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Section 4: Two-Lane Rural Highways

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Overview

The term “two-lane rural highway” as used in this chapter, refers to roadways in un-developed areas that have one-lane of traffic in each direction. Access to these facilities is controlled through driveway locations and intersecting roadways. Two-lane rural highways usually do not have any type of median barrier.

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Basic Design Features

This subsection includes information on the following basic design features for two-lane rural highways:

Additional information on structure widths may be obtained in TxDOT’s Bridge Design - LRFD and the Bridge Project Development Manual.

Table 3-6 shows references for the geometric design criteria for two-lane rural highways.

Anchor: #i1638061Table 3-6. Geometric Design Criteria for Rural Two-Lane Highways

Geometric Design Element

Reference or Design Value

Design Speed

Table 3-7

Minimum Horizontal Radius

Table 2-4, Table 2-5

Maximum Grade (%)

Table 2-9

Stopping Sight Distance

Table 2-1, Figure 2-3

Width of Travel Lanes

Table 3-8

Width of Shoulders

Table 3-8

Vertical Clearance for New Structures

Table 2-11

Clear Zone

Table 2-12

Passing Sight Distance

Table 3-9

Superelevation

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

Turning Radii

See Chapter 7, Minimum Designs for Truck and Bus Turns



Table 3-7 shows minimum design speed for rural two-lane highways. See Chapter 2 for definition of level and rolling terrain as well as guidance on choosing an appropriate design speed.

Anchor: #i1635162Table 3-7: Minimum Design Speed for Rural Two-lane Highways

Functional Class

Terrain

Minimum Design Speed (mph) for future ADT of:

< 400

400-2000

> 2000

Arterial

Level

70

Rolling

60

Collector

Level

501

50

60

Rolling

402,4

404

50

Local3

Level

402,4

50

50

Rolling

304

404

404

Notes:

  1. A 40 mph minimum design speed may be used where roadside environment or unusual design considerations dictate (e.g., significant horizontal curvature due to mountainous or hilly terrain).
  2. A 30 mph minimum design speed may be used where roadside environment or unusual design considerations dictate (e.g., significant horizontal curvature due to mountainous or hilly terrain).
  3. Applicable only to off-system routes that are not functionally classified at a higher classification.
  4. When determining applicable radii and superelevation, Table 2-3 and Table 2-4 (for high-speed and non-urban conditions) should be used even though listed speeds are considered low-speed.


Anchor: #i1642858Table 3-8: Width of Travel Lanes and Shoulders on Rural Two-lane Highways

Functional Class

Design Speed (mph)

Minimum Width 1,2, 9, 10(ft) for future ADT of:

< 400

400-2000

> 2000

Arterial

LANES (ft)12

All

12

SHOULDERS (ft)3

All

47

8

108

Collector

LANES (ft)12

30

10

11

12

35

10

11

12

40

10

11

12

45

10

11

12

50

10

12

12

55

10

12

12

60

11

12

12

65

11

12

12

70

11

12

12

75

11

12

12

80

11

12

12

SHOULDERS (ft)5

All

24, 7

8

108

Local6

LANES (ft)12

30

10

11

12

35

10

11

12

40

10

11

12

45

10

11

12

50

10

11

12

SHOULDERS (ft)5

All

2

47, 11

8

Notes:

  1. Minimum surfacing width is 24-ft for all on-system state highway routes.
  2. On high riprapped fills through reservoirs, a minimum of two 12-ft lanes with 8-ft shoulders should be provided for roadway sections. For arterials with 2,000 or more ADT in reservoir areas, two 12-ft lanes with 10-ft shoulders should be used.
  3. On arterials, shoulders fully surfaced.
  4. On collectors, use minimum 4-ft shoulder width at locations adjacent to roadside barrier.
  5. Shoulders must be fully surfaced on collectors with 1,500 or more ADT. Shoulder surfacing not required but is desirable even if only for partial width, on collectors with less than 1,500 ADT and all local roads.
  6. Applicable only to off-system routes that are not functionally classified at a higher classification.
  7. 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.
  8. In highly constrained conditions 8-ft may be used.
  9. Minimum width of new or widened structures should accommodate the approach roadway including shoulders.
  10. A minimum surfacing width of 36-ft should be considered on horizontal curves with a radius less than 1910-ft (i.e. greater than 3 degrees of curvature). See FHWA Handbook for Designing Roadways for the Aging Population for further recommendations on minimum pavement width in horizontal curves when considering older drivers.
  11. Mailbox Turnouts should be considered on local roads with 400 or more ADT. Refer to TxDOT standard drawings for details regarding the design and construction of mailbox turnouts.
  12. See Ch. 3 Section 3 for TWLTL criteria.


