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Section 7: Cross-Sectional Elements

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Overview

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

Pavement design is covered in TxDOT’s Pavement Manual.

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Pavement Cross Slope

The operating characteristics of vehicles on crowned pavements are such that for 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 sufficiently 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 frequent rainfall events with high intensities, steeper cross slopes may be used as discussed in AASHTO’s A Policy on Geometric Design of Highways and Streets.

Highways with three or more lanes inclined in the same direction should desirably have an increasing cross slope as the distance from the crown line increases to facilitate pavement drainage. In these cases, the first two lanes adjacent to the crown line may be sloped flatter than normal-typically at 1.5 percent but not less than 1 percent. The cross slope of each successive pair of lanes (or single lane if that is the outside lane) outward from the crown should be increased by 0.5 to 1.0 percent from the cross slope of the adjacent lane. A cross slope should not normally exceed 3 percent on a tangent alignment unless there are three or more lanes in one direction of travel. In areas of intense rainfall and where three or more lanes are provided in one direction of travel, the maximum cross slope should be limited to 4 percent.

On bridge structures with three or more lanes in one direction, maintain a constant slope of 2.5 percent, transitioning before and after the bridge accordingly.

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 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 percent. Maximum shoulder slope should not exceed 10 percent. Following are recommended cross slopes for various types of shoulders:

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  • Bituminous and concrete-surface shoulders on tangents should be sloped from 2 to 6 percent. Often the slope rate is identical to that used on the travel lanes, for constructability, smooth transition, and ease of use during construction and maintenance traffic control.
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  • Gravel or crushed rock shoulders should be sloped from 4 to 6 percent.
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  • Turf shoulders should be sloped at about 8 percent.

For hurricane shoulder evacuation lanes (Evaculanes) and for facilities where widening is anticipated to accommodate the ultimate typical section (i.e. using the proposed shoulders as future traffic lanes) the shoulder cross-slope should be designed in accordance with the criteria for traffic lanes.

Pavement cross slopes on all roadways should not be less than 1 percent. A cross-slope should not normally exceed 3 percent on a tangent alignment unless there are three or more lanes in one direction of travel. Where 3 or more lanes are provided in one direction the cross slope should not exceed 4 percent. Cross-slopes greater than 2 percent should be limited to use in areas of intense rainfall.

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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, with design width dependent on the type and location of the highway or street facility. When determining the minimum median width, consideration should be given to the demand for U-turn movements based on local access requirements. Refer to Chapter 7, Figure 7-47 for minimum median width required to accommodate various design vehicles.

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 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 clear zone for run-off-the-road vehicle encroachments. Refer to Chapter 3 Section 5, Multi-Lane Rural Highways Section for additional information on medians in rural areas.

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

Urban freeways generally include narrower, flush medians with continuous longitudinal barriers. Refer to Appendix A Section 8, Median Barrier for recommendations on the use of median barriers. Medians vary in width, up to 30-ft, with 24-ft commonly used. Refer to Chapter 3 Section 6, Freeways for additional information on medians on freeways.

For low-speed urban arterial streets, flush or curbed medians are used. A width of 16-ft will effectively accommodate left-turning traffic for either raised (turn lane plus raised median) or flush medians. However, where pedestrian refuge is a consideration for raised medians, allowances for a 6-ft width raised median, measured from the back of curb, is preferred, (6-ft measured from the face of curb is the minimum requirement). Refer to Chapter 7 Section 3, Pedestrian Facilities for additional guidance. The continuous two-way left-turn lane (TWLTL) design is appropriate where a high frequency of mid-block left turns exists or are anticipated. 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.

Median encroachment countermeasures should be considered where appropriate. High severity injuries and fatalities are a result of cross median crashes on highspeed roadways. Reducing median encroachment reduces cross median crashes and fixed object crashes in the median. The following guidelines below are for reducing the frequency and severity of median related crashes on divided highways:

Design Guidance to Reduce Consequences of Median Encroachments

Countermeasures to Reduce Likelihood of Median Encroachments

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Lane Widths

For high-speed facilities such as all freeways and most rural arterials, lane widths should be 12-ft minimum. For low-speed urban streets, 11-ft or 12-ft 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

Shoulders are of considerable value on high-speed facilities such as freeways and rural highways. Wide, surfaced shoulders provide a suitable, all-weather area for stopped vehicles to be clear of the travel lanes. Shoulders, in addition to serving as emergency parking, 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 values for shoulder widths for the various classes of highways are shown in the appropriate subsequent portions of this manual.

