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FOREST SERVICE HANDBOOK

NATIONAL HEADQUARTERS (WO)

WASHINGTON, DC

FSH 7709.56 – ROAD PRECONSTRUCTION HANDBOOK

CHAPTER 40 – DESIGN

Amendment No.: 7709.56-2014-1

Effective Date: July 3, 2014

Duration: This amendment is effective until superseded or removed.

Approved: GREGORY C. SMITH

 Acting Associate Deputy Chief, NFS

Date Approved: 06/27/2014

Posting Instructions: Amendments are numbered consecutively by handbook number and 

calendar year. Post by document; remove the entire document and replace it with this 

amendment. Retain this transmittal as the first page(s) of this document. The last amendment to 

this handbook was 7709.56-2011-5 to FSH 7709.56_40.

New Document 7709.56_40 70 Pages

Superseded Document(s) by 

Issuance Number and 

Effective Date

7709.56_40 

(Amendment 7709.56-2011-5, 08/19/2011 )

72 Pages

Digest: 

40 - Revises, updates, and sets forth new direction throughout the entire chapter.
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FSH 7709.56 – ROAD PRECONSTRUCTION HANDBOOK 

CHAPTER 40 – DESIGN

Table of Contents

40.2 - Objective........................................................................................................................... 4

40.3 - Policy................................................................................................................................ 4

40.5 - Definitions........................................................................................................................ 4

40.7 - References ........................................................................................................................ 6

41 - DESIGN CRITERIA ................................................................................................. 7

42 - DESIGN ELEMENTS AND DESIGN STANDARDS.............................................. 11

42.1 - Application of Design Standards.................................................................................... 12

42.2 - Coordination of Design Elements .................................................................................. 12

42.3 - Number of Lanes............................................................................................................ 13

42.4 - Road Structure ................................................................................................................ 13

42.41 - Traveled Way............................................................................................................ 13

42.42 - Shoulder Width......................................................................................................... 17

42.43 - Turnouts.................................................................................................................... 17

42.44 - Turnarounds.............................................................................................................. 22

42.45 - Curve Widening........................................................................................................ 22

42.46 - Clearance .................................................................................................................. 28

42.47 - Fill Widening............................................................................................................ 28

42.48 - Clearing Widths........................................................................................................ 28

42.49 - Daylighting ............................................................................................................... 28

42.5 - Speed and Sight Distance ............................................................................................... 29

43 - ALIGNMENT.......................................................................................................... 34

43.1 - Horizontal Alignment..................................................................................................... 34

43.2 - Vertical Alignment ......................................................................................................... 39

43.3 - Intersections.................................................................................................................... 42

43.4 - Railroad Grade Crossings............................................................................................... 43

43.5 - Roadside Design............................................................................................................. 43

44 - ROAD DRAINAGE ................................................................................................ 46

44.1 - Traveled Way Surface Shapes........................................................................................ 48

44.2 - Surface Cross Drains...................................................................................................... 49

44.3 - Ditches............................................................................................................................ 51

44.4 - Culverts........................................................................................................................... 51

44.5 - Subdrainage Systems...................................................................................................... 55

44.6 - Drainage Systems on Stored Roads................................................................................ 57

45 - EROSION CONTROL AND WATERSHED PROTECTION................................... 57

45.1 - Permanent Erosion Control and Watershed Protection .................................................. 58

45.2 - Temporary Erosion Control and Watershed Protection ................................................. 59

45.3 - Aquatic Habitat Protection ............................................................................................. 59

45.4 - Disposal of Waste........................................................................................................... 59

45.5 - Retaining Structures ....................................................................................................... 59

46 - UTILITIES AND OTHER EXISTING USES, RIGHTS-OF-WAY, AND 

CONSTRUCTION EASEMENTS ....................................................................... 60
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CHAPTER 40 – DESIGN

47 - MATERIALS .......................................................................................................... 62

47.1 - Slopes ............................................................................................................................. 62

47.2 - Pavement Structure......................................................................................................... 63

47.3 - Compaction..................................................................................................................... 68

47.4 - Geosynthetics ................................................................................................................. 69
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FSH 7709.56 – ROAD PRECONSTRUCTION HANDBOOK 

CHAPTER 40 – DESIGN

This chapter provides guidance for design of roads. Design implies the concept of alternative 

solutions. It is the responsibility of the designer to apply engineering judgment to develop and 

evaluate alternatives that best fit project objectives. 

40.2 - Objective

The objective of this chapter is to provide guidance for the selection of design elements and 

standards in order to meet design criteria and resource management prescriptions as set forth in 

the road management objectives (RMOs) (FSM 7714). Meeting RMOs involves collaboration

with engineering peers, other Forest Service specialists, such as realty specialists, landscape 

architects, hydrologists, and fisheries biologists, and relevant State and Federal agencies.

40.3 - Policy

The geometric design of National Forest System (NFS) roads, managed as public roads, is 

subject to this chapter. Additional guidance for roads at level of service F and above is contained 

in the “Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT<400)” and 

“A Policy on Geometric Design of Highways and Streets,” issued by the American Association 

of State Highway and Transportation Officials (AASHTO), hereinafter referred to as the 

“AASHTO Guidelines”.

40.5 - Definitions

See FSM 7721.05 for additional definitions pertaining to development of roads. Forest Service 

definitions for local, collector, and arterial roads are found in FSM 7705 and differ from the

definitions of those terms in the AASHTO Guidelines. Most NFS roads fit the definition of a 

local or minor collector road in the AASHTO Guidelines.

Administrative National Forest System Road. Any National Forest System road that is 

not a public road.

Average Daily Traffic (ADT). The daily number of vehicles on a route, determined by 

dividing the annual number of vehicles on the route by 365.

Clearance. The distance from the edges of the traveled way to vertical and horizontal 

obstructions. 

Critical Vehicle. A vehicle type, typically the largest on a road by weight, size, or unique 

configuration, whose limited use on the road is necessary to fulfill the RMOs. 

Design Criterion. A requirement derived from management area direction, such as safety 

requirements, level of service, traffic types and volumes, the environmental 

considerations, and the economic constraints that governs the selection of design 

elements and design standards.
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FSH 7709.56 – ROAD PRECONSTRUCTION HANDBOOK 

CHAPTER 40 – DESIGN

Design Element. A physical characteristic of a road (such as traveled way width, 

shoulder, slope, curve widening, and pavement structure) that is considered in its design.

Design Exception. A proposed variation in design that would employ a less restrictive 

standard than the AASHTO Guidelines. 

Design Speed. The maximum speed that the design vehicle can safely maintain along a 

road or road segment when the design features of the road, rather than the operational 

limitations of the design vehicle, are taken into account. (The design speed is used to 

determine the design elements and design standards of a road.) 

Design Standard. The definitive length, width, depth, slope, or grade of a design element.

Design Vehicle. A vehicle type that frequently travels a road, that is not subject to 

restrictions on use of that road, and that travels at the road’s design speed and that 

determines the design standards for a particular design element for the road.