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Access Control

The installation of access driveways along two-lane rural highways must be in accordance with the Access Management Manual.

Frontage roads or parallel service roads to serve small rural business communities or other developments should not be permitted along two-lane rural highways. To a driver unfamiliar with the local area, a frontage road takes on the appearance of a separate roadway of a multilane divided facility, thus resulting in the assumption that the two-way, two-lane highway is a one-way roadway. Where individual driveways are located within deep cut or high fill areas, driveways may be routed parallel to the highway for short distances to provide for a safe, economical junction with the highway.

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Transitions to Four-Lane Divided Highways

Typical transitions from two-lane to four-lane divided highways are discussed in Transitions to Four-Lane Divided Highways, Multi-Lane Rural Highways, and illustrated in Figure 3-16.

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Passing Sight Distances

Passing sight distance is the distance required by a driver to make a passing maneuver without cutting off the passed vehicle and before meeting an opposing vehicle. Therefore, passing sight distance is applicable to two-lane highways only (including two-way frontage roads).

Recommended passing sight distances are based on the following assumptions:

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  • The speeds of the passing and opposing vehicles are equal and represent the design speed of the highway;
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  • The passed vehicle travels at a uniform speed and the speed difference between the passing and passed vehicles is 12 mph;
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  • The passing vehicle has sufficient acceleration capability to reach the specified speed difference relative to the passed vehicle by the time it reaches the critical position, which generally occurs about 40 percent of the way through the passing maneuver;
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  • The lengths of the passing and passed vehicles are 19-ft;
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  • The passing driver's perception-reaction time in deciding to abort passing a vehicle is 1-s;
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  • If the passing maneuver is aborted, the passing vehicles will use a deceleration rate of 11.2 ft/s2, the same deceleration rate used in stopping sight distance design criteria;
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  • For a completed or aborted pass, the space headway between passing and passed vehicles is 1-s; and
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  • The minimum clearance between passing and opposing vehicles at the point at which the passing vehicle return to its normal lane is 1-s.

In the design of two-lane highways, minimum or greater passing sight distance should be provided wherever practical, since less than minimum distances reduces the safety and level of service of the roadway. For rolling terrain, provision of climbing lanes may be a more economical alternative than achieving a vertical alignment with adequate passing sight distance.

The minimum passing sight distance for a two-lane road is about twice the minimum stopping sight distance at the same design speed. To meet those greater sight distances, clear sight areas on the inside of curves should be provided. For cut sections, designing for passing sight distance should be limited to tangents and very flat curves. Even in level terrain, providing passing sight distance would need a clear area inside each curve that would, in some instances, extend beyond the normal right-of-way line.

Minimum passing sight distance values for design of two-lane highways are shown in Table 3-9. These distances are for design purposes only and should not be confused with other distances used as warrants for striping no-passing zones as shown in the Texas Manual on Uniform Traffic Control Devices (TMUTCD). For the design of typical two-lane rural highways, except for level terrain, provision of near continuous passing sight distance is impractical. However, the designer should attempt to increase the length and frequency of passing sections where economically feasible.

Anchor: #i1064360Table 3-9: Passing Sight Distance

K-Values for Determining Length of Crest Vertical Curve for Various Passing Sight Distances (PSD)

Design Speed (mph)

Minimum PSD for Design (ft)

Minimum K-Value1

20

400

57

25

450

72

30

500

89

35

550

108

40

600

129

45

700

175

50

800

229

55

900

289

60

1000

357

65

1100

432

70

1200

514

75

1300

604

80

1400

700

Note:

  1. Based on eye height = 3.5 ft and object height = 3.5 ft; K = PSD2 / 2800.


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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 climbing lanes. A design waiver is required for speed change lanes that do not meet minimum length or width criteria.

