examples. Medians in rural areas are normally depressed to form a swale in the center and are constructed without curbs. The type of median used in an urban area

BARRIER MEDIAN

—Shoulder Width – Barrier Width —

DEPRESSED MEDIANS

84′

Rounding
60′
loundlng

Rounding

Rounding

FIGURE 2.27 Typical designs for medians. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission) depends on the traffic volume, speed, degree of access, and available right-of-way. On major streets with numerous business drives, a median consisting of an additional lane, striped as a continuous two-way left turn lane, is appropriate. A solid 6-in-high (0.15-m) lane concrete median may be used in low-speed areas (where the design speed is 40 mi/h (64 km/h) or less) and where an all-paved section is desired and a wider median cannot be justified. Barrier medians are normally recommended for urban facilities when the design speed is over 40 mi/h (64 km/h). However, care must be
exercised when barrier medians are used on expressways with unsignalized at-grade intersections because of sight distance limitations and end treatments of the barrier.

U-Turn Median Openings. U-turn median openings may be provided on express­ways, freeways, or interstate highways with nonbarrier medians where space permits and there is a need. U-turns may be needed for proper operation of police and emer­gency vehicles, as well as for equipment engaged in physical maintenance, traffic service, and snow and ice control. U-turn crossings should not be constructed in barrier-type medians. When U-turn median openings are permitted, it is intended they be spaced as close to 3-mi (4.8-km) intervals as possible. Crossings should be located at points approximately 1000 ft (305 m) beyond the end of each interchange speed change lane.

An example of a typical U-turn median opening is shown in Fig. 2.28, which indi­cates geometric features applicable to crossings located in medians of widths ranging from 40 to 84 ft (12 to 26 m). Turning radius should be modified proportionately for medians of varying widths. Tapers should be 200 ft (61 m) in length for all median widths. The profile grade line should normally be an extension of the cross slope of the shoulder paving, rounded at the lowest point.

This section is concerned with the design of the slopes, ditches, parallel channels, and interchange grading. It incorporates into the roadside design the concepts of vehicular safety developed through dynamic testing. Designers are urged to consider flat foreslopes and backslopes, wide gentle ditch sections, and elimination of barriers.

Slopes. Several combinations of slopes and ditch sections may be used in the grading of a project. Details and use of these combinations are discussed in subsequent paragraphs. In general, slopes should be made as flat as possible to minimize the necessity for barrier protection and to maximize the opportunity for a driver to recover

control of a vehicle after leaving the traveled way. Regardless of the type of grading used, projects should be examined in an effort to obtain flat slopes at low costs. For example, fill slopes can be flattened with material that might otherwise be wasted, and backslopes can be flattened to reduce borrow.

To better understand the various types of grading, it is necessary to become familiar with the concept of a clear zone. Clear zone is defined as the unobstructed, relatively flat area provided beyond the edge of the traveled way for the recovery of errant vehicles and includes any shoulders or auxiliary lanes (Ref. 1, 2). Chapter 6 discusses the road­side safety aspects of designing for the clear zone, including the use of barriers to shield objects in the clear zone. In the following paragraphs, four types of roadside grading are described. The designer must select the appropriate one for the roadway being designed.

Safety grading is the shaping of the roadside using 6:1 or flatter slopes within the clear zone area, and 3:1 or flatter foreslopes and recoverable ditches beyond the clear zone. Safety grading is used on interstate highways, other freeways, and expressways. Figures 2.17 and 2.18 show many of these details.

Clear zone grading is the shaping of the roadside using 4:1 or flatter foreslopes and traversable ditches within the clear zone area. Foreslopes of 3:1 may be used but are not measured as part of the clear zone distance. Clear zone grading is recommended

(A)

Treated width includes that portion of the shoulder improved with stabilized aggregate or better.

(B) Minimum barrier clearance.

(C) 3’on interstate, other freeways and expressways.

(D) Treated shoulder width may equal graded shoulder width in some cases.

