5.1 Design Considerations

5.1.1 Introduction

Local roads primarily serve to provide access to abutting land. Local roads provide high access and limited mobility. Design criteria for local roads is of lower order due to the relatively low traffic volumes and high proportion of highway mileage. However, they should be planned, located, and designed to be suitable for predictable traffic operations and should be consistent with the development and environment abutting the ROW.
This chapter discusses the features and design criteria for local roads and includes the following topics:
5.1 Design Considerations
  • 5.1.1 Introduction;
  • 5.1.2 Target Design Values;
  • 5.1.3 Design Speed;
  • 5.1.4 Design Traffic Volumes;
  • 5.1.5 Levels of Service;
  • 5.1.6 Travel Lane Width and Number of Lanes;
  • 5.1.7 Shoulders;
  • 5.1.8 Speed Lane Changes;
  • 5.1.9 Parking Lanes;
  • 5.1.10 Intersections;
  • 5.1.11 Medians;
  • 5.1.12 Curbs;
  • 5.1.13 Cross Slope and Superelevation;
  • 5.1.14 Grades;
  • 5.1.15 Roadside Design;
  • 5.1.16 Structures;
  • 5.1.17 Vertical Clearance; and
  • 5.1.18 Bicycle and Pedestrian Facilities.
5.2 Rural Design Element Characteristics
5.3 Rural Town, Suburban, Urban and Urban Core Design Element Characteristics
This chapter is organized with an introductory section on the general design considerations for local roads, followed by separate design discussions for local roads in rural, rural town, suburban, urban, and urban core contexts.

5.1.2 Target Design Values

Table 5-1
shows target design values for Local Roads in each context classification. These design values may be adjusted based on the anticipated operating speed and a performance-based practical design evaluation of the corridor as discussed in
Chapter 2
.
The target design values shown are for new construction of local roads that are off-system routes not functionally classified at a higher classification. Projects to improve existing local roads differ from new construction in that the performance of the existing local road can guide the selection of design values.
The following subsections provide additional discussion for each design element and factors that may influence these values.
The design value thresholds for design exceptions and design waivers are presented in Appendix A and B respectively.
Table 5-1: Target Design Values for Local Roads
Design Element
Rural
Rural Town
Suburban
Urban
Urban Core
Reference
Roadway
Design Speed
1
40 to 50 MPH
25 to 30 MPH
40 MPH
25 to 30 MPH
20 to 25 MPH
See 5.1.3
Lane Width
12-ft
11 to 12-ft
12-ft
11-ft
11-ft
See 5.1.6
Shoulder Width (Uncurbed)
8-ft
4-ft
10-ft
8-ft
2-ft
See 5.1.7
Offset to Face of Curb
N/A
1-ft
2-ft
2-ft
2-ft
See 4.10.12
Curb Parking Lane Width
2
N/A
9-ft
N/A
9-ft
9-ft
See 4.10.16
Bike Lane Width
See 18.4
Speed Change Lane Width
12-ft
11 to 12-ft
12-ft
11-ft
11-ft
See 4.10.2
Median Width
See 4.10.15
Horizontal Curve Radius (Minimum)
See 4.7
Cross Slope on a Tangent
2%
See 4.10.4
Cross Slope on a Tangent (Maximum)
3%
See 4.10.4
Superelevation Rate
See Table 4-3 through Table 4-7
See 4.7.3
Minimum Grade (PGL)
0.25% (lined ditch channels), 0.3% (curbed facilities), or 0.5% (unpaved ditches)
See 4.8.1
Maximum Grade (Level)
6%
8%
8%
8%
8%
See 4.8.1
Maximum Grade (Rolling)
8%
8%
8%
8%
8%
See 4.8.1
Vertical Clearance
16.5-ft
See 4.8.6
Design Loading Structural Capacity
HL-93
AASHTO LRFD Bridge Design Specifications
Stopping Sight Distance (SSD)
See 4.11.1
Design Vehicle
S-BUS-40
See 4.3.2
Level of Service (LOS Design Year)
See 5.1.5
See 4.3.4.1
Roadside
Clear Zone (Curbed)
See 4.10.7
Clear Zone (Uncurbed)
See 4.10.7
Sidewalk Width
N/A
6 to 8-ft
6 to 8-ft
6 to 8-ft
6 to 8-ft
See 19.2.2
Sidewalk Offset
N/A
Varies
See 19.3.2
Border Width
3
N/A
10-ft
10-ft
10-ft
10-ft
See 4.10.9
Notes
Denotes that this is a design exception condition if the specified values in Appendix A are not met.
Denotes that this is a design waiver condition if the specified values in Appendix B are not met.
1
The selected Design Speed should meet the anticipated operating speed of the facility during non-peak hours.
2
If a bike lane is adjacent to a parking lane, the parking lane width should be increased to 12-ft to allow car doors to be open without bicyclist conflict. See Section 4.10 for additional guidance.
3
A 2-ft border width may be used where available ROW is limited or too costly.