Shoulder widths on bridge structures are measured from the nominal face of rail to the edge of traveled way. For additional guidance in reference to current standard bridge railings in Texas, reference the TxDOT Bridge Railing Manual and the applicable Bridge Railing Standard.

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. Subsequent sections of this manual identify appropriate shoulder widths for the various classes of highway and street facilities.

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Pavement Taper Lengths

The following equations define minimum taper lengths where lanes and / or shoulders are reduced, opened, or shifted. These equations define minimum taper lengths. The project conditions (e.g., higher traffic or truck volumes) may indicate the need for additional lengths or appropriate horizontal curvature. For guidance on the length of tapers for turn lanes, acceleration lanes, or deceleration lanes, reference the respective facility type in Chapter 3.

L = WS, for S ≥ 45 mph

L = WS2/60, for S < 45 mph

Where:

L = Length of taper, ft

W = Width of offset, ft

S = Posted speed, mph

When more space is available, a longer than minimum taper distance can be beneficial.

Lane Reduction Transition Taper (L)

Lane-reduction transition tapers are used where the number of through travel lanes is reduced due to narrowing of the roadway or section of on-street parking. The minimum length of a lane reduction transition taper is L.

Approach Taper for Obstructions (L)

Approach tapers for obstructions are used where the width of a through travel lane is reduced because of a fixed obstruction within a paved roadway. An approach taper for an obstruction is required upstream and downstream of the obstruction. The minimum length of an approach taper for obstruction is L.

Lane-Opening Taper (½ L)

Lane-opening tapers are used to where a through travel lane is added without a lateral shift of the through traffic. The minimum length of a lane-opening taper is ½ L.

Shoulder Taper (⅓ L)

Shoulder tapers are used where the improved shoulders are reduced or increased in width. The minimum length of a shoulder taper is ⅓ L.

Shifting Taper

Shifting tapers are used to perform a lateral shift of the through traffic during temporary traffic control activities (i.e. construction, maintenance, and incident management). Shifting tapers should not be used to address permanent changes in horizontal alignment. When a change in horizontal alignment is required the design criteria from Chapter 2, Section 5 should be used. For additional information on temporary traffic control refer to the Texas MUTCD.

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Curb and Curb with Gutters

Although curbs do not have a significant re-directional capacity, curbs are intended to discourage motorists from deliberately leaving the roadway. Curb designs are classified as vertical or mountable. Vertical curbs are defined as those having a vertical or nearly vertical face 6-in or higher. Mountable curbs are defined as those having a mountable face less than 6-in in height. Mountable curbs, especially those with heights of 4-in or less, can be readily traversed by a motorist when necessary. A preferable height for mountable curbs at some locations may be 4-in or less because higher curbs may drag the underside of some vehicles. Refer to the current TxDOT Curb Standard (CCCG) for illustrations on the various TxDOT standard curb types.

Curbs are used primarily on frontage roads, cross roads, and low-speed streets in urban areas. In general, curbs are not desirable along high-speed roadways. They should not be used with high-speed through 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 mountable type, preferably 4-in or less in height.

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Roadside Design

Of particular concern in the design process is mitigating the number of single-vehicle, run-off-the-road crashes which occur even on the safest facilities. The configuration and condition of the roadside greatly affect the extent of damages and injuries for these crashes.

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

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  • 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.
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  • For existing highways, treatment of obstacles should be considered in the following order:
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    • Remove the obstacle.
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    • Redesign the obstacle so that it can be safely traversed.
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    • Relocate the obstacle to a point where it is less likely to be struck.
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    • Make the obstacle breakaway.
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    • 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.
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    • Delineate the obstacle.
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  • 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 substantially increases safety and usefulness.
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  • For improved safety performance, highway geometry and traffic control devices should confirm drivers' expectations. Unexpected situations (e.g., 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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Slopes and Ditches

Side slopes

Side slopes 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 travel-way 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. Crash data indicates that approximately 75 percent of reported encroachments do not exceed a lateral distance of 30-ft 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 clear zone width associated with these slopes are further discussed in Clear Zone.