FIGURE 2.13 Cross sections of shoulders showing graded and treated shoulder widths. Conversions: 2 ft = 0.61 m, 3 ft = 0.91 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission) for undivided rural facilities where the design speed exceeds 50 mi/h (80 km/h), the design hourly volume is 100 or greater, and at least one of the following conditions exists:

• The wider cross section is consistent with present or future planning for the facility.

• The project is new construction or major reconstruction involving significant length.

• The wider cross section can be provided at little or no additional cost.

Figure 2.19 shows examples of clear zone grading and traversable ditches.

Standard grading is the shaping of the roadside using 3:1 or flatter foreslopes and normal ditches. Standard grading is used on undivided facilities where the conditions for the use of safety grading or clear zone grading do not exist. The designer should ensure that any obstacles within the clear zone receive proper protection. Figure 2.20 shows examples of standard grading and normal ditches.

HIGH SIDE OF SUPERELEVATED SECTIONS

Barrier grading is the shaping of the roadside when a barrier is required for slope protection. Normally, 2:1 foreslopes and normal ditch sections are used. Figure 2.20 includes an example of barrier grading.

Rounding of Slopes. Slopes should be rounded at the break points and at the intersection with the existing ground line to reduce the chance of a vehicle’s becoming airborne and to harmonize with the existing topography. Rounding at various locations is illustrated in Figs. 2.17 to 2.20.

Special Median Grading. Figure 2.21c shows some examples of median grading when separate roadway profiles are used.

HIGH SIDE OF SUPERELEVATED SECTIONS

The break at the edge of the pavement shall not exceed 7%.

FIGURE 2.16 Recommended cross slopes and grade brakes for turf shoulders. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission)

Rock and Shale Slopes. In rock or shale cuts, the maximum rate of slope should be determined by a soils engineer. In deep rock or shale cuts where slopes are steeper than 1:1, a 10-ft-wide (3.05-m) bench should be provided between the top of the ditch back – slope and the toe of the rock face as illustrated in Fig. 2.21a. In shale cuts, the designer should not use backslopes steeper than 2:1 unless excessive waste would result. In any event, 2:1 slopes should be used for all shale cut sections less than 20 ft (7 m) in depth, and the bench should be omitted. In this discussion, depth of cut is measured from the top of shale or rock to the ditch flow line. Backslopes steeper than 2:1 should not be used in rock cuts until the depth exceeds 16 ft (5 m). In such cases the bench may be omitted.

Curbed Streets. Figure 2.22 shows typical slope treatments next to curbed streets.

Driveways and Crossroads. At driveways or crossroads, where the roadside ditch is within the clear zone distance and where clear zone grading can be obtained, the ditch and pipe should be located as shown on Fig. 2.23.

CUT SECTION

URBAN INTERSTATE, OTHER FREEWAYS AND EXPRESSWAYS

I • 8:1 I

9-6- oitch

If backslope exceeds 3:1, use 40′ radius as shown above.

SHALLOW CUT OR LOW FILL

•*

Recoverable Oitch

Slope transition between low fill design and medium fill design shall be such that the flowline of the roadside ditch does not turn toward the roadway.

K – Clear zone – sj

Ml]}

MEDIUM FILL

Application of these sections may vary to avoid frequent slope changes and to maintain reasonably straight ditches.

FIGURE 2.17 Cross sections showing safety grading for four different conditions. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission)

Ditches. When the depth or velocity of the design discharge accumulating in a roadside or median ditch exceeds the desirable maximum established for the various highway clas­sifications, a storm sewer will be required to intercept the flow and carry it to a satisfactory outlet. If right-of-way and earthwork considerations are favorable, a deep, parallel side ditch (see Fig. 2.21b) may be more practical and should be considered instead of a storm sewer. In some cases where large areas contribute flow to a highly erodible soil cut, an intercepting ditch may be considered near the top of the cut to intercept the flow from the

outside and thereby relieve the roadside ditch. Constant-depth ditches (usually 18 in or 0.46 m deep) are desirable. Where used, the minimum pavement profile grades should be 0.24 to 0.48 percent. Where flatter pavement grades are necessary, separate ditch profiles are developed and the ditch flow line elevations shown on each cross section.

