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Section 2: Design Characteristics

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Pavement Design

Pavement rehabilitation includes all pavement-related work undertaken to extend the service life of an existing facility. This includes placement of additional surfacing material and/or other work necessary to return an existing roadway, including shoulders, to a condition of structural and/or functional adequacy. The following are some examples of pavement rehabilitation work:

The existing pavement condition and deficiencies should be identified for 3R projects. Design strategies selected to correct deficiencies will vary from seal coats to overlays to complete pavement structure reconstruction. Projects that consist only of seal coats or overlays, and do not meet the design guidelines presented in this chapter, are not eligible for rehabilitation funding.

Reference TxDOT’s Pavement Manual for additional information related to pavement rehabilitation.

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Geometric Design

Geometric design guidelines are provided for the following roadways in the tables indicated.

To measure bridge width on bridges without curbs, measure to the nominal face of rail. Reference TxDOT’s Bridge Railing Manual and Bridge Railing Standards for the nominal widths of specific rail types and additional guidance. To measure bridge width on bridges with curbs, measure to the face of curb.

Anchor: #i1003288Table 4-1: 3R Minimum Design Guidelines for Rural Multilane Highways (Nonfreeway)1

Design Element

Guideline for Highway Class

6-Lane Divided

4-Lane Divided

4-Lane Undivided

Design Speed (mph)2

50

50

50

Lane Width (ft)

11

11

11

Outside Shoulder Width (ft)

4

4

4

Inside Shoulder Width (ft)

4

2

N/A

Turn Lane Width (ft)3

10

10

10

Clear Zone (ft)4

16

16

16

Bridges: Width to be retained (ft)5

42

28

52

Notes:

  1. These values are intended for use on rehabilitation projects. However, the designer may select higher values to provide consistency with adjoining roadway sections, to provide consistency with prevailing conditions on similar roadways in the area or to provide operational improvements at specific locations.
  2. Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter. Projects should be designed to target, as close as possible, the existing or anticipated posted speed as practical.
  3. For two-way left turn lanes, 11-ft to 14-ft usual.
  4. A clear zone is the unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. Clear Zone is measured from the edge of travel lane to the obstruction. For low-speed rural collectors, and all rural local roads, a clear zone of 10-ft is allowable in constrained circumstances.
  5. Where structures are to be modified, bridges should meet approach roadway width as a minimum. (Approach roadway width is the total width of the lanes and shoulders.) Greater bridge widths may be appropriate if the rehabilitation project increases roadway life significantly or if higher design values are selected for the remainder of the project. Existing structure widths less than those shown may be retained if the total lane width is not reduced across or near the structure.


Anchor: #i1003397Table 4-2: 3R Minimum Design Guidelines for Rural Two-Lane Highways1

Design Element

Guidelines for Current ADT

0 – 399

400 -1,499

1,500 or more

Design Speed (mph)2

30

30

40

Shoulder Width (ft)

0

1

3

Lane Width (ft)

10

11

11

Surfaced Roadway Width (ft)

20

24

28

Turn Lane Width (ft)3

10

10

10

Clear Zone (ft)4

7

7

10

Bridges: Width to be retained (ft)5

20

24

246

Notes:

  1. These values are intended for use on rehabilitation projects. However, the designer may select higher values to provide consistency with adjoining roadway sections, to provide consistency with prevailing conditions on similar roadways in the area or to provide operational improvements at specific locations.
  2. Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter. Projects should be designed to target as close to the anticipated posted speed as practical.
  3. For two-way left turn lanes, 11-ft to 14-ft usual.
  4. 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 is measured from the edge of travel lane to the obstruction. If a high design speed (50 mph or greater) is selected for project, the clear zone is 16-ft.
  5. Where structures are to be modified, bridges should meet approach roadway width as a minimum. (Approach roadway width is the total width of the lanes and shoulders). Greater bridge widths may be appropriate if the rehabilitation project increases roadway life significantly or if higher design values are selected for the remainder of the project. Existing structure widths less than those shown may be retained if the total lane width is not reduced across or near the structure.
  6. For current ADT exceeding 2,000, minimum width of bridge to be retained is 28-ft.


