6.1 Design Considerations

6.1.1 Introduction

A collector roadway will usually serve moderate traffic volumes and provides a circulation link between local roadways and arterial highways. Design criteria presented in this chapter apply to new construction and reconstruction projects
for collector highways on the State Highway System.

This chapter discusses the features and design criteria for collectors and includes the following sections and subsections:

6.1 Design Considerations

6.1.1 Introduction;

6.1.2 Target Design Values;

6.1.3 Design Speed;

6.1.4 Design Traffic Volumes;

6.1.5 Levels of Service;

6.1.6 Travel Lane Width and Number of Lanes;

6.1.7 Shoulders;

6.1.8 Speed Change Lanes;

6.1.9 Parking Lanes;

6.1.10 Intersections;

6.1.11 Medians;

6.1.12 Curbs;

6.1.13 Cross Slope and Superelevation;

6.1.14 Grades;

6.1.15 Roadside Design;

6.1.16 Structures;

6.1.17 Vertical Clearance;

6.1.18 Grade Separations, Interchanges, and Ramps;

6.1.19 Pedestrian and Bicycle Facilities; and

6.1.20 Access Control.

6.2 Rural Design Elements

6.2.1 General Characteristics; and

6.2.2 Typical Sections.

6.3 Rural Town Design Elements

6.3.1 General Characteristics; and

6.3.2 Typical Sections.

6.4 Suburban, Urban, and Urban Core Design Elements

6.4.1 General Characteristics; and

6.4.2 Typical Sections.

This chapter is organized with an introductory section on the general design considerations for collectors, followed by separate design discussions for local roads in rural, rural town, suburban, urban, and urban core contexts.

6.1.2 Target Design Values

Table 6-1

shows target design values for Collectors for each context classification. These design values may be adjusted based on the anticipated target speed and a performance-based practical design evaluation of the corridor as discussed in

Chapter 2

The subsections following

Table 6-1

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 6-1: Target Design Values for Collectors
Design Element			Rural	Rural Town	Suburban	Urban	Urban Core	Reference
Roadway		Design Speed¹	50 to 70 MPH	35 to 40 MPH	45 to 55 MPH	35 to 40 MPH	25 to 30 MPH	See 6.1.3
		Travel Lane Width	12-ft	11 to 12-ft	12-ft	11-ft	11-ft	See 6.1.6
		Shoulder Width (Uncurbed)²	10-ft²	3 to 8-ft²	8-ft²	3 to 8-ft²	3 to 8-ft²	See 6.1.7
		Offset to Face of Curb	See 4.10.12	2-ft	See 4.10.12	2-ft	2-ft	See 4.10.2
		Curb Parking Lane Width⁵	N/A	10-ft	N/A	10-ft	10-ft	See 4.10.16
		Bike Lane Width	N/A	5-ft	N/A	5-ft	5-ft	See 18.4
		Speed Change Lane Width	12-ft	12-ft	12-ft	11-ft	11-ft	See 4.10.2
		Median Width	See 4.10.15
		Horizontal Curve Radius	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)	4%	8%	6%	8%	9%	See 4.8.1
		Maximum Grade (Rolling)	5%	9%	7%	9%	11%	See 4.8.1
		Vertical Clearance at New Structures³	16.5-ft³					See 4.8.6
		Design Loading Structural Capacity	HL-93					AASHTO LRFD Bridge Design Specs
		Stopping Sight Distance (SSD)	See 4.11.1
		Design Vehicle⁴	WB-67	WB-67	WB-67	SU-40⁴	CITY-BUS⁴	See 4.3.2
		Level of Service (LOS Design Year)	B	B	B	B	C	See 4.3.4.1
Roadside		Clear Zone (Curbed)	See 4.10.7
		Clear Zone (Uncurbed)	See 4.10.7
		Sidewalk Width	5-ft (if applicable)	5-ft	5-ft	6 to 8-ft	5 to 8-ft	See 19.3.2
		Sidewalk Offset	N/A	Varies 6-ft or more for curbed roadways Clear Zone width or more for uncurbed roadways				See 19.3.2
		Border Width	N/A	20-ft	20-ft	20-ft	15-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 target speed of the facility during non-peak hours.
	2	A 12-ft paved shoulder should be considered where the volume of truck traffic exceeds 250 DDHV. An 8-ft minimum paved shoulder should be considered for truck traffic volumes that are less than 250 and the highway is located on the Texas Highway Freight Network (THFN).
	3	18.5-ft vertical clearance is required for highways on the Texas Highway Freight Network (THFN). Specific railroad company guidelines may require additional vertical clearance requirements.
	4	Consider providing intersection turning radii for a WB-67 design vehicle on roadways or streets that are located on the Texas Freight Network and where high (> 10%) truck turning movements are known or expected.
	5	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.

6.1.3 Design Speed

The design speed for collectors can vary greatly between the five contexts. The design speed should reflect the anticipated target speed during non-peak hours.
However, the design speed should not exceed the limits of prudent construction, ROW, and socioeconomic costs. Minimum design speeds for collectors 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.

6.1.3.1 Rural

Design speeds for rural collectors may be reduced to 40 mph or higher in rolling terrain and 50 mph or higher in level terrain for facilities with an ADT < 2,000 VPD. See

Section 4.6

for guidance on different types of terrain.

6.1.3.2 Rural Town

Design speeds for collectors in the rural town context may vary from 30 to 45 mph.

6.1.3.3 Suburban

Design speeds for collectors in the suburban context range from 35 to 60 mph and will typically act as a transition segment between rural and urban or rural town sections of roadway.