Forest Road. A road wholly or partly within or adjacent to and serving the NFS that the 

Forest Service determines is necessary for the protection, administration, and utilization 

of the NFS and the use and development of its resources (36 CFR 212.1).

Highway-Legal Vehicle. Any motor vehicle that is licensed or certified under State law 

for general operation on all public roads in the state. Operators of highway-legal vehicles 

are subject to State traffic law, including requirements for operator licensing.

Level of Service. A road's significant traffic characteristics and operating conditions. 

National Forest System Road. A forest road other than a road which has been authorized 

by a legally documented right-of-way held by a State, county, or local public road 

authority (36 CFR 212.1). 

Passing Sight Distance. The distance required to enable the driver of one vehicle to pass 

another vehicle safely and comfortably. 

Public Road. A road that is:

1. Available, except during scheduled periods, extreme weather, or emergency 

conditions; 

2. Passable by four-wheel standard passenger cars; and 

3. Open to the general public for use without restrictive gates, prohibitive signs, or 

regulation other than restrictions based on size, weight, or class of registration. 

(23 U.S.C. 101(a)(27); 23 CFR 460.2(c) and 660.103).
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CHAPTER 40 – DESIGN

Seasonal Average Daily Traffic (SADT). The daily number of vehicles on a route 

intended for seasonal use determined by dividing the number of vehicles using the route 

during the season of use by the number of days in the season of use.

Travel way. Synonymous with the term “Traveled Way,” (sec. 42.4, ex 01).

40.7 - References

The following publications are referenced throughout this chapter.

1. AASHTO. Guide for Design of Pavement Structures. 1993. 700p.

2. AASHTO. A Policy on Geometric Design of Highways and Streets. 5th ed. 2004. 

872p.

3. AASHTO. Guidelines for Geometric Design of Very Low-Volume Local Roads 

(ADT<400). 1

st ed. 2001. 94p.

4. AASHTO. Roadside Design Guide. 3

rd ed. 2002. 44p.

5. Glennon, J. Design and Traffic Control Guidelines for Low-Volume Rural Roads. 

National Cooperative Highway Research Program (NCHRP) Report 214. TRB, National 

Research Council, Washington, D.C., Oct. 1979, Appendix F.

6. Mohney, J., et al. 1994. Retaining Wall Design Guide. EM-7170-14. Washington, 

D.C. U.S. Department of Agriculture, Forest Service. 542p.

7. Steward, J., Williamson, R., and Mohney, J., 1977. Guidelines For Use of Fabrics in 

Construction and Maintenance of Low-Volume Roads. FHWA-TS-78-205. FHWA, 

Washington, D.C. 

8. U.S. Department of Agriculture, Forest Service. 1999. Dust Palliative Selection and 

Application Guide. 9977 1207-SDTDC. San Dimas Technology Development Center, 

San Dimas, CA.

9. U.S. Department of Agriculture, Forest Service. 1997. Relief Culverts. 9777 1812-

SDTDC. San Dimas Technology Development Center, San Dimas, CA.

10. U.S. Department of Agriculture, Forest Service. 1997. Traveled Way Surface 

Shape. 9777 1808-SDTDC. San Dimas Technology Development Center, San Dimas, 

CA. 

11. U.S. Department of Agriculture, Forest Service. 1997. Water/Road Interaction 

Technology Series. 9777 1806-SDTDC. San Dimas Technology Development Center,

San Dimas, CA.
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CHAPTER 40 – DESIGN

12. U.S. Department of Agriculture, Forest Service. 1977. National Forest Landscape 

Management, Volume 2, Chapter 4, “Roads.” Agriculture Handbook 483. Washington, 

D.C. 62p.

13. U.S. Department of Agriculture, Forest Service. 1994. Slope Stability Reference 

Guide for National Forests in the United States, Volume II. Forest Service Engineering 

Staff, Washington, D.C. EM 7170-13. 359p.

14. U.S. Department of Agriculture, Forest Service. 1996. Earth and Aggregate 

Surfacing Design Guide for Low-Volume Roads. FHWA-FLP-96,001. Washington, 

D.C. EM-7170-16. 302 p.

15. U.S. Department of Defense, U.S. Army Corps of Engineers. 1983. Revised 

Procedures for Pavement Design Under Seasonal Frost Conditions. Cold Regions 

Research and Engineering Laboratory, Office of the Chief of Engineers, Washington, 

D.C. Special Report 83-27.

16. U.S. Department of Transportation, Federal Highway Administration (FHWA). 

1992. Soil and Base Stabilization and Associated Drainage Considerations, Volumes I 

and II. FHWA-SA-93-004.

17. U.S. Department of Agriculture, Forest Service. 2012. National Best Management 

Practices for Water Quality Management on National Forest Lands, Volume 1, “Road 

Management Activities.” Washington, D.C.: U. S. Department of Agriculture, Forest 

Service . FS-990a. 104p.

41 - DESIGN CRITERIA

This section discusses design criteria and explains how it serves as a basis for selection and 

application of design elements and design standards. Design criteria are outputs of travel

planning (FSH 7709.55, ch. 10).

1. Environmental and Resource Considerations. Environmental and resource

considerations include factors such as topography, climate, and soils that affect road

alignment, gradients, sight distance, quantities of cut and fill, slope selection, drainage, 

and pavement structure. Forest Service environmental requirements usually result in 

more restrictive design elements than the AASHTO Guidelines. Recreational uses, such 

as hiking, riding, and over-snow vehicle use, may require restricting use of roads by

highway-legal vehicles.
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2. Safety Considerations. Safety considerations include factors such as possible hazards 

and corresponding corrective action and the need for traffic control devices and 

maintenance. Safety considerations affect the selection of geometric elements and 

standards, such as design speed. Direction on safety requirements for National Forest 

System (NFS) roads is found in FSM 7733.

3. Traffic Requirements. Traffic requirements include volume, composition, 

distribution, and whether the route is a public road. Traffic requirements are used in the 

design of turnouts, road widths, surfacing, safety features, and traffic control devices. 

4. Levels of Service. Levels of service A through F in the AASHTO Guidelines are for 

roads with average daily traffic (ADT) greater than 400. Section 41, exhibit 01, adds 

levels of service G through J which uses the design guidance of this chapter. The exhibit 

provides expected traffic characteristics and operating conditions of the road. Use this 

information in making design element decisions such as:

a. Number of lanes.

b. Turnout spacing.

c. Lane widths.

d. Type of driving surface.

e. Sight distances.

f. Design speed.

g. Clearance.

h. Horizontal and vertical alignment.

i. Curve widening.

j. Turnarounds.
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41 – Exhibit 01

Levels of Service G Through J for ADT Less Than 400*

G H I J

Flow Free flow with adequate 

parking facilities.

Periodically congested, 

such as during peak 

logging or recreational 

periods.

Interrupted by limited 

passing opportunities or 

slowed by road condition.

