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Section 3: Suburban Roadways

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Overview

The term “suburban roadway” refers to high-speed roadways (speed limit or operating speeds of at least 50 mph is expected) that serve as transitional roads between low-speed urban streets and high-speed rural highways. Suburban roadways are typically 1 to 3 miles in length and have light to moderate driveway densities (approximately 10 to 30 driveways per mile). Because of their location, suburban roadways have both rural and urban characteristics. For example, these sections may 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.

See Chapter 2 for additional guidance on choosing an appropriate design speed.

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Item

Functional Class

Desirable

Minimum

Design Speed (mph)

All

60

50

Horizontal Radius

All

See Table 2-4 and 2-5

Maximum Grade (%)

All

See Table 2-9

Stopping Sight Distance

All

See Table 2-1, Figure 2-3

Width of Travel Lanes (ft.)

Arterial

Collector

Local

12

12

12

111

112

112

Curb Parking Lane Width (ft.)

All

None

Shoulder Width (ft.)6,7

All

10

4

Width of Speed Change Lanes(ft.)3

All

12

10

Offset to Face of Curb (ft.)

All

2

1

Median Width

All

See Medians, Urban Streets

Border Width (ft.)

Arterial

Collector

Local

20

20

10

15

15

10

Right-of-Way Width (ft.)

All

Variable4

Sidewalk Width (ft.)

All

6-85

5

Superelevation

All

See Chapter 2, Superelevation Rate, Superelevation Transition Length, Superelevation Transition Placement, Superelevation Transition Type

Clear Zone

All

See Table 2-12

Vertical Clearance for New Structures (ft.)

All

See Table 2-11

Turning Radii

All

See Chapter 7, Minimum Designs for Truck and Bus Turns

Notes:

  1. In highly restricted locations, 10-ft permissible.
  2. In industrial areas 12-ft usual, and 11-ft minimum for restricted right-of-way 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. A 5-ft minimum clear space for bicyclists should be provided on bridges being replaced or rehabilitated. Off-system Bridges, with current ADT greater than 400 ADT, where this addition may represent an unreasonable increase in cost may be excepted from the bicycle clear space requirement. See Ch. 6 Section 1 for specific off-system bridge requirements for current ADT of 400 or less.
  7. Where right turn lanes are present on uncurbed facilities, a 4-ft fully surfaced shoulder must be provided.


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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. The potential for severe crashes is also increased due to the high-speed differentials. Driver expectancy can be violated because through traffic drivers traveling at high speeds do 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 crashes. In addition, potential crash experience can be reduced by separating conflicting traffic movements with the use of turn bays and/or turn lanes. See TxDOT’s Access Management Manual for additional information.

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Medians

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

Raised Medians. See Raised Medians, Urban Streets.

Two-Way Left-Turn Lanes. Two-way left-turn lanes (TWLTL) are applicable on suburban roadways with moderate traffic volumes and low to moderate demands for left turns. For suburban roadways, TWLTL facilities should be between 14-ft and 16-ft wide.

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

A site can be considered suitable for the use of a TWLTL when a suburban roadway meets the following criteria:

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  • Future ADT volume of greater than 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; and
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  • Side roads plus driveway density of 10 or more entrances per mile.

When the above two conditions are met, the site should be considered suitable for the use of a TWLTL.

All 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 conflicts 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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Clear Zones

Table 2-12 presents the general clear zone 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, Freeways; Grade Separations and Interchanges, Urban Streets; and Chapter 10 of AASHTO's A Policy on Geometric Design of Highways and Streets.

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Right-of-Way Width

Right-of-way width is the area necessary to accommodate the various cross-sectional elements, including widths of travel and turning lanes, bicycle lanes, shoulders, median, sidewalks, sidewalk offsets, slopes, and borders. 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.

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Intersections

Due to high operating speeds (50 mph 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 may necessitate reduction in the values given for rural highways. For additional information regarding intersection design, see Intersections Urban Streets.

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Speed Change Lanes

Speed Change Lanes are defined as acceleration or deceleration lanes for left or right turns, exit or entrance acceleration or deceleration lanes, or a climbing lane. A design waiver is required for speed change lanes with lengths and widths that do not meet minimum criteria.

Speed change lanes may be provided as space for deceleration/acceleration to/from intersecting side streets with significant volumes and high operating speeds. 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 3-5 for length of right-turn lanes.)

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Parking

Parking adjacent to the curb on suburban roadways should not be allowed.

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