Design Values

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.

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 used on bridge header banks or ditch side slopes, both of which would likely require rip-rap. Slopes greater than 1V:2.5H require evaluation for slope stability per TxDOT’s Geotechnical Manual.

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 near the toe of these slopes. Particular care should be taken in the treatment of man-made appurtenances such as culvert ends. See Appendix A, Section 2 for additional information on considerations for barrier need.

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-in below the subgrade crown to insure stability of the base course. For additional information, see Drainage Facility Placement.

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Lateral Offset to Obstructions

The distance from the edge of the traveled way, beyond which a roadside object will not be perceived as an obstacle and result in a motorist's reducing speed or changing vehicle position on the roadway, is called the lateral offset. It is generally desirable that there be uniform clearance between traffic and roadside features such as bridge railings, parapets, retaining walls, and roadside barriers. In an urban environment, right of way is often limited and is characterized by sidewalks, enclosed drainage, numerous fixed objects (e.g., signs, utility poles, luminaire supports, fire hydrants, sidewalk furniture, etc.), and traffic making frequent stops.

Uniform alignment enhances highway safety by providing the driver with a certain level of expectation, thus reducing driver concern for and reaction to those objects. The lateral offset to obstructions helps to:

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  • Avoid impacts on vehicle lane position and encroachments into opposing or adjacent lanes;
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  • Improve driveway and horizontal sight distances;
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  • Reduce the travel lane encroachments from occasional parked and disabled vehicles;
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  • Improve travel lane capacity; and
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  • Minimize contact from vehicle mounted intrusions (e.g., large mirrors, car doors, and the overhang of turning trucks).

Where a curb is present, the lateral offset is measured from the face of curb (FOC) and must be a minimum of 1.5-ft. A minimum of 1-ft lateral offset should be provided from the toe of barrier to the edge of traveled way. A guard fence placed in the vicinity of a curb per the current TxDOT guardrail standard does not violate the minimum lateral offset requirement.

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Clear Zone

A clear recovery area, or clear zone, should be provided along highways. Clear zone requirements for 4R projects are shown in Table 2-12. A clear zone is the unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. The clear zone includes shoulders, bicycle lanes, and auxiliary lanes, except those auxiliary lanes that function like through lanes. Such a recovery area should be clear of unyielding objects where practical or shielded by crash cushions or barrier.

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Location

Functional Classification

Design Speed (mph)

Avg. Daily Traffic2

Clear Zone Width (ft)1,3,4,5

-

-

-

-

Minimum

Desirable

Rural

Freeways

All

All

30 (16 for ramps)

Rural

Arterial

All

≤ 750

≥ 750

16

30

30

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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 and collector-distributor)

Urban

All (Curbed)

≥ 50

All

Use above suburban criteria insofar as available border width permits.

Urban

All (Curbed)7

≤ 45

All

4 from FOC

6 from FOC

Urban

All (Uncurbed)

≥ 50

All

Use above suburban criteria.

Urban

All (Uncurbed)

≤ 45

All

106

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Notes:

  1. Devices such as traffic signal supports, railroad signal/warning device supports, and controller cabinets must be located as far from travel lanes as feasible. If not feasible to place outside of the clear zone, these devices may be excluded from clear zone requirements. Other non-breakaway devices must be located outside the prescribed clear zone or these devices must be protected with barrier.
  2. Average ADT over project life (i.e., 0.5 x (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:4H or flatter. Where fill slopes are steeper than 1V:4H it is desirable to provide a 10 ft 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 clear zone provisions is not required.
  7. For curbed facilities with a shoulder, bike lane or any buffer in addition to the curb offset, the minimum measurement begins at the edge of the through travel lane. The clear zone criteria is met if either 10-ft from the through travel lane or the distance measured from the FOC is met.


The clear zone values shown in Table 2-12 are measured from the edge of travel lane. These are appropriate design values for all cut sections (see Section 8, Drainage Facility Placement, for cross sectional design of ditches within the clear zone 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, a 1V:6H or flatter slope is preferred for both errant vehicle performance and slope maintainability. For slopes steeper than 1V:4H, errant vehicles have a reduced chance of recovery, therefore it is preferable to provide an obstacle-free area of 10-ft beyond the toe of steep side slopes even when this area is outside the clear zone.

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