Slow or blocked flow. 

Two-way traffic is 

difficult and may require 

backing to pass.

Volume Uncontrolled; will 

accommodate expected 

traffic volume.

Occasionally controlled 

during heavy use periods.

Erratic; frequently 

controlled as capacity is 

reached.

Intermittent, usually 

controlled, and limited to 

a single purpose.

Vehicle Types Mix of vehicles normally 

found on public roads.

Mix of vehicles normally 

found on public roads.

Controlled mix: 

accommodate all vehicle 

types normally found on 

NFS roads. Some use 

may be controlled by 

vehicle type.

Single use; not designed 

for mixed traffic. Some 

vehicles may not be able 

to negotiate the road. 

Critical Vehicles Clearances are adequate 

to allow free travel. 

Overload permits are 

required.

Traffic controls needed 

where clearances are 

marginal. Overload 

permits are required.

Special provisions may 

be needed. Some 

vehicles will have 

difficulty negotiating 

some road segments.

May require special 

temporary road 

treatments, such as filling 

of ditches or water bar 

removal. Loads may 

have to be removed and 

walked in.

Safety Safety features are part of 

the design.

Safety is a high priority in 

design. Some protection 

is accomplished by traffic 

management.

Protection may be 

provided by traffic 

management and 

restrictions rather than by 

road design.

The need for protection is 

minimized by low speeds 

and traffic management 

restrictions.

Traffic Management Normally limited to 

regulatory, warning, and 

guide signs and permits.

Employed to reduce 

traffic volume and 

conflicts.

Traffic restrictions are 

frequently needed during 

periods of high volume 

for the primary use.

Used to discourage or 

prohibit traffic other than 

for a single purpose.

User Efficiency Important and high. Generally less important 

and may be somewhat 

lower than for level of 

service G.

Not important; may be 

traded for lower 

construction costs.

Not a consideration.

Alignment Design speed is the 

predominant factor within 

feasible topographical

limitations.

Influenced more strongly 

by topography than by 

speed and efficiency.

Generally dictated by 

topographical features 

and environmental 

factors. Design speeds 

are generally low.

Dictated by topography, 

environmental factors, 

and design and critical 

vehicle limitations. 

Speed is not important.

Road Surface Stable and smooth, with 

little or no dust during the 

normal season of use.

Stable for the 

predominant traffic 

during the normal season 

of use. Periodic dust 

control for heavy use or 

environmental reasons. 

Smoothness is 

commensurate with the 

design speed.

May not be stable under 

all traffic or weather 

conditions during the 

normal season of use. 

Surface rutting, 

roughness, and dust may 

be present, but are 

controlled for 

environmental or 

investment protection.

Rough and irregular. 

Travel with lowclearance vehicles is 

difficult. Stable during 

dry conditions. Rutting 

and dusting controlled for 

vehicle safety, driver 

visibility, and soil and 

water protection.

* For roads with greater than 400 ADT, refer to AASHTO Guidelines Levels of Service A Through F.
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5. Vehicle Characteristics. Vehicle characteristics describe the types of vehicles using a

road.

a. Design Vehicle. Design vehicles determine the design standards for each design 

element, from the types and configuration of vehicles using the road. Analyze each 

design element for its design vehicle and check for passage for the critical vehicle. A

single design vehicle type rarely controls the design standards for all design elements

of a road. 

Section 41, exhibit 02 displays examples of typical relationships between design 

elements and design vehicles. 

41 - Exhibit 02

Typical Relationships Between Design Elements and Design Vehicles

Design Element Design Vehicle

Horizontal alignment. Large recreational vehicle (such as a motor home, 

tour bus, or pick-up and trailer combination) or

logging truck.

Thickness of pavement 

structure:

 Campground or picnic 

area.

 Road with heavy 

commercial traffic.

 Garbage or other service truck, large 

recreational vehicle, or bus.

 Log truck, dump truck, or belly dump.

Curve widening:

 Lateral clearance.

 Large recreational vehicle with trailer or log 

truck.

 Articulated truck or fifth wheel truck.

Gradient Large recreational vehicle.

b. Critical Vehicles. A critical vehicle’s convenience and/or speed of travel are not 

considerations in road design. The use of a critical vehicle may require a permit, a 

pilot car, temporary traffic control, and restrictions on other traffic. Examples of 

critical vehicles include:

(1) A log yarder on a timber access road.

(2) A truck with a lowboy trailer carrying an oversized load of construction or 

mining equipment.
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(3) Wheel or track-mounted equipment using the road for construction and 

maintenance purposes.

(4) A motor coach bus used to bring large groups to a recreation site.

Special design features, operating considerations, or a combination of both may be 

required to accommodate critical vehicles according to a road’s level of service. 

Consider both the equipment needed to construct and maintain the road, and the 

traffic it will carry when designing a road for critical vehicles.

6. Road Users. The selection of design elements and design standards should be based 

on a road user who is considered to be a safe and prudent driver. A prudent driver is a 

person operating within their physical and mental limitation, with a properly equipped

and maintained vehicle, and who always exercises due care for the road, traffic, lighting,

and weather conditions (AASHTO’s “Guidelines for Geometric Design of Very LowVolume Local Roads (ADT<400)”). 

7. Economics. Design NFS roads to serve the projected traffic and to meet RMOs at the 

lowest overall cost, consistent with environmental protection and safety considerations. 

Economics are a primary consideration in the determination and selection of design 

standards. Economic analysis of design alternatives should balance risk to the investment 

against risks to the environment from the selected design; this will include and is not 

limited to life cycle costs, including costs of initial construction, operation, maintenance, 

environmental mitigation, and closure or decommissioning. Road maintenance 

projections, performed with appropriated funds, should be based on expenditures made 

on similar roads with similar traffic within current annual road maintenance 

appropriations.

8. Other Considerations. Follow applicable best management practices (BMPs)

established for compliance with the non-point source provisions of the Clean Water Act,

other applicable laws, and their implementing regulations.

42 - DESIGN ELEMENTS AND DESIGN STANDARDS

National Forest System (NFS) roads with an objective road maintenance level of 3, 4, or 5 meet 

the definition of a public road and are subject to Federal safety requirements for public roads. 

These NFS roads are also subject to the requirements in FSM 7705, FSM 7731, FSM 7733, and 

FSH 7709.59. Users can expect to drive these roads in any typical design vehicle type, including 

standard passenger cars. They can expect some degree of consideration regarding comfort, 

convenience, and speed of travel. 

NFS roads, with an objective maintenance level of 1 or 2, meet the definition of an 

administrative NFS road. User comfort, convenience, and speed of travel are not management 

considerations for these roads, and no provision is made to warn users regarding hazards on the 
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road. The design of administrative NFS roads should reflect that users of these roads assume 

risks associated with using roads where hazards like pot holes, washouts, and fallen trees may be 

present and do so knowing no warning is provided. 

