Section 4: Two-Lane Rural Highways
Anchor: #i1083249Overview
The general geometric features for two-lane rural highways are provided in this section and are summarized in the following tables and figures:
- Figure 3-5: Typical cross section
- Table 3-6: Minimum Design Speed for Rural Two-lane Highways: Minimum design speed
- Table 3-7. Geometric Design Criteria for Rural Two-Lane Highways: Basic design criteria and cross sectional elements
- Table 3-8: Width of Travel Lanes and Shoulders on Rural Two-lane Highways: Lane and shoulder widths
-
Table
3-9: Minimum Structure Widths For Bridges to Remain in Place on
Rural Two-lane Highways: Minimum structure widths that may
remain in place.
Additional information on structure widths may be obtained in the Bridge Design - LRFD and the Bridge Project Development Manual.
Anchor: #CIHBDAJFTable 3-6: Minimum Design Speed for Rural Two-lane Highways (US Customary)
Functional
-
Minimum Design Speed (mph) for future ADT of:
Class
Terrain
< 400
400-1500
1500-2000
> 2000
Arterial
Level
Rolling
70
60
Collector
Level
Rolling
501
402
50
40
50
40
60
50
Local3
Level
Rolling
402
30
50
40
50
40
50
40
(Metric)
Functional
-
Minimum Design Speed (km/h) for future ADT of:
Class
Terrain
< 400
400-1500
1500-2000
> 2000
Arterial
Level
Rolling
110
100
Collector
Level
Rolling
801
602
80
60
80
60
100
80
Local3
Level
Rolling
602
50
80
60
80
60
80
60
1 A 40 mph [60 km/h] minimum design speed may be used where roadside environment or unusual design considerations dictate.
2 A 30 mph [50 km/h] minimum design speed may be used where roadside environment or unusual design considerations dictate.
3 Applicable only to off-system routes that are not functionally classified at a higher classification.
Anchor: #CIHEEIEITable 3-7. Geometric Design Criteria for Rural Two-Lane Highways (US Customary)
Geometric Design Element
Functional Class
Reference or Design Value
Design Speed
All
Minimum Horizontal Radius
All
Max. Gradient
All
Table 2-9
Stopping Sight Distance
All
Table 2-1
Width of Travel Lanes
All
Width of Shoulders
All
Vertical Clearance, New Structures
All
16.5 ft1
Horizontal Clearance
All
Table 2-11
Pavement Cross Slope
All
Chapter 2, Pavement Cross Slope
(Metric)
Geometric Design Element
Functional Class
Reference or Design Value
Design Speed
All
Minimum Horizontal Radius
All
Max. Gradient
All
Table 2-9
Stopping Sight Distance
All
Table 2-1
Width of Travel Lanes
All
Width of Shoulders
All
Vertical Clearance, New Structures
All
5.0 m1
Horizontal Clearance
All
Table 2-11
Pavement Cross Slope
All
Chapter 2, Pavement Cross Slope
1 Exceptional cases near as practical to 16.5 ft [5.0 m] but never less than 14.5 ft [4.4 m].
Anchor: #CIHHGBFFTable 3-8: Width of Travel Lanes and Shoulders on Rural Two-lane Highways (US Customary)
Functional Class
Design Speed (mph)
Minimum Width 1,2(ft) for future ADT of:
-
-
< 400
400-1500
1500-2000
> 2000
Arterial
LANES (ft)
-
All
12
-
SHOULDERS (ft)
-
All
43
43 or 83
83
8 - 103
Collector
LANES (ft)
-
30
10
10
11
12
-
35
10
10
11
12
-
40
10
10
11
12
-
45
10
10
11
12
-
50
10
10
12
12
-
55
10
10
12
12
-
60
11
11
12
12
-
65
11
11
12
12
-
70
11
11
12
12
-
75
11
12
12
12
-
80
11
12
12
12
-
SHOULDERS (ft)
-
All
24,5
45
85
8 - 105
Local6
LANES (ft)
-
30
10
10
11
12
-
35
10
10
11
12
-
40
10
10
11
12
-
45
10
10
11
12
-
50
10
10
11
12
-
SHOULDERS (ft)
-
All
2
4
4
8
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.