Climbing Lanes. It is desirable to provide an extra lane on the upgrade side of a two-lane highway as a climbing lane where the grade, traffic volume, and heavy vehicle volume combine to degrade traffic operations.

A climbing lane should be considered when one of the following three conditions exist:

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  • 10 mph or greater speed reduction is expected for a typical heavy vehicle;
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  • Level-of-service E or F exists on the upgrade; or
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  • A reduction of two or more levels of service is experienced when moving from the approach segment to the upgrade.

    For low-volume roadways there is minimal delay and a climbing lane may not be justified. For this reason, a climbing lane should only be considered on roadways with the following traffic conditions:

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  • Upgrade traffic flow rate is greater than 200 vehicles per hour or
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  • Upgrade truck flow rate is greater than 20 vehicles per hour.

The upgrade flow rate is estimated by multiplying the anticipated or existing design hour volume by the directional distribution factor for the upgrade direction and dividing the result by the peak hour factor (see Traffic Characteristics, Chapter 2 and the Highway Capacity Manual for definitions of these terms). To calculate the upgrade truck flow rate, multiply the upgrade flow rate by the percentage of trucks in the upgrade direction.

The beginning of a climbing lane should be introduced near the foot of the grade. The climbing lane should be preceded by a tapered section with a desirable taper ratio of 25:1 that should be at least 300-ft long.

Attention should also be given to the location of the terminal point of the climbing lane. Ideally, the climbing lane should be extended to a point beyond the crest where a typical truck could reach a speed that is within 10 mph of the speed of other vehicles. In addition, climbing lanes should not end just prior to an obstruction such as a restrictive width bridge. The climbing lane should be followed by a tapered section with a ratio of 50:1.

For projects in new locations, or where an existing highway will be regraded, consider improving the grade line in lieu of providing a climbing lane. Refer to Chapter 3 of AASHTO’s A Policy on Geometric Design of Highways and Streets for more information regarding the design of climbing lanes. Figure 3-7 shows a cross section for climbing lanes on rural highways.

Cross Sections for Arterial and Collector
Two-Lane Rural Highways. (click in image to see full-size image) Anchor: #i1018565grtop

Figure 3-7. Cross Sections for Arterial and Collector Two-Lane Rural Highways.

Left-Turn Deceleration Lanes. The additional expense of adding left-turn lanes on two-lane highways at intersecting cross roads is often not justified due to low volumes. For certain moderate or high-volume two-lane highways with heavy left-turn movements, however, left-turn lanes may be justified in view of reduced road user crash costs. Table 3-10 provides recommendations for when left-turn lanes should be considered for a typical two-lane highway intersection. Lengths of left-turn deceleration lanes are provided in Table 3-12.

In instances on three-leg intersections where a left turn lane is not warranted due to low major roadway volume, but separation of through and turning traffic is still desired due to moderate to high left turn volume, a bypass lane can be installed (see Figure 3-8). For width of left-turn deceleration lanes see Table 3-1.

Anchor: #i1653274Table 3-10. Guide for Left-Turn Lane Warrants on Two-Lane Highways in Rural Areas1

Left-Turn Lane Peak-Hour Volume (veh/hr)

Three-Leg Intersection Major-Road Peak-Hour Volume (veh/hr/ln) for a Bypass Lane

Three-Leg Intersection Major-Road Peak-Hour Volume (veh/hr/ln) for a Left-Turn Lane

Four-Leg Intersection Major-Road Peak-Hour Volume (veh/hr/ln) for a Left-Turn Lane

5

50

200

150

10

50

100

50

15

< 50

100

50

20

< 50

50

< 50

25

< 50

50

< 50

30

< 50

50

< 50

35

< 50

50

< 50

40

< 50

50

< 50

45

< 50

50

< 50

50 or More

< 50

50

< 50

Note:

  1. These guidelines apply where the major road is uncontrolled and the minor-road approaches are stop- or yield-controlled. Both the left-turn peak-hour volume and the major-road volume warrants should be met as shown in Figure 3-8.


