Roadway Design Manual: Cross Sectional Elements

Section 6: Cross Sectional Elements

Overview

This section includes information on the following cross sectional design elements:

Pavement design is covered in TxDOT’s Pavement Design Guide.

Pavement Cross Slope

The operating characteristics of vehicles on crowned pavements are such that on cross slopes up to 2 percent, the effect on steering is barely perceptible. A reasonably steep lateral slope is desirable to minimize water ponding on flat sections of uncurbed pavements due to imperfections or unequal settlement. With curbed pavements, a steep cross slope is desirable to contain the flow of water adjacent to the curb. The recommended pavement cross slope for usual conditions is 2 percent. In areas of high rainfall, steeper cross slopes may be used (see A Policy on Geometric Design of Highways and Streets, AASHTO).

On multilane divided highways, pavements with three or more lanes inclined in the same direction desirably should have greater slope across the outside lane(s) than across the two interior lanes. The increase in slope in the outer lane(s) should be at least 0.5 percent greater than the inside lanes (i.e., slope of 2.5 percent). In these cases, the inside lanes may be sloped flatter than normal, typically at 1.5 percent but not less than 1.0 percent.

For tangent sections on divided highways, each pavement should have a uniform cross slope with the high point at the edge nearest the median. Although a uniform cross slope is preferable, on rural sections with a wide median, the high point of the crown is sometimes placed at the centerline of the pavement with cross slopes from 1.5 to 2 percent. At intersections, interchange ramps or in unusual situations, the high point of the crown position may vary depending upon drainage or other controls.

For two lane roadways, cross slope should also be adequate to provide proper drainage. The cross slope for two lane roadways for usual conditions is 2 percent and should not be less than 1.0 percent.

Shoulders should be sloped sufficiently to drain surface water but not to the extent that safety concerns are created for vehicular use. The algebraic difference of cross slope between the traveled way and shoulder grades should not exceed 6 to 7 percent. Maximum shoulder slope should not exceed 10 percent. Following are recommended cross slopes for various types of shoulders:

Bituminous and concrete-surface shoulders should be sloped from 2 to 6 percent (often the slope rate is identical to that used on the travel lanes).
Gravel or crushed rock shoulders should be sloped from 4 to 6 percent.
Turf shoulders should be sloped at about 8 percent.

Pavement cross slopes on all roadways, exclusive of superelevation transition sections, of less than 1 percent will require design exception documentation.

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Slopes and Ditches

Sideslopes. Sideslopes refer to the slopes of areas adjacent to the shoulder and located between the shoulder and the right-of-way line. For safety reasons, it is desirable to design relatively flat areas adjacent to the travelway so that out-of-control vehicles are less likely to turn over, vault, or impact the side of a drainage channel.

Slope Rates. The path that an out-of-control vehicle follows after it leaves the traveled portion of the roadway is related to a number of factors such as driver capabilities, slope rates, and vehicular speed. Accident data indicates that approximately 75 percent of reported encroachments do not exceed a lateral distance of 30 ft [9 m] from the travel lane edge where roadside slopes are 1V:6H or flatter - slope rates that afford drivers significant opportunity for recovery. Crash test data further indicates that steeper slopes (up to 1V:3H) are negotiable by drivers; however, recovery of vehicular control on these steeper slopes is less likely. Recommended horizontal clearance width associated with these slopes are further discussed in Horizontal Clearances to Obstructions.

Design Values. Design guide values for the selection of earth fill slope rates in relation to height of fill are shown in Table 2-10. Particularly difficult terrain or restricted right-of-way width may require deviation from these general guide values. Where conditions are favorable, it is desirable to use flatter slopes to enhance roadside safety.

Anchor: #BGBDJBIBTable 2-10: Earth Fill Slope Rates
Height of Fill	Usual Max Slope Rate, Vertical:Horizontal¹
-	Type of Terrain
-	Flat or Gently Rolling	Rolling
0 - 5 ft [0 - 1.5 m]	1V:6H	1V:4H
5 - 10 ft [1.5 - 3.0 m]	1V:4H	1V:4H
10 – 15 ft [3.0 - 4.5 m]	1V:4H	1V:3H
15 ft and over [4.5 m and over]	Subject to Stability Requirements
¹ Deviation permitted for restrictive right-of-way or particularly difficult terrain conditions.