Commercial users may temporarily improve some design elements of objective maintenance

during periods of heavy commercial use. For example, levels 1 and 2 roads may improve to 

standards normally associated with levels 3 to 5 roads, such as providing for a smooth and dust 

free road surfaces. These roads are not suitable for use as public roads unless the need for signs 

and traffic control devices has been assessed by a qualified engineer and any signs and devices 

identified by the engineer are in place and functional.

Roadside features and other design considerations, depending upon the traffic type and volume, 

are the determining factor as to whether NFS roads are single-lane or double-lane. The 

AASHTO Guidelines are primarily oriented towards double-lane roads. This chapter provides 

more specific information on single lane roads then the AASHTO Guidelines. 

Consider the range of values associated with each design element, and select the value

appropriate for the road segment. The AASHTO Guidelines and this chapter establish minimum 

standards for each design element. Any exception to a requirement for a design standard or 

design element, in the AASHTO Guidelines, must have prior written approval from the regional 

director of Engineering. 

Proficient designs are the result of engineering judgment, rather than rigid application of

techniques or automated systems. Appropriate techniques and systems include use of computers, 

hand designs, field designs, and flag lines. Use the technique or system most suited to a

particular project. Decisions about which technique or system to use for a project depend upon 

many factors, including cost, user safety, and impacts on NFS resources. A combination of 

approaches may be appropriate for a single project.

42.1 - Application of Design Standards

Standards for certain design elements can be found in matrix form or graphics in this chapter, 

such as lane width, stopping site distance, or curve widening. Other standards can be found 

within the text under topic headings, such as number of lanes, or within the general body of the 

text, such as maximum design speed. Exceptions to design standards should be handled on a 

case-by-case basis with application of engineering judgment and appropriate approvals. 

42.2 - Coordination of Design Elements

Do not consider or select a single design element independent of the others. Coordinated design 

elements will meet RMOs and generally result in the least resource impacts and costs.
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Improved safety occurs with coordinated design elements. Consider surfacing for improving 

traction coefficient around curves or for stopping sight distance. Consider longer stopping sight 

distances than required in some areas based on other elements and local conditions, as well as 

turnouts on double lane roads or adjusting cut slopes for additional visibility. Avoid, to the 

extent practicable, combinations such as designing short stopping sight distances on horizontal or 

vertical curves; or designing sharp curves at the end of long tangents. A coordinated road 

design alleviates the extensive use of traffic warning signs due to changing and unexpected road 

conditions.

42.3 - Number of Lanes

Where the estimated seasonal ADT for the design life is 100 to 250, a double-lane road should be 

considered. Where the estimated ADT for the design life is over 250, a double-lane road 

generally is the minimum design standard. Where the estimated ADT for the design life is less 

than 100, a single-lane road is generally preferable. Single-lane roads may have economic and 

environmental advantages. Decisions regarding the number of lanes may depend largely on 

environmental factors and costs. 

Design and construct NFS roads as single-lane or double-lane. Design widths will be established 

in accordance with the AASHTO Guidelines for double-lane roads or this chapter for single-lane

roads. Bicycle lanes and pedestrian walkways should meet the AASHTO Guidelines.

The decision to construct or reconstruct a road as single-lane or double-lane should be made and 

documented during travel planning. Additional information obtained during the design process 

may necessitate reevaluation of the planning decision. Modifications of planning decisions 

should be documented as amendments to the planning documents. Construction and 

reconstruction decisions may depend largely on environmental factors and costs. 

42.4 - Road Structure

The following relates to design of various elements of a roadway. Road structure terminology is 

found in section 42.4, exhibit 01.

42.41 - Traveled Way 

For double-lane roads, provide sufficient lane width for vehicles traveling in opposite directions 

to pass each other. The types of vehicles, users, and the volume of traffic determine whether to 

construct or reconstruct a single-lane or double-lane road. The primary consideration for 

determining the width of the traveled way is the types of vehicles that will use the road. 

Secondary considerations are the design speed, traffic volume, anticipated driver skill, and the 

presence or absence of shoulders and ditches.

Use the minimum traveled way widths for the conditions shown in exhibit 02 of this section. 
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Do not construct or reconstruct a traveled way that is less than 10 feet wide for a single-lane 

road.

Each region should evaluate local conditions where off-highway haul is a factor and should

establish regional standards for traveled way width where necessary (FSM 7731.04a).

a. Single-Lane Roads. For minimum widths, see section 42.4, exhibit 02 of this section

and for additional information, see AASHTO’s “A Policy on Geometric Design of 

Highways and Streets,” chapter 5, “Special Purpose Roads.”

A ditch may permit a narrower traveled way width because a ditch provides the necessary 

lateral clearance on one side.

In the absence of a ditch, a reduction below the minimum traveled way width shown in 

section 42.4, exhibit 02 should only occur in unusual circumstances with the 

understanding that such widths will probably result in higher construction costs, higher 

maintenance costs, and significantly reduced road utility. Use of widths which are less 

than those shown in section 42.4, exhibit 02 should be approved by the Regional Director 

of Engineering.

Traveled way widths should not exceed 14 feet, plus curve widening, unless wider lane 

widths are needed to accommodate off-highway haul and have been approved in a 

regional handbook supplement.

Limit design speed on single-lane roads to a maximum of 40 miles per hour.

b. Double-Lane Roads. See AASHTO’s “A Policy on Geometric Design of Highways 

and Streets” or AASHTO’s “Guidelines of Geometric Design of Very Low-Volume 

Local Roads (ADT<400)” for appropriate widths for double-lane roads.

c. Critical Vehicles. Check traveled way and shoulder widths to ensure that critical 

vehicles can traverse the road. Critical vehicles do not have to traverse the road at or 

even approaching the speeds of design vehicles. On roads with an asphalt or concrete 

surface, it is often appropriate to design widening to accommodate critical vehicles that

are managed as an unpaved shoulder.
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CHAPTER 40 – DESIGN

42.4 - Exhibit 01

Road Structure Terms
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CHAPTER 40 – DESIGN

42.4 - Exhibit 02

Traveled Way Widths for Single-Lane Roads

Design Speed (in miles per hour)

Type and Size of Vehicle 20 or Less 20 - 30 30 or More

Minimum Traveled Way Width (in feet)

Recreational, administrative, and service

vehicles, 6.5 to 8.0 feet wide.

10 12 14

Commercial 

hauling and 

commercial 

passenger vehicles,

including buses, at 

least 8 feet wide.

Roads with ditch and 

roads without ditch where 

ground cross slope is less

than 25 percent. 

12 12 14

Road without ditch with 

ground cross slope greater 

than 25 percent.