Parallel Channels. Where it is determined that a stream intercepted by the roadway improvement is to be relocated parallel to the roadway, the channel should be located beyond the limited access line (or highway easement line) in a separate channel ease­ment. This arrangement locates the channel beyond the right-of-way fence, if one is to be installed. Figure 2.21b shows a parallel channel section. This does not apply to con­ventional intercepting erosion control ditches located at the top of cut slopes in rolling terrain.

In areas of low fill and shallow cut, protection along a channel by a wide bench is usually provided. Fill slope should not exceed 6:1 when this design is used, and maxi­mum height from shoulder edge to bench should generally not exceed 10 ft (3.05 m). If it should become necessary to use slopes steeper than 6:1, guiderail may be necessary

CUT SECTION

**■*

Traversable Ditch
(See below)

**For fill heights over!6′ use barrier gradlnc

Normal Ditch

TRAVERSABLE DITCH

FIGURE 2.19 Examples of clear zone grading and traversable ditches. Conversion: 1 ft = 0.305 m. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission)

and fill slopes as steep as 2:1 may be used. In cut sections 5 ft (1.52 m) or more in depth, earth barrier protection can be provided. This design probably affords greater protection where very deep channels are constructed and requires less excavation. Where the sections alternate between cut and fill and it is desired to use but a single design, earth barrier protection is less costly if waste excavation material is available. Likewise, bench protection is less costly if borrow is needed on the project as a whole.

Earth bench or earth barrier protection provided adjacent to parallel channels should not be breached for any reason other than to provide an opening for a natural or relocated stream that requires a drainage structure larger in rise than 42 in (1.07 m). Outlet pipes from median drains or side ditches should discharge directly into the par­allel channel.

Channels and toe-of-slope ditches, used in connection with steep fill slopes, are both removed from the normal roadside section by benches. The designer should establish control offsets to the center of each channel or ditch at appropriate points that govern alignment so the flow will follow the best and most direct course to the outlet. Bench width should be varied as necessary.

STANDARD GRADING

FILL

Slope may be flatter than l^^b’V Exceeds 16 if excess material and right – 0r> » ^ of-way are available at little cost. ‘•*/

Interchange Grading. Interchange interiors should be contour-graded so that maximum safety is provided and the least amount of guiderail is required. Figures 2.24 and 2.25 show examples. The generous use of flat slopes (6:1 or flatter) will also be easier for main­tenance crews to work with. Sight distance is critical for passenger vehicles on ramps as they approach entrance or merge areas. Therefore, sight distance should be unobstructed by landscaping, earth mounds, or other barriers on the merging side of the vehicle.

Crossroads. At a road crossing within an interchange area, bridge spill-through slopes should be 2:1, unless otherwise required by structure design. They should be flattened to 3:1 or flatter in each corner cone and maintained at 3:1 or flatter if within the interior of an interchange. Elsewhere in interchange interiors, fill slopes should not exceed 3:1.

Ramps. Roadside design for ramps should be based on Fig. 2.17 or 2.18, depending on the mainline grading concept.

Gore Area. Gore areas of trumpets, diamonds, and exteriors of loops adjacent to the exit point should be graded to obtain slopes of 6:1 or flatter, which will not endanger a vehicle unable to negotiate the curvature because of excessive speed.

Trumpet Interiors. Interior areas of trumpets (Fig. 2.24) should be graded to slopes not in excess of 8:1, sloping downward from each side of the triangle to a single, rounded low point. Roadside ditches should not be used. Exteriors should be graded in accordance with mainline or ramp standards.

To be used on clear zone grading projects where the roadside ditch flowline is located within the clear zone distance

FIGURE 2.23 Slopes and ditches at driveway and crossroad in cut or low fill for use on clear zone grading projects where ditch is within clear zone distance. (From Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission)

Loop Interiors. In cut, the interior of a loop should be graded to form a normal ditch section adjacent to the lower part of the loop, and the backslope should be extended to intersect the opposite shoulder of the upper part of the loop. This applies unless the character and the amount of material or the adjacent earthwork balances indicate that the cost would be prohibitive. Roadside cleanup and landscaping should be provided in undisturbed areas of loop interiors. If channels are permitted to cross the loop interior, slopes should not be steeper than 4:1. Figure 2.25 shows an example.