Front Slope. The slope adjacent to the shoulder is called the front slope. Ideally, the front slope should be 1V:6H or flatter, although steeper slopes are acceptable in some locations. Rates of 1V:4H (or flatter) facilitate efficient operation of construction and maintenance equipment. Slope rates of 1V:3H may be used in constrained conditions. Slope rates of 1V:2H are normally only used on bridge header banks or ditch side slopes, both of which would likely require rip-rap.
When the front slope is steeper than 1V:3H, a longitudinal barrier may be considered to keep vehicles from traversing the slope. A longitudinal barrier should not be used solely for slope protection for rates of 1V:3H or flatter since the barrier may be more of an obstacle than the slope. Also, since recovery is less likely on 1V:3H and 1V:4H slopes, fixed objects should not be present in the vicinity of the toe of these slopes. Particular care should be taken in the treatment of man-made appurtenances such as culvert ends.
Back Slope. The back slope is typically at a slope of 1V:4H or flatter for mowing purposes. Generally, if steep front slopes are provided, the back slopes are relatively flat. Conversely, if flat front slopes are provided, the back slopes may be steeper. The slope ratio of the back slope may vary depending upon the geologic formation encountered. For example, where the roadway alignment traverses through a rock formation area, back slopes are typically much steeper and may be close to vertical. Steep back slope designs should be examined for slope stability.

Design. The intersections of slope planes in the highway cross section should be well rounded for added safety, increased stability, and improved aesthetics. Front slopes, back slopes, and ditches should be sodded and/or seeded where feasible to promote stability and reduce erosion. In arid regions, concrete or rock retards may be necessary to prevent ditch erosion.

Where guardrail is placed on side slopes, the area between the roadway and barrier should be sloped at 1V:10H or flatter.

Roadside drainage ditches should be of sufficient width and depth to handle the design run-off and should be at least 6 inches [150 mm] below the subgrade crown to insure stability of the base course. For additional information, see Drainage Facility Placement.

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Median Design

A median (i.e., the area between opposing travel lane edges) is provided primarily to separate opposing traffic streams. The general range of median width is from 4 ft to 76 ft [1.2 m to 22.8 m], with design width dependent on the type and location of the highway or street facility.

In rural areas, median sections are normally wider than in urban areas. For multi-lane rural highways without access control, a median width of 76 ft [22.8 m] is desirable to provide complete shelter for trucks at median openings (crossovers). These wide, depressed medians are also effective in reducing headlight glare and providing a horizontal clearance for run-off-the-road vehicle encroachments.

Where economically feasible, freeways in rural areas should also desirably include a 76 ft [22.8 m] median. Since freeways by design do not allow at-grade crossings, median widths need not be sufficient to shelter crossing trucks. In this regard, where right-of-way costs are prohibitive, reduced median widths (less than 76 ft [22.8 m]) may be appropriate for certain rural freeways. Statistical studies have shown that over 90 percent of median encroachments involve lateral distances traveled of 48 ft [14.4 m] or less. In this regard, depressed medians on rural freeways sections should be 48 ft [14.4 m] or more in width.

Urban freeways generally include narrower, flush medians with continuous longitudinal barriers. For urban freeways with flush median and six or more travel lanes, full (10 ft [3.0 m]) inside shoulders should be provided to provide space for emergency parking. Median widths vary up to 30 ft [9.0 m], with 24 ft [7.2 m] commonly used. For projects involving the rehabilitation and expansion of existing urban freeways, the provision of wide inside shoulders may not be feasible. Under these circumstances documentation for narrower shoulders should be submitted and a design exception requested.

For low-speed urban arterial streets, flush or curbed medians are used. A width of 16 ft [4.8 m] will effectively accommodate left-turning traffic for either raised or flush medians. Where the need for dual left turns are anticipated at cross streets, the median width should be 28 ft [8.4 m]. The two-way (continuous) left-turn lane design is appropriate where there exists (or is expected to exist) a high frequency of mid-block left turns. Median types for urban arterials without access control are further discussed in Chapter 3, Section 2, Urban Streets.