3On arterials, shoulders fully surfaced.
4On collectors, use minimum 4 ft shoulder width at locations where roadside barrier is utilized.
5For collectors, shoulders fully surfaced for 1,500 or more ADT. Shoulder surfacing not required but desirable even if partial width for collectors with lower volumes and all local roads.
6 Applicable only to off-system routes that are not functionally classified at a higher classification.
Anchor: #i1064085Table 3-8: Width of Travel Lanes and Shoulders on Rural Two-lane Highways (Metric)
Functional Class
Design Speed (km/h)
Minimum Width 1,2(m) for future ADT of:
-
-
< 400
400-1500
1500-2000
> 2000
Arterial
LANES (m)
-
All
3.6
-
SHOULDERS (m)
-
All
1.23
1.23 or 2.43
2.43
2.4 - 3.03
Collector
LANES (m)
-
50
3.0
3.0
3.3
3.6
-
60
3.0
3.0
3.3
3.6
-
70
3.0
3.0
3.3
3.6
-
80
3.0
3.0
3.6
3.6
-
90
3.0
3.0
3.6
3.6
-
100
3.3
3.3
3.6
3.6
-
110
3.3
3.3
3.6
3.6
-
120
3.3
3.6
3.6
3.6
-
130
3.3
3.6
3.6
3.6
-
SHOULDERS (m)
-
All
0.64,5
1.25
2.45
2.4-3.05
Local6
LANES (m)
-
50
3.0
3.0
3.3
3.6
-
60
3.0
3.0
3.3
3.6
-
70
3.0
3.0
3.3
3.6
-
80
3.0
3.0
3.3
3.6
-
SHOULDERS (m)
-
All
0.6
1.2
1.2
2.4
1 Minimum surfacing width is 7.2 m for all on-system state highway routes.
2 On high riprapped fills through reservoirs, a minimum of two 3.6 m lanes with 2.4 m shoulders should be provided for roadway sections. For arterials with 2,000 or more ADT in reservoir areas, two 3.6 m lanes with 3.0 m shoulders should be used.
3 On arterials, shoulders fully surfaced.
4 On collectors, use minimum 1.2 m shoulder width at locations where roadside barrier is utilized.
5 For collectors, shoulders fully surfaced for 1,500 or more ADT. Shoulder surfacing not required but desirable even if partial width for collectors with lower volumes and all local roads.
6 Applicable only to off-system routes that are not functionally classified at a higher classification.
The following notes apply to Table 3-8:
- Minimum width of new or widened structures should accommodate the approach roadway including shoulders.
- See Table 3-9 for minimum structure widths that may remain
in place.
Anchor: #CIHEAFEDTable 3-9: Minimum Structure Widths For Bridges to Remain in Place on Rural Two-lane Highways (US Customary)
Functional Class
Roadway Clear Width 1 (ft) for ADT of:
-
< 400
400-1500
1500-2000
> 2000
Local
20
22
24
28
Collector
22
22
24
28
Arterial
Traveled Way + 6 ft
(Metric)
Functional Class
Roadway Clear Width 1 (m) for ADT of:
-
< 400
400-1500
1500-2000
> 2000
Local
6.0
6.6
7.2
8.4
Collector
6.6
6.6
7.2
8.4
Arterial
Traveled Way + 1.8 m
1 Clear width between curbs or rails, whichever is lesser, is considered to be at least the same as approach roadway clear width. Approach roadway width includes shoulders.
Basic Design Features
This subsection includes information on the following basic design features for two-lane rural highways:
- Access Control
- Transitions to Four-Lane Divided Highways
- Passing Sight Distances
- Speed Change Lanes
- Intersections
Access Control
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.
The installation of access driveways along two-lane rural highways shall be in accordance with the TxDOT Access Management Manual.