12 14 14
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CHAPTER 40 – DESIGN

42.42 - Shoulder Width

Shoulders may be necessary to provide:

a. Lateral support for the outside edge of asphalt or concrete pavement. The minimum 

shoulder width that can be economically constructed, generally 1 to 2 feet on each side, is 

adequate.

b. Parking areas when needed for specific activities or attractions along a road. The need 

for parking areas may be determined during travel planning or project planning. Provide 

a minimum usable width of 8 to 10 feet for parallel parking areas. Normally, parking 

areas are not provided along NFS roads for stalled or disabled vehicles.

c. Space for installations such as drainage structures, berms, guardrails, signs, and 

roadside utilities. The design of these devices or facilities determines the required width.

d. An increase in roadway width on the approach to a bridge or other structure so as to 

match the traveled way width of the structure. 

e. A recovery zone for vehicles straying from the traveled way on roads designed under

AASHTO’s “Guidelines of Geometric Design of Very Low-Volume Local Roads 

(ADT<400),” where a risk analysis indicates the costs are justified.

f. Additional width to provide for passage by a critical vehicle.

42.43 - Turnouts

The purpose of turnouts on a single-lane road is to provide for safe passing of vehicles.

a. Turnout Spacing. Intervisible turnouts (where a motorist at a turnout can see the next 

turnout) should be provided on public roads. In situations where intervisible turnouts 

cannot be provided, signing should be used to advise motorists of the situation.

Turnout spacing is determined by applying engineering judgment to information derived 

from exhibit 03, exhibit 04, and the Turnout Spacing Equation, all of which are shown 

below. 

Exhibit 03 displays the increased vehicle travel times that result from passing oncoming

vehicles at turnouts for four different turnout spacing as a function of the number of 

vehicles passing over a road per hour. (When converting ADT to VPH, use a 10-hour 

day.)
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CHAPTER 40 – DESIGN

42.4 – Exhibit 03

Increased Travel Time for Various Turnout Spacings
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CHAPTER 40 – DESIGN

Exhibit 04 contains turnout spacing requirements and operational constraints for levels of service

G through J. 

42.4 - Exhibit 04

Turnout Spacing

Level of 

Service Turnout Spacing Operational Constraints

G Turnouts are intervisible, unless 

compliance with cost or 

environmental criteria would be 

prohibitive. Closer spacing may 

contribute to efficiency and 

convenience. Maximum spacing is 

1,000 feet.

Traffic: mixed

Capacity: up to 25 VPH

Design Speed: up to 40 MPH

Delays: 20 sec./mile or less

H Intervisible turnouts are highly 

desirable, but may be precluded by 

excessive cost or environmental 

constraints. Maximum spacing is 

1,000 feet.

Traffic: mixed

Capacity: up to 25 VPH

Design Speed: up to 25 MPH

Delays: should be 30 sec./mile or less

Use signs to warn noncommercial 

users of commercial traffic. Road 

segments without intervisible turnouts 

should be signed.

I When the environmental impact is 

low and the investment is 

economically justifiable, additional 

turnouts may be constructed.

Maximum spacing is 1,000 feet.

Traffic: some mixed

Capacity: up to 20 VPH

Design Speed: up to 20 MPH

Delays: up to 60 sec./mile

Road should be managed to minimize 

conflicts between commercial and 

noncommercial users.

J Generally, only naturally occurring 

turnouts, such as additional width 

on ridges or other available areas 

on flat terrain, are provided.

Traffic: not intended for mixed use

Capacity: generally 10 VPH or less

Design Speed: 15 MPH or less

Delays: at least 60 sec./mile expected 

Road should be managed to restrict 

concurrent use by commercial and 

non-commercial users.
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CHAPTER 40 – DESIGN

Turnout Spacing Equation:

T=DS/36, where:

T = increase in travel time for the interrupted vehicle as a percentage

D = delay time per mile for the interrupted vehicle in seconds (sec. 42.4,ex. 04)

S = design speed in miles per hour

Process for determining turnout spacing:

1. Begin by solving the Turnout Spacing Equation for T. 

2. Use the graph in exhibit 03 of this section to determine the turnout spacing required to 

accommodate the number of vehicles passing over the road per hour (VPH). 

3. Then use engineering judgment to apply the turnout spacing considerations for levels 

of service displayed in exhibit 04 of this section.

a. The variables of spacing, VPH, delay time, and design speed should each be analyzed 

based on their relative importance. Different combinations should be investigated to 

determine their effect on safety, cost, and the environment. The objective is to provide 

turnout spacing to attain the appropriate traffic flow for the intended level of service.

b. In locating turnouts, consider the types of road users, safety, and cost. Desirable 

physical locations of turnouts are: 

1. On the outside of cuts, such as the outside of a curve around a point of a ridge;

2. On the low side of fills, such as the upper side of curves across ravines; 

3. At the runout point between through cuts and fills; and 

4. On roads intended for logging traffic, so that empty haul vehicles or other traffic 

can use the turnouts while a loaded vehicle maintains speed and course. 

c. Turnouts should not be included in designs solely to provide for waste or storage of 

excavated material. Turnouts may be used to provide openings for scenic vistas.

d. Standards for minimum turnout widths and lengths are shown in exhibit 05 of this 

section. When turnouts are located on curves, provide curve widening in addition to the 

turnout width (42.45).

e. When the design alignment and grade require numerous turnouts, consider 

constructing a two-lane road, if practicable based on cost and safety considerations.
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CHAPTER 40 – DESIGN

42.4 – Exhibit 05

Turnout Widths and Lengths

Level of 

Service Turnout Width Turnout and Transition Lengths

G 10 feet Design vehicle length or at least 75

feet, whichever is greater; at least 50-

foot transitions at each end.

H 10 feet Design vehicle length; at least 50-foot 

transitions at each end.

I Minimum total width of the 

traveled way and turnout equals 

the width of two design vehicles 

plus 4 feet.

Design vehicle length; at least 30-foot 

transitions at each end.

J Minimum total width of the 

traveled way and turnout equals 

the width of two design vehicles 

plus 4 feet.

Empty truck length; at least 30-foot 

transitions at each end.

For tangent and curved section turnouts, measurement of the length and transition starts

at the centerline of the road. For additional information on these turnouts, see 

AASHTO’s “A Policy on Geometric Design of Highways and Streets,” chapter 5, 

“Special Purpose Roads,” especially exhibits 5-17 and 5-18.
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CHAPTER 40 – DESIGN

42.44 - Turnarounds

Turnaround designs should consider both critical and design vehicles. Turnarounds should be 

provided at or near the end of roads, at points where the level of service or road standards 

change, and at closure points such as gates and barricades. Normally, when the need for 

turnarounds has been considered in project planning, meeting that need will not require 

additional extensive construction. A terminal facility can often be used as a turnaround with 

minor alterations.

Intermediate turnarounds are usually not necessary on most roads and should be provided only 

when existing facilities, such as turnouts, do not provide adequate room for users to turn around.

Consider placing signs if intermediate turnarounds create a hazard to other users. Design 

turnarounds to allow design vehicles to turn reasonably safely. To the extent practicable, design 

turnarounds to allow design vehicles to reverse direction in one backing and turning maneuver.

RMOs may require additional turnarounds for fire staging, maintenance, or other access needs. 

Additional turnarounds for construction purposes may be cost-effective.