When flush median designs are selected, it should be expected that some crossing and turning movements can occur in and around these medians. Full pavement structure designs will usually be carried across flush medians to allow for traffic movements.

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Lane Widths

For high-speed facilities such as all freeways and most rural arterials, lane widths should be 12 ft [3.6 m] minimum. For low-speed urban streets, 11 ft or 12 ft [3.3 m or 3.6 m] lanes are generally used. Subsequent sections of this manual identify appropriate lane widths for the various classes of highway and street facilities.

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Shoulder Widths

Wide, surfaced shoulders provide a suitable, all-weather area for stopped vehicles to be clear of the travel lanes. Shoulders are of considerable value on high-speed facilities such as freeways and rural highways. Shoulders, in addition to serving as emergency parking areas, lend lateral support to travel lane pavement structure, provide a maneuvering area, increase sight distance of horizontal curves, and give drivers a sense of safe, open roadway. Design shoulder widths for the various classes of highways are shown in the appropriate subsequent portions of this manual.

On urban collector and local streets, parking lanes may be provided instead of shoulders. On arterial streets, parking lanes decrease capacity and generally are discouraged.

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Sidewalks and Pedestrian Elements

Sidewalks provide distinct separation of pedestrians and vehicles, serving to increase pedestrian safety as well as to enhance vehicular capacity. Sidewalks are typically an integral part of the transportation system in central business districts. In rural and suburban areas, sidewalks are most justified at points of community development such as at schools, recreation areas, and local businesses, that result in pedestrian concentrations near or along the highways. In typical suburban development, there are initially few pedestrian trips because there are few closely located pedestrian destinations. However, when pedestrian demand increases with additional development, it may be more difficult and more costly to go back and install pedestrian facilities if they were not considered in the initial design. Early consideration of pedestrian facility design during the project development process may also greatly simplify compliance with accessibility requirements established by the Americans with Disabilities Act Accessibility Guidelines (ADAAG) and the Texas Accessibility Standards (TAS).

Sidewalk Location. For pedestrian comfort, especially adjacent to high speed traffic, it is desirable to provide a buffer space between the traveled way and the sidewalk as shown in Figure 2-11(A). For curb and gutter sections, a buffer space of 3 ft [915 mm] or greater between the back of the curb and the sidewalk is desirable. For rural sections without curb and gutter, sidewalks should be placed between the ditch and the right of way line if practical.

Sidewalk Width. Sidewalks should be wide enough to accommodate the volume and type of pedestrian traffic expected in the area. The minimum clear sidewalk width is 5 ft [1525 mm]. Where a sidewalk is placed immediately adjacent to the curb as shown in Figure 2-11(B), a sidewalk width of 6 ft [1830 mm] is desirable to allow additional space for street and highway hardware and allow for the proximity of moving traffic. Sidewalk widths of 8 ft [2440 mm] or more may be appropriate in commercial areas, along school routes, and other areas with concentrated pedestrian traffic.

Where necessary to cross a driveway while maintaining the maximum 2 percent cross slope, the sidewalk width may be reduced to 4 ft [1220 mm] if insufficient space is available to locate street fixtures (elements such as sign supports, signal poles, fire hydrants, manhole covers, and controller cabinets that are not intended for public use) outside the 5 ft [1525 mm] minimum clear width. The width may be reduced to 4 ft [1220 mm] for a length of 2 ft [610 mm] maximum, provided that reduced width segments are separated by at least 5 ft [1525 mm] in length.

Street Crossings. Intersections can present formidable barriers to pedestrian travel. Intersection designs which incorporate properly placed curb ramps, sidewalks, crosswalks, pedestrian signal heads and pedestrian refuge islands can provide a pedestrian-friendly environment. Desirably, drainage inlets should be located on the upstream side of crosswalks and sidewalk ramps. Refuge islands enhance pedestrian comfort by reducing effective walking distances and pedestrian exposure to traffic. Islands should be a minimum of 5 ft [1525 mm] wide to afford refuge to people in wheelchairs. A minimum 5 ft [1525 mm] width should be cut through the island for pedestrian passage, or curb ramps with a minimum 5 ft x 5 ft [1525 mm x 1525 mm] landing provided in the island.