Anchor: #CIHIHAGCTransitions 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 Multi-Lane Rural Highway Intersection.
Anchor: #CIHDEBHFPassing Sight Distances
Passing sight distance is the length of highway 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 conditions:
- 3.5 ft [1,080 mm] driver eye height
- 3.5 ft [1,080 mm] object height
- 10 mph [15 km/h] speed differential between the passing vehicle and vehicle being passed
In the design of two-lane highways, minimum or greater passing sight distance should be provided wherever practical, since less than minimum distances reduce capacity and adversely affect level of service. For rolling terrain, provision of climbing lanes may be a more economical alternative than achieving a vertical alignment with adequate passing sight distance.
Minimum passing sight distance values for design of two-lane highways are shown in Table 3-10. 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. For the design of typical two-lane rural highways, except for level terrain, provision of near continuous passing sight distance (2,680 ft at 80 mph [815 m at 130 km/h]) is impractical. However, the designer should attempt to increase the length and frequency of passing sections where economically feasible.
|
(US Customary) |
||
|---|---|---|
|
K-Values for Determining Length of Crest Vertical Curve for Various Passing Sight Distances |
||
|
Design Speed (mph) |
Minimum Passing Sight Distance for Design (ft) |
K-Value1 |
|
20 |
710 |
180 |
|
25 |
900 |
289 |
|
30 |
1090 |
424 |
|
35 |
1280 |
585 |
|
40 |
1470 |
772 |
|
45 |
1625 |
943 |
|
50 |
1835 |
1203 |
|
55 |
1985 |
1407 |
|
60 |
2135 |
1628 |
|
65 |
2285 |
1865 |
|
70 |
2480 |
2197 |
|
75 |
2580 |
2377 |
|
80 |
2680 |
2565 |
|
(Metric) |
||
|
K-Values for Determining Length of Crest Vertical Curve for Various Passing Sight Distances |
||
|
Design Speed (km/h) |
Minimum Passing Sight Distance for Design (m) |
K-Value1 |
|
30 |
200 |
46 |
|
40 |
270 |
84 |
|
50 |
345 |
138 |
|
60 |
410 |
195 |
|
70 |
485 |
272 |
|
80 |
540 |
338 |
|
90 |
615 |
438 |
|
100 |
670 |
520 |
|
110 |
730 |
617 |
|
120 |
775 |
695 |
|
130 |
815 |
769 |
|
1K = Length of Crest Vertical Curve ÷ Algebraic Difference in Grades |
||
Anchor: #CIHDEDHB
Speed Change Lanes
There are three kinds of speed change lanes: climbing lanes, left-turn lanes, and right-turn lanes.
Climbing Lanes. It is desirable to provide a climbing lane, as an extra lane on the upgrade side of a two-lane highway 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:
- 10 mph [15 km/h] or greater speed reduction is expected for a typical heavy truck
- level-of-service E or F exists on the upgrade
- a reduction of two or more levels of service is experienced
when moving from the approach segment to the upgrade.
For low-volume roadways, only an occasional car is delayed, and a climbing lane may not be justified economically. For this reason, a climbing lane should only be considered on roadways with the following traffic conditions:
- upgrade traffic flow rate in excess of 200 vehicles per hour or
- upgrade truck flow rate in excess of 20 vehicles per hour.
The upgrade flow rate is predicted by multiplying the predicted 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). The upgrade truck flow rate is obtained by multiplying 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 desirably with a ratio of 25:1, but at least 150 ft [50 m] long.
Attention should also be given to the location of the climbing lane terminal. Ideally, the climbing lane should be extended to a point beyond the crest where a typical truck could attain a speed that is within 10 mph [15 km/h] 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 desirably with a ratio of 50:1.
For projects on new location or where an existing highway will be regraded, the economics of providing an improved grade line in lieu of providing climbing lanes should be investigated. 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 3-5 shows cross sections for climbing lanes on rural highways.
Figure 3-5. (US). Cross Sections for Arterial and Collector Two-Lane Rural Highways. Click US Customary or Metric to see a PDF of the image.