42.45 - Curve Widening

Widening of the traveled way is required on some curves to provide for off-tracking of tractor

trailers and for some light vehicle-trailer combinations. Curve widening to accommodate the 

design vehicle is considered a part of the traveled way.

In most cases, the design should consider several types of vehicles, most commonly:

a. Tractor trailer combinations where the fifth wheel is located directly over the drive 

wheels, such as a lowboy or a gravel truck.

b. Tractor trailer combinations with the towing pivot point offset to the rear of the drive 

wheels, such as logging trucks with a stinger to facilitate making short radius turns.

c. Tractor trailer combinations that have two fifth wheels and accessory axles.

d. Special configurations for other commercial vehicles such as tractors with double or 

triple trailers.

e. Large recreational vehicles such as motor homes, tour buses, and pick-up and trailer 

combinations.

f. Log yarding and construction equipment vehicles arranged in operational mode or 

travel configuration.
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CHAPTER 40 – DESIGN

Curve widening is affected by the type of vehicle, radius of curvature, and the central or delta 

angle.

Generally, the need for curve widening increases as the radius decreases. Shorter curves require 

less curve widening than longer curves.

The following discussion outlines the relationship of curve widening to the level of service 

(sec. 41, ex. 01).

42.4 – Exhibit 06

Relationship to Level of Service

Level of 

Service

Curve Widening

G Provided to accommodate the design vehicle at the design speed for each curve. 

Curve widening for design vehicles should be provided in each lane of doublelane roads. Curve widening for highway-legal critical vehicles should be 

provided by the use of other design elements, if planned, such as turnouts and 

shoulders. Curve widening for non-highway legal critical vehicles should be 

provided only to the extent necessary for them to traverse the road safely with 

operational controls, such as permit-required pilot cars, in effect. 

H Same as for level of service G.

I Same as for level of service G, except curve widening for highway legal critical 

vehicles may be reduced to only what is necessary to traverse the road safely 

with operational controls.

J Generally provided only for the design vehicle. Loads carried by critical 

vehicles may be off-loaded and walked to the project or transferred to vehicles 

capable of traversing the road. Temporary widening to permit the passage of 

critical vehicles, such as temporary filling of the ditch at narrow sections, may 

also be used. 

On the level of service I and J roads, it may be desirable to design turnouts where 

significant curve widening would otherwise be required, such as on short-radius curves. 

In these situations, the minimum lane width should be the greater of the:

1. Single-lane width, plus design vehicle curve widening, plus turnout width.

2. Single-lane width plus critical vehicle curve widening.
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CHAPTER 40 – DESIGN

The turnout should extend from the point of curvature (PC) to the point of tangency (PT),

with tapering provided as appropriate for the design vehicle.

Use section 42.4, exhibits 06, 07 and 08 and the following equations (sec 42.4, ex. 09), as 

appropriate, to determine curve widening. The curves in the graphs were developed from 

the central and delta angle variations of vehicles using off-tracking equations. Exhibit 07

applies to lowboy configurations, and exhibit 08 applies to logging truck configurations. 

For other configurations, see AASHTO’s “A Policy on Geometric Design of Highways 

and Streets.”

42.4 – Exhibit 07

Lowboy Configurations

It is not necessary to widen all curves. If the minimum width as determined by the

equation below is equal to or less than the traveled way width, no curve widening is 

required.
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CHAPTER 40 – DESIGN

42.4 – Exhibit 08

Logging Truck Configuration 

*Add curve widening (CW) to lane width.

The following field-checked equation, which is reasonably accurate for a radius of 

50 feet or more, yields the minimum lane width (MLW), which includes 2 feet for offtracking corrections.

MLW = 14 feet + curve widening (CW), where CW is determined using the equation 

below.

MLW may be reduced to 14 feet plus curve widening for level of service I roads open to 

commercial hauling or commercial passenger vehicles (sec. 42.4, ex. 02) and to 12 feet 

plus CW on other level of service I roads where analysis indicates that adequate user 

safety and utility can be maintained at the reduced width.
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CHAPTER 40 – DESIGN

42.4 – Exhibit 09

Equation to Determine Curve Widening

 

































   















 

0.216 0.015

2 2

1

L

R

CW R R L e

,

where:

R = centerline radius in feet,

e = base for natural logarithms,

∆ = central angle in degrees, and

L = the value computed below.

For a lowboy or standard tractor trailer,

2

3

2

2

2

L  L1  L  L

,

where:

L1 = the wheel base of the tractor in feet,

L2 = the distance from the fifth wheel to the middle of the rear duals for the

 first trailer in feet, and

L3 = the distance from the fifth wheel to the middle of the rear duals for the

 second trailer in feet.

For a logging truck with a stinger, 

2

3

2

2

2

L  L1  L  L

,

where:

L1 = the wheel base of the tractor in feet,

L2 = the length of the stinger measured from the middle of the tractor rear,

 duals to the end of the stinger in feet, and

L3 = the bunk-to-bunk distance, minus the length of the stinger in feet.
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CHAPTER 40 – DESIGN

Curve widening should be uniformly tapered before the PC and after the PT for the 

following lengths:

Curve Widening Taper Lengths

Radius (in feet) Taper Length (in feet)

<70 60

70-85 50

86-100 40

>100 30

Carefully consider the adequacy of curve widening when designing roads with reverse or 

compound curves or curves separated by tangents shorter than the curve widening 

transition length.

Limited initial studies on reverse curves indicate that minimum lane width at the point of 

reverse curvature should be:

1. Fourteen feet for level of service I roads open to commercial hauling or commercial 

passenger vehicles.

2. Twelve feet for other level of service I roads + 2/3 (curve widening for the first curve) 

+ 2/3 (curve widening for the second curve).

Curve widening for compound curves should be calculated using the total of the 

deflection angles of both curves. Use the greater of the individually calculated curve 

widening for adjacent curves, as the curve widening at the point of compound curvature 

(PCC).

For more information on vehicle tracking, refer to AASHTO’s “A Policy on Geometric 

Design of Highways and Streets,” chapter 2.
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CHAPTER 40 – DESIGN

42.46 - Clearance

The desirable minimum horizontal clearance is 4 feet for level of service G and H roads. The 

desirable minimum horizontal clearance varies from 0 to 4 feet on level of service I and J roads, 

based on local factors determined during road planning and design. The minimum vertical 

clearance on level of service G, H, and J roads is 14 feet. At higher speeds, consider increasing

horizontal and vertical clearances. 

Critical vehicles, with unique features, such as a spar on a log yarder or construction equipment 

vehicle usually require special clearances. The completed design should accommodate critical 

and design vehicles or include provisions for operational controls to accommodate them.

42.47 - Fill Widening

Depending on the construction technique specified, some shoulder material settling can be 

expected. Fill widening may be used to provide additional width during construction so that the 

intended traveled way width remains stable. 

