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Section 3: Suburban Roadways

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Overview

The term “suburban roadway” refers to high-speed roadways that serve as transitions between low-speed urban streets and high-speed rural highways. Suburban roadways are typically 1 to 3 miles [1.6 to 4.8 kilometers] in length and have light to moderate driveway densities (approximately 10 to 30 driveways per mile [5 to 20 driveways per kilometer]). Because of their location, suburban roadways have both rural and urban characteristics. For example, these sections typically maintain high speeds (a rural characteristic) while utilizing curb and gutter to facilitate drainage (an urban characteristic). Consequently, guidelines for suburban roadways typically fall between those for rural highways and urban streets.

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Basic Design Features

This subsection includes information on the following basic design features for suburban roadways:

Table 3-5 shows tabulated basic geometric design criteria for suburban roadways. The basic design criteria shown in this table reflect minimum and desired values that are applicable to new location, reconstruction or major improvement projects.

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(US Customary)

Item

Functional Class

Desirable

Minimum

Design Speed (mph)

All

60

50

Minimum Horizontal Radius

All

See Tables 2-3 and 2-4

Maximum Gradient (%)

All

See Table 2-9

Stopping Sight Distance

All

See Table 2-1

Width of Travel Lanes (ft)

Arterial

Collector

12

12

111

102

Curb Parking Lane Width (ft)

All

None

Shoulder Width (ft)

All

10

4

Width of Speed Change Lanes3(ft)

All

11-12

10

Offset to Face of Curb (ft)

All

2

1

Median Width

All

See Medians, Urban Streets

Border Width (ft)

Arterial

Collector

20

20

15

15

Right-of-Way Width (ft)

All

Variable4

Sidewalk Width (ft)

All

6-85

5

Superelevation

All

See Chapter 2, Superelevation

Horizontal Clearance

All

See Table 2-11

Vertical Clearance for New Strs. (ft)

All

16.5

16.56

Turning Radii

All

See Chapter 7, Minimum Designs for Truck and Bus Turns

1 In highly restricted locations, 10 ft permissible.

2 In industrial areas 12 ft usual, and 11 ft minimum for restricted R.O.W. conditions. In non-industrial

areas, 10 ft minimum.

3 Applicable when right or left-turn lanes are provided.

4 Right-of-way width is a function of roadway elements as well as local conditions.

5 Applicable for commercial areas, school routes, or other areas with concentrated pedestrian traffic.

6 Exceptional cases near as practical to 16.5 ft but never less than 14.5 ft. Existing structures that provide

at least 14 ft may be retained.



Anchor: #i1063576Table 3-5: Geometric Design Criteria for Suburban Roadways

(Metric)

Item

Functional Class

Desirable

Minimum

Design Speed (km/h)

All

100

80

Minimum Horizontal Radius

All

See Tables 2-3 and 2-4

Maximum Gradient (%)

All

See Table 2-9

Stopping Sight Distance

All

See Table 2-1

Width of Travel Lanes (m)

Arterial

Collector

3.6

3.6

3.31

3.02

Curb Parking Lane Width (m)

All

None

Shoulder Width (m)

All

3.0

1.2

Width of Speed Change Lanes3 (m)

All

3.3-3.6

3.0

Offset to Face of Curb (m)

All

0.6

0.3

Median Width

All

See Medians, Urban Streets

Border Width (m)

Arterial

Collector

6.0

6.0

4.5

4.5

Right-of-Way Width (m)

All

Variable4

Sidewalk Width (m)

All

1.8-2.45

1.5

Superelevation

All

See Chapter 2, Superelevation

Horizontal Clearance

All

See Table 2-11

Vertical Clearance for New Strs. (m)

All

5.0

5.06

Turning Radii

All

See Chapter 7, Minimum Designs for Truck and Bus Turns

1 In highly restricted locations, 3.0 m permissible.

2 In industrial areas 3.6 m usual, and 3.3 m minimum for restricted R.O.W. conditions. In non-industrial

areas, 3.0 m minimum.

3 Applicable when right or left-turn lanes are provided.

4 Right-of-way width is a function of roadway elements as well as local conditions.

5 Applicable for commercial areas, school routes, or other areas with concentrated pedestrian traffic.

6 Exceptional cases near as practical to 5.0 m but never less than 4.4 m. Existing structures that provide at

least 4.3 m may be retained.