Left-Turn Deceleration Lanes. Left-turn lanes on two-lane highways at intersecting crossroads generally are not economically justified. 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 accident costs. Figure 3-6 provides recommendations for when left-turn lanes should be considered based on traffic volumes.
Example: Traffic northbound on a highway has 350 vph with 10 percent left turns included. The southbound traffic volume is 200 vph. The design speed on the highway is 60 mph [100 km/h]. Beginning at the opposing volume (southbound in this case) of 200 vph, using the 10 percent left turn column and 60 mph [100 km/h] design speed section, a value of 330 vph advancing volume (northbound) is found in the table. Because the northbound volume of 350 vph exceeds the table value of 330 vph, a left turn lane should be considered at the intersection.
Lengths of left-turn deceleration lanes are provided in Table 3-13.
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 3-6 shows typical geometry for a rural two-lane highway with left-turn bays at a crossroad intersection.
Figure 3-6. Typical Two-Lane Highway Intersection with Left-Turn Lanes. Click here to see a PDF of the image.
|
Opposing Volume (vph) |
Advancing Volume (vph) |
|||
|---|---|---|---|---|
|
- |
5 % Left Turns |
10 % Left Turns |
20 % Left Turns |
30 % Left Turns |
|
40 mph [60 km/h] Design Speed |
||||
|
800 |
330 |
240 |
180 |
160 |
|
600 |
410 |
305 |
225 |
200 |
|
400 |
510 |
380 |
275 |
245 |
|
200 |
640 |
470 |
350 |
305 |
|
100 |
720 |
515 |
390 |
340 |
|
50 mph [80 km/h] Design Speed |
||||
|
800 |
280 |
210 |
165 |
135 |
|
600 |
350 |
260 |
195 |
170 |
|
400 |
430 |
320 |
240 |
210 |
|
200 |
550 |
400 |
300 |
270 |
|
100 |
615 |
445 |
335 |
295 |
|
60 mph [100 km/h] Design Speed |
||||
|
800 |
230 |
170 |
125 |
115 |
|
600 |
290 |
210 |
160 |
140 |
|
400 |
365 |
270 |
200 |
175 |
|
200 |
450 |
330 |
250 |
215 |
|
100 |
505 |
370 |
275 |
240 |
Right-Turn Deceleration Lanes. Shoulders 10 ft [3.0 m] wide alongside the traffic lanes generally provide sufficient area for acceleration or deceleration of right-turning vehicles. Where the right turn lane is being constructed in addition to the through lanes and shoulders, the minimum right turn lane width is 10 ft [3.0 m] with a 2 ft [0.6 m] surfaced shoulder. Where speed change lanes are used, they should be provided symmetrically along both sides of the highway for both directions of traffic, thus presenting 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. In this regard, right-turn speed change lanes are generally inappropriate for “tee” intersection design except where a four lane (2 through, 1 median left turn, 1 right acceleration/deceleration) section is provided.
Section 2, Figure 3-4 shows the lengths for right-turn deceleration lanes.
The length of a right-turn deceleration lane is the same as that for a left-turn lane (see Table 3-13). Right turn lanes shorter than the lengths given in Table 3-13 may be acceptable on some low volume rural highways.
Right-Turn Acceleration Lanes. Right-turn acceleration lanes may be appropriate on some two-lane rural highways – for example on high volume highways where significant truck percentages are encountered. See Table 3-8 for acceleration distances and taper lengths.
Anchor: #CIHJDHJIIntersections
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 as well as areas with 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.
Desirably, the roadways should cross at approximately right angles. Where crossroad skew is flatter than 60 degrees to the highway, the crossroad should be re-aligned to provide for a near perpendicular crossing. The higher the functional classification, the closer to right-angle the crossroad intersection should be.
Minimum Designs for Truck and Bus Turns in Chapter 7 provides information regarding the accommodation of various types of truck class vehicles in intersection design. Further information on intersection design may also be found in AASHTO’s A Policy on Geometric Design of Highways and Streets.