Curb Ramps and Landings. Curb ramps must be provided in conjunction with each project where the following types of work will be performed:

resurfacing projects, including overlays and seal coats, where a barrier exists to a sidewalk or path
construction of curbs, curb and gutter, and/or sidewalks
installation of traffic signals which include pedestrian signals
installation of pavement markings for pedestrian crosswalks

A sidewalk curb ramp and level landing will be provided wherever a public sidewalk crosses a curb or other change in level. The maximum grade for curb ramps is 8.3 percent. The maximum cross slope for curb ramps is 2 percent. Flatter grades and slopes should be used where possible and to allow for construction tolerances. The minimum width of curb ramps is 4 ft [1220 mm], exclusive of flared sides. Where a side of a curb ramp is contiguous with a public sidewalk or walking surface, it will be flared with a slope of 10 percent maximum.

Where a perpendicular or diagonal curb ramp is provided, a landing must be provided at the top of the ramp run. The slope of the landing will not exceed 2 percent in any direction. The landing should have a minimum clear dimension of 5 ft x 5 ft [1525 mm x 1525 mm] square or a 5 ft [1525 mm] diameter circle and will connect to the continuous passage in each direction of travel as shown in Figure 2-12. Landings may overlap with other landings.

Where a parallel curb ramp or driveway crossing is provided (i.e., the sidewalk ramps down to a landing at street level), the designer is encouraged to use a slope of 5 percent or less to avoid the need for detectable warnings or handrails on the sloped portions of the sidewalk. A minimum 5 ft x 5 ft [1525 mm x 1525 mm] landing should be provided at the entrance to the street for parallel curb ramps.

At marked crossings, the bottom of a curb ramp run should be wholly contained within the markings of the crosswalk. Beyond the curb line, there should be a minimum 4 ft x 4 ft [1220 mm x 1220 mm] maneuvering space wholly contained within the crosswalk (marked or unmarked) and outside the path of parallel vehicular traffic.

Manhole covers, grates, and obstructions should not be located within the curb ramp, maneuvering area, or landing.

The perpendicular curb ramp configuration, as shown in Figure 2-13, should be used on all new sidewalk or curb ramp installations and all sidewalk or curb ramp reconstruction projects. Studies have shown that the diagonal configuration shown in Figure 2-14 may inadvertently direct visually impaired persons into the center of the intersection. Existing diagonal curb ramp installations may be allowed to remain only on projects where no work is planned for the existing sidewalks. Perpendicular ramps may be placed within the curb radius; however, they must be wholly contained within the crosswalk and they must meet the slope requirements in ADAAG/TAS.

The standard sheet PED may be referenced for additional information on the configuration of curb ramps.

Cross Slope. Sidewalk cross slope will not exceed 1:50 (2 percent). Due to construction tolerances, it is recommended that sidewalk cross slopes be shown in the plans at 1.5 percent to avoid exceeding the 2 percent limit when complete. Cross slope requirements also apply to the continuation of the pedestrian route through the cross walk. Sidewalks immediately adjacent to the curb or roadway may be offset to avoid a non-conforming cross slope at driveway aprons by diverting the sidewalk around the apron as shown in Figure 2-11.

Street Furniture. Special consideration should be given to the location of street furniture (items intended for use by the public such as benches, public telephones, bike racks, and parking meters). A clear ground space at least 2.5 ft x 4 ft [760 mm x 1220 mm] with a maximum slope of 2 percent must be provided and positioned to allow for either forward or parallel approach to the element in compliance with ADAAG/TAS. The clear ground space must have an accessible connection to the sidewalk and must not encroach into the 5 ft [1525 mm] minimum sidewalk width by more than 2 ft [610 mm].

ADAAG/TAS. Specific design requirements to accommodate the needs of persons with disabilities are established by the ADAAG/TAS. A request for a design variance for any deviations from TAS requirements must be submitted to the Design Division for forwarding to the Texas Department of Licensing and Regulation for approval.