Engineering judgment should be employed to determine whether to use fill widening. Consider 

the requirements of BMPs for compliance with non-point source pollution potential in such 

judgments. The engineer should also consider other measures besides fill widening to achieve

stable roadways, such as flattening constructed slopes, compacting soils, using “select” borrow 

instead of “common” material; contractual requirements to revegetate slopes should be kept 

current as excavation progresses. Fill widening over 2 feet should not be used. 

Do not use fill widening in place of widening the shoulders, the traveled way, curves, or other 

parts of the road structure that require stability.

42.48 - Clearing Widths

Provide the minimum clearing widths consistent with RMOs. To minimize costs, especially on 

short-term level of service J roads, consider the reduction of clearing widths through the:

1. Design of fills to encroach on live trees to an acceptable height depending on species 

and size.

2. Reduction or elimination of clearings above the cut slope, where cut slope stability 

and road operation and maintenance allow.

42.49 - Daylighting

When the road design provides for a through cut with a low cut bank on the downhill side, it is 

often desirable to "daylight" the downhill side of the cut. (for example, extend the roadbed width 

at finished grade slope and elevation to the downhill side to where it “daylights” with the ground 

surface beyond the proposed cut.) Daylighting can increase construction efficiency and reduce 
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CHAPTER 40 – DESIGN

maintenance costs. Daylighting should not be indiscriminately specified, since excessive use of 

this practice can increase construction costs. Daylighting criteria should be determined and 

documented when other design standards are established for the road.

42.5 - Speed and Sight Distance

1. Design Speed. The design of road speed is an iterative process between RMOs and 

road design elements and has a direct impact on cost, safety, driver comfort, and 

convenience. In new construction design, layout the horizontal and vertical road 

alignment, and check it against design criteria such as level of service, maximum grades, 

and curve requirements. Determine what speed the geometry allows. Compare this with 

the speed to accommodate RMOs. Adjust the design elements as necessary, considering 

roadside design features such as recovery and clear zones. Consider that consistent road 

designs reduce unexpected conditions and the need of warning devices. 

For reconstruction projects, field-verify the design speed for the existing road segments, 

and research crash data to identify the need for any adjustments in the design speed. 

Perform a site visit to determine if any visual clues are available, such as skid marks, 

damaged guardrail, or damaged trees.

Terrain and other physical constraints may dictate a change in the design speed for 

certain road segments. For example, a change in the design speed may be warranted for 

road segments traversing major changes in topography or at intersecting roads. Consider 

posting signs on level of service G and H roads to warn users of short segments with a 

different design speed.

Avoid road designs on level of service G and H roads, that allow most segments to be 

driven at more than the design speed, to the extent practicable. If these designs are used, 

analyze the segments that can be driven safely only at the design speed to determine 

whether redesign could economically eliminate the hazard by providing for safely 

negotiating them at higher speeds. If not, use traffic control devices or other means to 

address the potential hazard to the extent practicable. 

Other factors besides the design speed, such as weather, time of day, driver proficiency, 

level of maintenance, and change in design elements, also affect the safe speed along a 

road or road segment. 

a. Level of Service G and H Roads. Speed is an important consideration in 

evaluating the convenience and economics of level of service G and H roads.

When determining the design speed, evaluate user costs, which usually increase as the 

design speed decreases. The design speed establishes the minimum sight distance for 
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CHAPTER 40 – DESIGN

stopping and passing and the minimum radius of curvature, may establish the 

gradient, and may establish the type of running surface. Evaluate alternative 

combinations of horizontal and vertical alignment to obtain the greatest sight distance 

within economic and environmental constraints (sec. 43).

b. Level of Service I and J Roads. The design speed for level of service I and J roads 

are not as significant as for level of service G and H roads. For level of service I and 

J roads with a design speed of 15 miles per hour or less where cost is a minor factor, 

shift the emphasis in design from speed to vehicle operational controls. In 

determining the design speed for level of service I and J roads:

(1) Fit the vertical and horizontal alignments to the natural terrain as closely as 

possible without considering the design speed.

(2) Consider soil types and environmental constraints as well as the standard design 

criteria.

(3) Analyze the vertical and horizontal alignments to determine the design speed.

(4) Apply additional design standards as appropriate to accommodate the design 

vehicle upon the determination of the design speed. For example, if the desired 

design speed has been identified in the planning process, compare it to the design 

speed determined in paragraph 1b(3). If improvements to the design are necessary to 

accommodate the desired design speed, weigh the cost of these improvements against 

the benefits of the desired design speed; this should be based on construction, 

operating cost, and maintenance. Modify the design or employ traffic control devices 

or other measures, to the extent practicable and as appropriate, at those locations if 

the speed is likely to create safety problems.

2. Sight Distance. Evaluate sight distance when designing crest and sag vertical curves, 

horizontal curves, intersections, and passing areas (secs. 43.1 and 43.2).

Sight distance is an important factor to consider in road design because it cannot easily be 

judged by drivers. Inadequate sight distance does not readily cause the driver to slow to a 

safe driving speed. Reduce the design speed or include traffic controls, to the extent 

practicable if the sight distances appropriate for the design speed cannot be provided.

On two-way single-lane roads, provide minimum sight distance equal to twice the 

stopping sight distance for the design speed. Provide sight distance on one-way singlelane roads equal to or greater than the stopping sight distance for the design speed.
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3. Stopping Sight Distance. The stopping sight distance varies with the type of vehicle, 

skill of the driver, condition of the road surface, geometry of the road at a given point 

(primarily gradient), and number of lanes (single or double). For example, the stopping 

sight distance needed for a two-way single-lane road, where opposing traffic is confined 

to one lane, is commonly twice as much as that required for a double-lane road.

a. Use the following equation to compute stopping sight distance for double-lane 

roads. This equation takes into account varying road surface conditions, for example, 

loose gravel, wet earth, wet asphalt, and wet-packed gravel.

Stopping Sight Distance Equation

 SSD = stopping sight distance in feet;

V = approach or initial speed in miles per hour;

T = perception reaction times:

• 2.5 seconds for level of service G and H roads,

• 2.0 seconds for level of service I and J roads; and

a = driver deceleration (ft/sec2)

 (14 ft/sec2for trucks; 21 ft/sec2for cars).

Use the following table for stopping sight distances, calculated using the equation in 

paragraph 3a, for trucks with a deceleration rate of 14 ft./sec.2.

Truck Stopping Sight Distance (in feet)

Double-Lane Single-Lane

Speed

(MPH)

T=2.0

(sec.)

T=2.5

(sec.)

T=2.0

(sec.)

T=2.5

(sec.)

10 37 44.5 74 89

15 61.5 72.5 123 145

20 89.5 104 179 208

25 121.5 139.5 243 279

30 157 179 314 358

35 197 222.5 394 445

40 240 269.5 480 539
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CHAPTER 40 – DESIGN

Perception-reaction time (T) can vary considerably. Two T values are used in this 

chapter. A T of 2.5 seconds is used for designing level of service G and H roads. 