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Access Control

A major concern for suburban roadways is the large number of access points introduced due to commercial development. These access points create conflicts between exiting/entering traffic and through traffic. In addition, the potential for severe accidents is increased due to the high-speed differentials. Driver expectancy is also violated because through traffic traveling at high speeds does not expect to have to slow down or stop. Research has shown that reducing the number of access points and increasing the amount of access control will reduce the potential for accidents. In addition, accident experience can be reduced by separating conflicting traffic movements with the use of turn bays and/or turn lanes. Reference can be made to TxDOT Access Management Manual for additional access discussion.

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Medians

Medians are desirable for suburban roadways with four or more lanes primarily to provide storage space for left-turning vehicles. The types of medians used on suburban roadways include raised medians and two-way left-turn lanes.

Raised Medians. Raised medians with curbing are used on suburban arterials where it is desirable to control left-turn movements. These medians should be delineated with curbs of the mountable type. Raised medians are applicable on high-volume roadways with high demand for left turns. For additional guidelines regarding the installation of raised medians, see Raised Medians, Urban Streets.

Two-Way Left-Turn Lanes. The two-way left-turn lanes (TWLTL) is applicable on suburban roadways with moderate traffic volumes and low to moderate demands for left turns. For suburban roadways, TWLTL facilities should minimally be 14 ft [4.2 m] and desirably 16 ft [4.8 m] in width.

The desirable value of 16 ft [4.8 m] width should be used on new location projects or on reconstruction projects where widening necessitates the removal of exterior curbs. The “minimum” value of 14 ft [4.2 m] width is appropriate for restrictive right-of-way projects and improvement projects where attaining “desirable” median lane width would necessitate removing and replacing exterior curbing to gain only a small amount of roadway width.

Criteria for the potential use of a continuous TWLTL on a suburban roadway are as follows:

  • future ADT volume of 3,000 vehicles per day for an existing two-lane suburban roadway, 6,000 vehicles per day for an existing four-lane suburban roadway, or 10,000 vehicles per day for an existing six-lane suburban roadway.
  • side road plus driveway density of 10 or more entrances per mile [6 or more entrances per kilometer].

When both conditions are met, the use of a TWLTL should be considered. For ADT volumes greater than 20,000 vehicle per day, or where development is occurring and volumes are increasing and are anticipated to reach this level, a raised median design should be considered.

Seven-lane cross sections should be evaluated for pedestrian crossing capabilities.

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Median Openings

As the number of median openings along a suburban roadway increase, the interference between through traffic and turning traffic increases. To reduce the interference between turning traffic and through traffic, turn bays should be provided at all median openings. Recommended minimum median opening spacings are based on the length of turn bay required. For additional information regarding the design of median openings, see Section 2, Urban Streets, Medians.

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Speed Change Lanes

Due to high operating speeds on suburban roadways, speed change lanes may be provided as space for deceleration/acceleration to/from intersecting side streets with significant volumes. For information regarding the design of left-turn (median) speed change lanes and right speed change lanes, see Section 2, Urban Streets, Speed Change Lanes. (See Table 3-3 for lengths of single left-turn lanes; Table 3-4 for lengths of dual left-turn lanes, Figure 3-4 for length of right-turn lanes.)

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Right of Way Width

Similar to urban streets, the width of right-of-way for suburban roadways is influenced by traffic volume requirements, lane use, cost, extent of ultimate expansion, and land availability. Width is the summation of the various cross-sectional elements, including widths of travel and turning lanes, shoulders, median, sidewalks, and borders.

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Horizontal Clearances

Table 2-11: Horizontal Clearances presents the general horizontal clearance guidelines for suburban roadways.

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Borders

See Borders Urban Streets.

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Grade Separations and Interchanges

See Grade Separations and Interchanges, Urban Streets.

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Intersections

Due to high operating speeds (50 mph [80 km/h] or greater) on suburban roadways, curve radii for turning movements should equal that of rural highway intersections. Space restrictions due to right-of-way limitations in suburban areas, however, may necessitate reduction in the values given for rural highways. For additional information regarding intersection design, see Intersections Urban Streets.

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Parking

Desirably, parking adjacent to the curb on suburban roadways should not be allowed.

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