5.1.3 Design Speed

The design speed for local roads should reflect the appropriate environmental and terrain conditions for the various context classifications. The type and extent of adjacent development will generally suggest lower design speeds for these types of roadways due to the availability of ROW. Minimum design speeds for local roads are shown in
Appendix A
.
See
Section 4.2
for additional guidance on selecting the appropriate design speed.
When determining applicable radii and superelevation for design speeds see
Table 4-3
for a summary of superelevation methodologies to be used.
5.1.3.1 Rural
Design speeds in the rural context may range from 40 mph (rolling terrain) to 50 mph (level terrain) for roads with greater than 400 VPD. For rural local roads with less than 400 VPD, 30 mph (rolling terrain) or 40 mph (level terrain) design speeds may be used.
Lower design speeds are applicable to roads with winding alignments in rolling terrain or where environmental conditions dictate. Higher design speeds are applicable to roads in level terrain or where environmental conditions are favorable.
5.1.3.2 Suburban
Design speeds for local roads in the suburban context range should ideally be 40 mph and will typically act as a transition segment between rural and urban or rural town sections of roadway.
5.1.3.3 Rural Town, Urban and Urban Core
Design speeds in these contexts are generally limited because of closely spaced intersections in a rural town or urban street grid. Since the function of local roads is to provide access to adjacent property in these contexts,
all design elements should be consistent with the character of activity adjacent to the street.
 Design speeds may be reduced to be as low as 20 mph for urban core and design elements should encourage speeds to be no greater than 30 mph.

5.1.4 Design Traffic Volumes

Traffic volume is a major factor for local roads serving industrial or commercial areas. Local roads should be designed to accommodate traffic projections for a 20-year period into the future for all contexts. The ADT projected to some future design year should be the design basis. For additional guidance on the selection of the appropriate periods for forecasting design traffic volumes, see
Section 4.3.1
and
AASHTO’s A Policy on Geometric Design of Highways and Streets
.

5.1.5 Levels of Service

The highest level of service practical and consistent with the anticipated conditions should be provided for local roads.
See
Section 4.3.4.1
for more information regarding level of service as it relates to facility design.

5.1.6 Travel Lane Width and Number of Lanes

Travel lane widths may vary from 10 to 12-ft depending on the context classification of the facility and the future ADT.
Minimum width of new or widened structures should accommodate the approach roadway width including shoulders.
5.1.6.1 Rural
Rural contexts should generally provide 12-ft lanes, especially on roads where the ADT > 2000 and substantial truck volumes are present or agricultural equipment frequently uses the road. For roadways with ADT between 400 and 2,000 and a design speed ≤ 50 mph a minimum of 11-ft lanes must be used. On very low-volume roadways (lower than 400 VPD), 10-ft lanes may be used.
Two travel lanes should be provided on local roads in rural areas. Additional lanes may be needed based on operational analysis.
5.1.6.2 Rural Town, Suburban, Urban and Urban Core
Rural town, suburban, urban, and urban core contexts commonly use 11 to 12-ft lane widths. In industrial areas, 12-ft lane widths should be provided. Industrial areas with constrained ROW may use 11-ft lane widths. In nonindustrial areas with constrained ROW, 10-ft lane width may be used. 9-ft minimum lane width may be used in residential areas.
Refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for guidance on number of lane widths in these contexts.

