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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 evaluate the project according to the additional guidelines presented in this chapter, are not eligible for rehabilitation funding.

Reference can be made to the Pavement Design Guide 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.

For the purposes of measuring Bridge Width on bridge structures, the bridge width on bridge structures without curbs is measured to the nominal face of rail. Reference the TxDOT Bridge Railing Manual and Bridge Railing Standards for the nominal widths of specific rail types and additional guidance. For Bridges with curbs, the bridge width is measured to the face of curb.

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

(US Customary)

Design Element

Highway Class

-

6-Lane Divided

4-Lane Divided

4-Lane Undivided

Design Speedb

50 mph

50 mph

50 mph

Lane Width

11 ft

11 ft

11 ft

Outside Shoulder

4 ft

4 ft

4 ft

Inside Shoulder

4 ft

2 ft

N/A

Turn Lane Widthc

10 ft

10 ft

N/A

Clear Zonee

16 ft

16 ft

16 ft

Bridgesd: Width to be retained

42 ft

28 ft

52 ft

a 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.

b Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter.

c For two-way left turn lanes, 11 ft – 14 ft usual.

d 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 in the vicinity of the structure.

e For low-speed rural collectors, and all rural local roads, a clear zone of 10 ft is allowable in constrained circumstances. The clear zone is measured from the edge of travel lane to the obstruction.



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

(US Customary)

Design Element

Current Average Daily Traffic

-

0 – 400

400 -1500

1500 or more

Design Speedb

30 mph

30 mph

40 mph

Shoulder Width

0 ft

1 ft

3 ft

Lane Width

10 ft

11 ft

11 ft

Surfaced Roadway

20 ft

24 ft

28 ft

Turn Lane Widthc

10 ft

10 ft

10 ft

Clear Zonef

7 ft

7 ft

10 ft

Bridgesd: Width to be retained

20 ft

24 ft

24 fte

a 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.

b Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter.

c For two-way left turn lanes, 11 ft – 14 ft usual.

d 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 in the vicinity of the structure.

e For current ADT exceeding 2000, minimum width of bridge to be retained is 28 ft.

f The clear zone is measured from the edge of travel lane to the obstruction.



Anchor: #i1003506Table 4-3: 3R Minimum Design Guidelines for Urban Streetsa All Functional Classes

(US Customary)

Design Element

Guideline

Design Speedb

30 mph

Lane Width

10 ft

Turn Lane Widthc

10 ft

Parallel Parking Lane Width

7 ft

Curb Offset

0 ft

Shouldersd,e

2 ft

Clear Zone

To back of curb or outside edge of shoulder

Bridges: Width to be retained

Approach roadway, not including shoulders

a 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.

b Considerations in selecting design speeds for the project should include the roadway alignment characteristics as discussed in this chapter.

c For two-way left turn lanes, 11 ft – 14 ft usual.

d Minimally 1 ft of shoulder surfaced where lane width is 10 ft thereby providing a 22 ft

surfacing width.

e Applicable to uncurbed streets.



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Design Values

Where the existing highway features comply with the design values given in this chapter, the designer may choose not to modify these features. However, where the existing features do not meet these values, upgrading should be to the values shown in this chapter. These values are intended for use on typical rehabilitation projects. 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.

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Alignment

Typically, 3R projects will involve minor or no change in either vertical or horizontal alignment. However, flattening of curves or other improvements may be considered 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, improvements in superelevation may also be a consideration.

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 upgrades are needed, but those upgrades are not included in the 3R project. See Chapter 1, Section 2 for specific geometric criteria requiring either a design exception or design waiver.

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Design Speed

The reconstruction of horizontal and vertical alignments should be considered when the suggested design speed of the particular roadway in question is not consistent with the existing geometrics. For rehabilitation purposes, the suggested minimum design speed for rural multilane highways is 50 mph [80 km/h]. The suggested minimum design speed for high volume rural two-lane highways and high volume rural frontage roads is 40 mph [60 km/h]. The suggested minimum design speed for low volume rural two-lane highways, low volume rural frontage roads, urban streets, and urban frontage roads is 30 mph [50 km/h].

This does not imply that roadways with alignments falling below these current design speed values are unsafe. Rather, these roadways were usually designed to values considered current at the time of construction or at a time when alignment criteria was nonexistent for that particular type of roadway. These roadways may experience enhanced safety and improved traffic operations if the proposed rehabilitation project can cost effectively make alignment improvements.

For roadways not meeting the suggested 3R design speeds, an evaluation should be done to examine high frequency crash locations to determine whether cost effective alignment revisions can be accomplished with the resources available.

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Side and Backslopes

Existing side and backslopes usually should be retained except where crown widening or grade changes create conditions that dictate otherwise.

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Lane Widths

Consideration should be given to increasing lane widths to 12 ft [3.6 m] in conjunction with rehabilitation projects where the highway is a high volume route utilized extensively by large trucks. This consideration should be factored in along with all of the other normal considerations that determine the scope of a project, including expected service life of the proposed rehabilitation work, long range plans for the route and the design standards of other nearby segments on the route.

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