Suggested Left-Turn Warrants Based on Results
from Benefit-Cost Evaluations for Intersections on Two-Lane Highways
in Rural Areas.
Source: AASHTO's A Policy on Geometric Design of Highways and Streets (click in image to see full-size image) Anchor: #FDQGULXFgrtop

Figure 3-8. Suggested Left-Turn Warrants Based on Results from Benefit-Cost Evaluations for Intersections on Two-Lane Highways in Rural Areas. Source: AASHTO's A Policy on Geometric Design of Highways and Streets

Example:

Three Leg Intersection

Left Turn Volume = 17 veh/hr

Major Road Volume = 150 veh/hr; 2 ln

Choose the next highest turn lane volume of 20 veh/h. The major road is 75 veh/h/ln, which is greater than 50, therefore a left turn lane is warranted.

Where used, left-turn lanes should be delineated with striping and pavement markers or jiggle bars. Passing should be restricted in advance of the intersection, and horizontal alignment shifts of the approaching travel lanes should be gradual. Figure 3-9 shows typical geometry for a rural two-lane highway with left-turn bays at an intersecting crossroad.

Typical Two-Lane Highway Intersection with
Left-Turn Lanes. (click in image to see full-size image) Anchor: #i1018567grtop

Figure 3-9. Typical Two-Lane Highway Intersection with Left-Turn Lanes.

Right-Turn Deceleration Lanes. Shoulders 10-ft wide alongside the traffic lanes generally provide sufficient area for acceleration or deceleration of right-turning vehicles. Where the right turn deceleration or acceleration lane is being constructed adjacent to the through lanes, the minimum lane width is 10-ft with a 2-ft surfaced shoulder. Speed change lanes should be symmetrical along both sides of the highway to provide drivers with a balanced section.

A deceleration-acceleration lane on one side of a two-lane highway, such as at a “tee” intersection, results in the appearance of a three-lane highway and may result in driver confusion. Therefore, right-turn speed change lanes are generally inappropriate for “tee” intersection design except where a four-lane section is provided. An example of this configuration is two through lanes, one median left turn lane and one right acceleration/deceleration lane.

Figure 3-5 shows an example of right-turn deceleration lanes.

The length of a right-turn deceleration lane is the same as that for a left-turn lane (see Table 3-12). On some low-volume rural highways, it may be acceptable to provide right turn lanes shorter than the lengths given in Table 3-12.

Right-Turn Acceleration Lanes. Right-turn acceleration lanes may be appropriate on some two-lane rural highways such as high-volume highways where significant truck percentages are encountered. See Table 3-13 for acceleration distances and taper lengths.

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Grade Separations and Interchanges

See Grade Separations and Interchanges, Freeways and Chapter 10 of AASHTO's A Policy on Geometric Design of Highways and Streets.

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Intersections

The provision of adequate sight distance is of utmost importance in the design of intersections along two-lane rural highways. At intersections, consideration should be given to avoid steep profile grades and limited horizontal or vertical sight distance. An intersection should not be situated just beyond a short crest vertical curve or a sharp horizontal curve. Where necessary, backslopes should be flattened and horizontal and vertical curves lengthened to provide additional sight distance. For more information on intersection sight distance, see Intersection Sight Distance in Chapter 2.

The roadways should intersect at approximately right angles and should not intersect less than 75 degrees. Where crossroad skew is less than 75 degrees to the highway, the crossroad should be re-aligned to provide for a near perpendicular crossing. As a general rule, the higher the functional classification, the closer the crossroad intersection should be to 90 degrees.

Chapter 7 provides additional information regarding the accommodation of various types of truck class vehicles in intersection design in the section on Minimum Designs for Truck and Bus Turns. Further information on intersection design may also be found in AASHTO’s A Policy on Geometric Design of Highways and Streets.

Transverse or In-Line Rumble Strips. See Chapter 7 Section 5 Rumble Strips for additional traffic calming measures at intersections.

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