6.1.3.4 Urban and Urban Core

The design speed for urban collectors should range from 30 to 45 mph. The design speed for urban core collectors should range from 25 to 30 mph.

6.1.4 Design Traffic Volumes

Collector roadways for all contexts should be designed to accommodate traffic projections for a 20-year period into the future. However, some elements may be based on shorter or longer design periods (e.g., 20-year period for flexible pavements and 30-year period for rigid pavements). For additional guidance on the selection of the appropriate periods for forecasting design traffic volumes, see

Section 4.3.1

AASHTO’s A Policy on Geometric Design of Highways and Streets

Projected volumes for other modes of transportation, such as pedestrians and or bicyclists, must also be estimated in the design of the facility.
While future estimates of transit and non-motorized use may not be available, the designer may use available planning and land use documents to assist in determining future levels of nonmotorized demand.

6.1.5 Levels of Service

Collectors and their auxiliary facilities should be designed for Level of Service C or better in the design year for all contexts. Level of Service D may be allowable for heavily developed or constrained areas.

See

Section 4.3.4.1

for more information regarding level of service as it relates to facility design.

6.1.6 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 anticipated target speed.

12-ft travel lanes are recommended on all roadways that are part of the Texas Highway Freight Network (THFN).

The number of lanes required to accommodate the anticipated traffic in the design year is determined by the level of service evaluation as discussed in the

HCM

. The typical range of lanes for collectors is one to two travel lanes in each direction.

6.1.6.1 Rural

Rural collectors should be designed to 12-ft due to the higher operating speeds usually associated with these types of facilities.

11-ft lanes may be used on two lane collectors that:

Have an operating speed of 45 MPH or less and a future ADT less than 2,000 VPD;

Have an operating speed of 60 MPH or higher and a future ADT less than 400 VPD; or

Have an operating speed of 70 MPH or less and a future ADT less than 1,500.

10-ft lanes may be used on two lane collectors that:

Have operating speeds of 55 MPH or less and future ADT’s less than 400 VPD.

6.1.6.2 Rural Town

Rural town collectors should provide 12-ft lanes due to the agricultural vehicles and trucks that will drive on these types of facilities. 11-ft lanes may be used in restrictive ROW conditions.

Lane widths may be reduced to 10-ft in highly constrained areas where both the following conditions are present:

Truck and bus volumes are low (less than 5%); and

The anticipated operating speed is less than or equal to 35 mph.

Engineering judgement must be exercised when determining if a 10-ft lane width is acceptable. Use of 10-ft travel lanes must be approved by the District and approval documented in the project file.

6.1.6.3 Suburban, Urban, Urban Core

Suburban, urban, and urban core collectors will commonly use 11-ft lanes on facilities with lower operating speeds as a traffic calming measure (i.e., 45 mph or less). If the anticipated target speed is 50 mph or greater and or truck volumes are significant (>10%), 12-ft lanes should be used.

Lane widths may be reduced to 10-ft in highly constrained areas where the conditions described in 6.1.6.2 are met.

6.1.7 Shoulders

Collector shoulder widths may vary from 2 to 10-ft depending on the context classification, ADT, and anticipated target speed of the facility.

A 5-ft minimum clear space for bicyclists should be provided on bridges being replaced or rehabilitated.

See

Section 4.10.3

for additional information on shoulders and shoulder surfacing requirements.

6.1.7.1 Rural

Outside shoulder widths on rural collectors can vary from 2 to 10-ft as outlined in the subsequent section. Rural collectors on the Texas Highway Freight Network (THFN) should provide 10-ft minimum outside shoulders.

6.1.7.1.1 Two-Lane Rural Collectors

The minimum outside shoulder must be 10-ft for all design speeds and the future ADT is greater than 2,000 VPD. The outside shoulder width may be reduced to a minimum of 8-ft in highly constrained situations for any ADT or if the future ADT is between 1,500–2,000 VPD. A minimum 6-ft shoulder must be provided if the future ADT is between 400–1,500 VPD and the design speed is 50 mph or greater. The outside shoulder width may be reduced to a minimum 4-ft if the future ADT is between 400–1,500 VPD and the design speed is 45 mph or less. A minimum of 2-ft outside shoulders may be used if future ADT volumes are less than 400 VPD. 4-ft minimum shoulders must be used at locations adjacent to roadside barrier.

6.1.7.1.2 Multi-Lane Rural Collectors

Outside shoulder widths may be reduced to 8-ft for highly constrained situations for both divided and undivided facilities

A minimum of 4-ft inside shoulders must be provided for 4 lane divided rural collectors. On 6 lane or more divided collectors, 10-ft inside shoulders should be provided. The inside shoulder width may be reduced to 4-ft in constrained situations.

When right or left turn lanes are present, a 4-ft minimum shoulder must be provided for the turn lane.

6.1.7.2 Suburban

Suburban collector shoulder widths can vary from 4 to 10-ft. It is preferred to have 8 to 10-ft shoulder widths on suburban roadways with high operating speeds (50 mph or greater).

When right or left turn lanes are present, a 4-ft minimum shoulder must be provided for the turn lane.

Due to the higher speeds frequently associated with this context, shoulders are usually provided instead of curbs. See

Section 4.10.12

for additional information on curbs.

6.1.7.3 Rural Town, Urban, and Urban Core

Rural town, urban, and urban core contexts will typically use curbs and maintain curb offsets instead of shoulders due to the lower operating speeds. See

Section 4.10.12

for curb offset requirements and additional information on curbs.

For uncurbed roadways, shoulder widths may vary from 3 to 8-ft. Uncurbed collectors on the Texas Highway Freight Network (THFN) should provide at least 8 ft shoulders for these contexts.