5.1.7 Shoulders

Local roads shoulder widths may vary from 2 to 8-ft depending on the context classification of the facility. Where right turn lanes are present on uncurbed facilities, a 4-ft fully surfaced shoulder must be provided.
Facilities must be assessed to determine if bicycle accommodation are required on the shoulders.
Bike accessible shoulders must be fully surfaced for the entire shoulder width.
See
Chapter 18
for additional guidance on bicycle accommodations.
See
Section 4.10.3
for additional information on shoulders and shoulder surfacing requirements.
See
Chapter 12
for specific off-system bridge requirements for current ADT of 400 VPD or less.
A 5-ft minimum clear space for bicyclists should be provided on bridges being replaced or rehabilitated.
5.1.7.1 Rural
Outside shoulder widths on rural local roads are normally 4 to 8-ft. For two-lane rural roads, shoulder widths may be reduced to 2-ft if future ADT volumes are less than 400 VPD.
Mailbox turnouts should be considered on local roads with 400 or more VPD. Refer to TxDOT standard drawings for details regarding the design and construction of mailbox turnouts.
5.1.7.2 Suburban
Uncurbed shoulders in the suburban context can range from 4 to 10-ft wide.
5.1.7.3 Rural Town, Urban and Urban Core
Uncurbed shoulders in rural town, suburban, urban, and urban core contexts may vary from 2 to 8-ft shoulder widths.
Parking lanes in rural town, urban and urban core contexts may be used instead of shoulders. See
Section 4.10.16
for guidance on the use of parking lanes.

5.1.8 Speed Change Lanes

The width of a speed change lane should match the through lane width of the facility. See
Section 4.10.2
and
AASHTO’s A Policy on Geometric Design of Highways and Streets
for further discussion on speed change lane widths and their use.

5.1.9 Parking Lanes

Guidance on Parking Lanes is presented in
Section 4.10.16
.

5.1.10 Intersections

Guidance on Intersections is presented in
Chapter 13
.

5.1.11 Medians

Guidance on medians is presented in
Section 4.10.15
.

5.1.12 Curbs

Guidance on Curbs and Curb Offsets is presented in
Section 4.10.12
.

5.1.13 Cross Slope and Superelevation

Guidance on Cross Slope and Superelevation is presented in
Sections 4.10.4
and
4.7
.

5.1.14 Grades

Guidance for maximum Grades for local roads is presented in
Section 4.8.1
.

5.1.15 Roadside Design

Local roads must meet the minimum clear zone requirements shown in
Table 4-20
. Additional guidance on roadside design is presented in
Section 4.10.11
.

5.1.16 Structures

Bridges, culverts, walls, tunnels, and other structures must be designed in accordance with the current
AASHTO LRFD Bridge Design Manual
. The design loading must be HL-93 design live load unless design for a special vehicle is specified or warranted.
See
Chapter 12
for specific off-system bridge requirements for current ADT of 400 VPD or less.
Refer to the latest
TxDOT Bridge Design Manual - LFRD
for additional design guidance for bridges on TxDOT facilities.

5.1.17 Vertical Clearance

Vertical clearance requirements are presented in
Section 4.8.6
.

5.1.18 Bicycle and Pedestrian Facilities

Guidance for Bicycle and Pedestrian Facilities are presented in
Chapter 18
and
19
, respectively. All projects should be assessed for pedestrian and bicycle accommodations that may be required.
For specific jurisdictional requirements confirm with local entity for additional design criteria.