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Figure 2-9. Sidewalks at Driveway Aprons. Click here to see a PDF of the image.

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Figure 2-10. Curb Ramps and Landings. Click here to see a PDF of the image.

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Figure 2-11. Perpendicular Curb Ramps. Click here to see a PDF of the image.

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Figure 2-12. Diagonal Curb Ramps (Not Recommended). Click here to see a PDF of the image.

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Curb and Curb and Gutters

Curb designs are classified as vertical or sloping. Vertical curbs are defined as those having a vertical or nearly vertical traffic face 6 inches [150 mm] or higher. Vertical curbs are intended to discourage motorists from deliberately leaving the roadway. Sloping curbs are defined as those having a sloping traffic face 6 inches [150 mm] or less in height. Sloping curbs can be readily traversed by a motorist when necessary. A preferable height for sloping curbs at some locations may be 4 inches [100 mm] or less because higher curbs may drag the underside of some vehicles.

Curbs are used primarily on frontage roads, crossroads, and low-speed streets in urban areas. They should not be used in connection with the through, high-speed traffic lanes or ramp areas except at the outer edge of the shoulder where needed for drainage, in which case they should be of the sloping type.

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Roadside Design

Of particular concern to the design engineer is the number of single-vehicle, run-off-the-road accidents which occur even on the safest facilities. About one-third of all highway fatalities are associated with accidents of this nature. The configuration and condition of the roadside greatly affect the extent of damages and injuries for these accidents.

Increased safety may be realized through application of the following principles, particularly on high-speed facilities:

A “forgiving” roadside should be provided, free of unyielding obstacles including landscaping, drainage facilities that create obstacles, steep slopes, utility poles, etc. For adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practicable for the specific highway and traffic conditions.
For existing highways, treatment of obstacles should be considered in the following order:
- Eliminate the obstacle.
- Redesign the obstacle so that it can by safely traversed.
- Relocate the obstacle to a point where it is less likely to be struck.
- Make the obstacle breakaway.
- Apply a cost-effective device to provide for redirection (longitudinal barrier) or severity reduction (impact attenuators). Barrier should only be used if the barrier is less of an obstacle than the obstacle it would protect, or if the cost of otherwise safety treating the obstacle is prohibitive.
- Delineate the obstacle.
Use of higher than minimum design standards result in a driver environment which is fundamentally safer because it is more likely to compensate for driver errors. Frequently, a design, including sight distances greater than minimum, flattened slopes, etc., costs little more over the life of a project and increases safety and usefulness substantially.
For improved safety performance, highway geometry and traffic control devices should merely confirm drivers' expectations. Unexpected situations, such as left side ramps on freeways, sharp horizontal curvature introduced within a series of flat curves, etc., have demonstrated adverse effects on traffic operations.

These principles have been incorporated as appropriate into the design guidelines included herein. These principles should be examined for their applicability at an individual site based on its particular circumstances, including the aspects of social impact, environmental impact, economy, and safety.

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Horizontal Clearances to Obstructions

A clear recovery area, or horizontal clearance, should be provided along high-speed rural highways. Such a recovery area should be clear of unyielding objects where practical or shielded by crash cushions or barrier. Table 2-11 shows criteria for horizontal clearances.