Anchor: #i1003506Table 4-3: 3R Minimum Design Guidelines for Urban Streets All Functional Classes1

Design Element

Guideline

Design Speed (mph)2

30

Lane Width (ft)

10

Turn Lane Width (ft)3

10

Parallel Parking Lane Width (ft)

7

Curb Offset for Curbed Streets (ft)

0

Shoulders for uncurbed streets (ft)4

2

Clear Zone (ft)

To back of curb or outside edge of shoulder

Bridges: Width to be retained (ft)

Approach roadway, not including shoulders

Notes:

  1. These values are intended for use on rehabilitation projects. However, the designer may select higher values to provide consistency with adjoining roadway sections, to provide consistency with prevailing conditions on similar roadways in the area or to provide operational improvements at specific locations.
  2. Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter. Projects should be designed to target as close to the anticipated posted speed as practical.
  3. For two-way left turn lanes, 11-ft to 14-ft usual.
  4. Minimally 1-ft of shoulder surfaced where lane width is 10-ft thereby providing a 22-ft surfacing width.


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Design Values

Where existing highway features comply with the design values given in this chapter, the designer may choose not to modify these features. However, where existing features do not meet these values, the designer should upgrade to the values shown in this chapter. These values are intended for use on typical non-freeway rehabilitation projects. The designer may select higher values to be consistent with adjoining roadway sections, to provide consistency with prevailing conditions on similar roadways in the area, or to provide operational improvements at specific locations.

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Alignment

Typically, 3R projects will involve minor or no change in vertical or horizontal alignment. However, consider flattening of curves or other improvements where a crash history indicates a concern or where existing curvature is inconsistent with prevailing conditions within the project or on similar roadways in the area. Where appropriate, consider superelevation improvements as well.

These types of isolated improvements are not considered substantial and thus may be included for consideration in 3R projects. Substantial changes in existing horizontal and/or vertical alignment improvements are considered reconstruction. Projects with substantial changes in existing alignment should be developed to reconstruction (4R) standards.

Design exceptions or design waivers for vertical or horizontal alignment on a 3R project will only be required when crash history or prevailing conditions indicate needed upgrades, but those upgrades are not included in the 3R project. See Chapter 1 Section 2 Design Exceptions, Design Waivers, Design Variances, and Texas Highway Freight Network (THFN) Design Deviations for specific geometric criteria required for a design exception or design waiver.

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Design Speed

Reconstruction of horizontal and vertical alignments should be considered when the design speed of the roadway in question is not consistent with the existing geometrics. For rehabilitation purposes, the minimum design speed for rural multi-lane highways is 50 mph. The minimum design speed for high-volume rural two-lane highways and high-volume rural frontage roads is 40 mph. The minimum design speed for low-volume rural two-lane highways, low-volume rural frontage roads, urban streets, and urban frontage roads is 30 mph.

For roadways that do not meet the minimum 3R design speeds, an evaluation should be done to examine high-frequency crash locations (and potential crash locations) to determine whether cost-effective alignment revisions can be accomplished with the resources available. These Projects should be designed to accommodate the posted speed where practical. When the posted speed cannot be met, additional traffic safety measures should be provided including, but not limited to, enhanced signage, pavement markings and delineation.

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Side and Backslopes

Existing side and backslopes should be retained except where crown widening, grade changes, or not meeting clear zone requirements create conditions that dictate otherwise.

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Lane Widths

Consideration should be given to increasing lane widths to 12-ft in conjunction with rehabilitation projects where the highway is a high-volume route utilized extensively by large trucks. Widening shoulders has generally been shown to have a greater reduction in crashes versus widening travel lanes. Factor in this consideration along with other considerations that determine the scope of a project, including expected service life of the proposed rehabilitation work, long-range plans for the route, and design standards of nearby segments on the route.

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