Diamond Interiors. If the location of the ramp intersection at the crossroad is relatively near the main facility, a continuous slope between the upper roadway shoulder and the lower roadway ditch will provide the best and most pleasing design. If the ramp

intersection at the crossroad is located a considerable distance from the main facility, then both ramp and mainline roadsides should have independent designs, until the slopes merge near the gore.

If the quadrant is entirely, or nearly so, in cut, the combination of a 3:1 backslope at the low roadway ditch and a gentle slope down from the high roadway shoulder will provide the best design in the wide portion of the quadrant. Approaching the gore, the slopes should transition to continuous 4:1 and 6:1 or flatter slopes. Quadrants located entirely in fill areas should have independently designed roadways for ramp, mainline, and crossroad. Each should be provided with normal slopes not greater than 3:1, with the otherwise ungraded areas sloped to drain without using ditches. If the quadrant is located part in cut and part in fill, the best design features a gentle fill slope at the upper roadway and a gentle backslope at the lower roadway, joined to a bench at the existing ground level that is sloped to drain. The combination of a long diamond ramp having gentle alignment with a loop ramp in the same interchange quadrant is not to be treated as a trumpet. Each ramp should be designed independently of the other in accordance with the suggested details set forth above.

2.3.1 Bridge Criteria

Although bridge engineering is discussed in Chap. 4, information on pertinent physical dimensions is presented here. Lateral clearance at underpasses and vertical clearance over roadways, as used in Ohio, are given in Table 2.25 for new and reconstructed bridges. The table notes provide a good insight into when variations from the standards are allowed.

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.

(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.

TABLE 2.22 Guide for Selecting Shoulders for Rural Areas’*1

Graded width, ft Rounding, ft (B)

Functional Design year ADT With barrier or foreslope steeper than 6:1 Without barrier 6:1 or flatter Treated for design speed, mi/h Guardrail offset, ft (from traveled way) (D) classification foreslope width, ft Type(C) > 50 < 50 Interstate, Other Freeways & Expressways All 17′ Rt. 9′ Med. (E) 12′ Rt. 4′ Med. (F) 12′ Rt. (G) 4′ Med. (F) Paved 10′ (H) Arterial (K) >4000 14′ 10′ 10′ PVD (I) 8′ 4′ 12′ 2001U000 12′ 8′ 8′ PVD (I) 8′ 4′ 10′ 1001-2000 10′ 8′ 6′ BIT. SRF. TRT.(L) 8′ 4′ 8′ 400-1000 10′ 8′ 6′ BIT. SRF. TRT.(L) 4′ 4′ 8′ <400 8′ 8′ 4′ STBL. AGG. 4′ 4′ 6′ Collector (K) >4000 12′ 8′ 8′(M) BIT. SRF. TRT. (L) 8′ 4′ 10′(N) 2001-4000 10′ 8′ 4′ BIT. SRF. TRT. (L) 8′ 4′ 8′(N) 1001-2000 8′ 640) 4′ STBL. AGG. 8′ 4′ 6′(N) 400-1000 6′ 4′ 4′ STBL. AGG. 4′ 4′ 4′ <400 6′ (P) (P) STBL. AGG 4′ 4′ 4′ Local >4000 12′ 8′ (Q) 8′(M) BIT. SRF. TRT.(L) 8′ 4′ 10′(N) 2001-4000 10′ 8′ (Q) 4′ BIT. SRF. TRT. (L) 8′ 4′ 8′(N) 1001-2000 8′ 640) 4′ STBL. AGG 8′ 4′ 6′(N) 400-1000 6′ 4′ 4′ STBL. AGG. 4′ 4′ 4′ <400 6′ (P) (P) STBL. AGG. 4′ 4′ 4′

(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.

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.