Anchor: #BGBFAHEETable 2-11: Horizontal Clearances
(US Customary)
Location	Functional Classification	Design Speed (mph)	Avg. Daily Traffic2	Horizontal Clearance Width (ft)^3,4,5
-	-	-	-	Minimum	Desirable
Rural	Freeways	All	All	30 (16 for ramps)
Rural	Arterial	All	0 - 750 750 - 1500 >1500	10 16 30	16 30 --
Rural	Collector	≥ 50	All	Use above rural arterial criteria.
Rural	Collector	≤45	All	10	--
Rural	Local	All	All	10	--
Suburban	All	All	<8,000	10⁶	10⁶
Suburban	All	All	8,000 - 12,000	10⁶	20⁶
Suburban	All	All	12,000 - 16,000	10⁶	25⁶
Suburban	All	All	>16,000	20⁶	30⁶
Urban	Freeways	All	All	30 (16 for ramps)
Urban	All (Curbed)	≥ 50	All	Use above suburban criteria insofar as available border width permits.
Urban	All (Curbed)	≤ 45	All	1.5 from curb face	3.0
Urban	All (Uncurbed)	≥ 50	All	Use above suburban criteria.
Urban	All (Uncurbed)	≤ 45	All	10	--
¹ Because of the need for specific placement to assist traffic operations, devices such as traffic signal supports, railroad signal/warning device supports, and controller cabinets are excluded from horizontal clearance requirements. However, these devices should be located as far from the travel lanes as practical. Other non-breakaway devices should be located outside the prescribed horizontal clearances or these devices should be protected with barrier. ²Average ADT over project life, i.e., 0.5 (present ADT plus future ADT). Use total ADT on two-way roadways and directional ADT on one-way roadways. ³ Without barrier or other safety treatment of appurtenances. ⁴ Measured from edge of travel lane for all cut sections and for all fill sections where side slopes are 1V:6H or flatter. Where fill slopes are steeper than 1V:6H it is desirable to provide an area free of obstacles beyond the toe of slope. ⁵ Desirable, rather than minimum, values should be used where feasible. ⁶ Purchase of 5 ft or less of additional right-of-way strictly for satisfying horizontal clearance provisions is not required.

Anchor: #i1067665Table 2-11: Horizontal Clearances
(Metric)
Location	Functional Classification	Design Speed (km/h)	Avg. Daily Traffic2	Horizontal Clearance Width (m)3,4,5
-	-	-	-	Minimum	Desirable
Rural	Freeways	All	All	9.0 (4.9 for ramps)
Rural	Arterial	All	0 - 750 750 - 1500 >1500	3.0 4.9 9.0	4.9 9.0 --
Rural	Collector	≥ 80	All	Use above rural arterial criteria.
Rural	Collector	≤ 70	All	3.0	--
Rural	Local	All	All	3.0	--
Suburban	All	All	<8,000	3.06	3.06
Suburban	All	All	8,000 - 12,000	3.06	6.06
Suburban	All	All	12,000 - 16,000	3.06	7.66
Suburban	All	All	>16,000	6.06	9.06
Urban	Freeways	All	All	9.0 (4.9 for ramps)
Urban	All (Curbed)	≥ 80	All	Use above suburban criteria insofar as available border width permits.
Urban	All (Curbed)	≤ 70	All	0.5 from curb face	1.0
Urban	All (Uncurbed)	≥ 80	All	Use above suburban criteria.
Urban	All (Uncurbed)	≤ 70	All	3.0	--
¹ Because of the need for specific placement to assist traffic operations, devices such as traffic signal supports, railroad signal/warning device supports, and controller cabinets are excluded from horizontal clearance requirements. However, these devices should be located as far from the travel lanes as practical. Other non-breakaway devices should be located outside the prescribed horizontal clearances or these devices should be protected with barrier. ²Average ADT over project life, i.e., 0.5 (present ADT plus future ADT). Use total ADT on two-way roadways and directional ADT on one-way roadways. ³Without barrier or other safety treatment of appurtenances. ⁴Measured from edge of travel lane for all cut sections and for all fill sections where side slopes are 1V:6H or flatter. Where fill slopes are steeper than 1V:6H it is desirable to provide an area free of obstacles beyond the toe of slope. ⁵Desirable, rather than minimum, values should be used where feasible. ⁶Purchase of 1.5 m or less of additional right-of-way strictly for satisfying horizontal clearance provisions is not required.

The horizontal clearance values shown in Table 2-11 are measured from the edge of travel lane. These are appropriate design values for all cut sections (see Drainage Facility Placement), for cross sectional design of ditches within the horizontal clearance area) and for all fill sections with side slopes 1V:4H or flatter. It should be noted that, while a 1V:4H slope is acceptable, that a 1V:6H or flatter slope is preferred for both errant vehicle performance and slope maintainability. For fill slopes steeper than 1V:4H, errant vehicles have a reduced chance of recovery and the lateral extent of each roadside encroachment increases. It is therefore preferable to provide an obstacle-free area beyond the toe of steep side slopes even when this area is outside the horizontal clearance.