Roads at these two levels of service involve mixed traffic, which can result in 

relatively complex driving situations. A T of 2.0 seconds is used for level of service I 

and J roads, where the driving situation is less complex.

The following equation takes into account the effects of road surface condition and 

gradient:

Stopping Site Distance with Grade and Surface Consideration

 SSD = stopping sight distance in feet;

V1 = approach or initial speed in miles per hour;

T = preception/reaction time:

 2.5 seconds for level of service G and H roads,

 2.0 seconds for level of service I and J roads;

ff = coefficient of friction between tires and roadway (expressed as a decimal); and

G = gradient (expressed as a decimal).

The following table displays friction and traction coefficients. The friction 

coefficients are based on the traction coefficients, which are reduced by 0.2 as a 

safety factor (ff = Tf - 0.2). Use these factors when calculating stopping sight 

distance using the equation above. 
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CHAPTER 40 – DESIGN

42.5 – Exhibit 01

Friction Coefficients, Ff, for Traction Coefficients, Tf

Material Dry Surface Wet Surface

ff = Tf – 0.2 Tf ff = Tf – 0.2

Concrete 0.75-0.90 0.55-0.70 0.55-0.70 0.35-0.50

Asphalt 0.55-0.70 0.35-0.50 0.40-0.70 0.20-0.50

Stabilized gravel 0.50-0.85 0.30-0.65 0.40-0.80 0.20-0.60

Loose gravel 0.40-0.70 0.20-0.50 0.36-0.75 0.16-0.55

Crushed rock 0.55-0.75 0.35-0.55 0.55-0.75 0.35-0.55

Wet earth 0.55-0.65 0.35-0.36 0.40-0.50 0.20-0.35

Dry-packed snow 0.20-0.55 0.00-0.35 -

Loosely packed

snow

0.10-0.60 0.00-0.40 -

Lightly sanded 

snow

0.29-0.31 0.09-0.11 -

Lightly sanded

snow, with chains

in use

0.34 0.14 -

Reference: Stryker, E. R. “Gradeability of Log Trucks.” Oregon State University, Table 3,

Appendix II, from Taborek (1957) and Western Highway Institute (1976).

On single-lane level of service G and H roads, where the sight distance is less than 

the stopping sight distance, the road should either have a double lane segment or the 

necessary sight distance should be provided by other means, such as by adjusting the 

cut slopes or selective clearing. (Only use selective clearing if it can be maintained 

on this and similar roads within the limits of expected levels of annual road 

maintenance appropriations). On level of service G and H roads, constructing 

turnouts to provide sight distance is not desirable. Level of service G and H roads are

intended for all users, including drivers who may be unfamiliar with driving NFS

roads. These users may not understand the need to drive into turnouts, when sight 

distance is restricted, to avoid potential head-on collisions with oncoming traffic.

4. Stopping Sight Distance on Horizontal Curves. The lateral clearance for stopping 

sight distance on horizontal curves is the offset distance from the centerline of the inside 

lane of a double-lane road or from the centerline of a single-lane road to a visual 

obstruction along the side of the road. Provide lateral clearance to allow drivers to see

obstructions on the road or approaching traffic when traveling around a horizontal curve.
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Stopping sight distance on horizontal curves is an important consideration on both singlelane and double-lane roads. Minimum lateral clearance is a function of design speed and 

radius of curvature. If appropriate lateral clearance can be provided by the traveled way, 

shoulders, and ditch, no further adjustments are needed. If additional clearance is 

needed, slopes, ditch widths, or clearing may be adjusted to provide the desired sight 

distance. Post signs to reduce speed or redesign the road when stopping distance is 

inadequate for the design speed of the road, to the extent practicable.

Stopping sight distance on horizontal curves is covered extensively in AASHTO’s “A 

Policy on Geometric Design of Highways and Streets,” which is not duplicated in this 

chapter. Due to the number of variables, it is not practical to include illustrations to 

depict all conditions. Sight distance is not limited to the distance between the PC and PT 

of a curve, but can also include part of the adjacent tangents. Steep grades and other 

variables, such as reaction time, road conditions, speed, central angle, and type of 

vehicle, may increase the stopping sight distance. AASHTO recommends a case-by-case 

analysis of the stopping sight distance using graphs.

When considering stopping sight distance on horizontal curves, also consider the 

stopping distance required due to the side friction factor. See section 43.1 for additional 

information on stopping sight distance on horizontal curves. For horizontal curves 

without superelevation, the side slipping factors may be more restrictive than the factors 

used for forward motion.

5. Passing Sight Distance. The passing sight distance allows a driver, when passing, not 

to interfere with the speed of an oncoming vehicle traveling at the design speed that

comes into view after the driver has begun passing. Passing sight distance is not a factor 

on single-lane roads because of the assumption that slower vehicles pull into turnouts to 

allow other vehicles to pass when necessary. For information on passing sight distance 

on double-lane roads, see AASHTO’s “A Policy on Geometric Design of Highways and 

Streets.”

43 - ALIGNMENT

43.1 - Horizontal Alignment

1. Curvature. Roads should be designed so that they are composed of sections of circular 

curves and tangents. For roads with a design speed of 15 miles per hour or less, it is 

acceptable to use a field-designed horizontal alignment that approximates the geometric 

requirements of circular curves and tangents. Check the horizontal and vertical 

alignment (sec. 43.2) to ensure that they allow other design elements, such as curve 

widening and stopping sight distance, and geometric design standards, such as ensuring a 

minimum 50-foot curve radius, to be addressed appropriately.
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The relationships of vehicle speed, radius of curvature, superelevation rate, and side 

friction factor are expressed in the following equation: 

Calculating Horizontal Curve Radius

R = curve radius in feet,

 V = vehicle speed in miles per hour,

 e = rate of superelevation, and

f = side friction factor.

Factors to consider in determining the minimum radius include:

a. Grade.

b. Vehicle speed.

c. Vehicle’s center of gravity.

d. Tire condition.

e. Driver comfort and proficiency.

f. Wet or dry conditions.

g. Variability of crushed rock and other road surfacing.

h. Considerations related to any intended use during wintry conditions, including 

characteristics of wintry precipitation; intended winter maintenance operations 

(plowing, abrasives, and so forth); and restrictions on users, including timing of use, 

allowable equipment, and use of traction devices.

i. Frequency of use.

Traction on dry pavement is lost at a side friction factor of approximately 0.4 or 0.5. 

National Cooperative Highway Research Program (NCHRP) Report 37 indicates that the 

traction coefficients for the side friction factor are approximately half of the traction 

coefficients for the forward motion. Another factor to consider is the driver’s comfort 

level. AASHTO recommendations for side friction factors vary with speed and pavement 
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condition. For more information on the side friction factor, see Clarkson H. Oglesby, 

Highway Engineering, 3d ed., AASHTO’s “A Policy on Geometric design of Streets and 

Highways,” and NCHRP Report 37.

Designers may approximate the relationship between Tf and fs as shown in the equation 

and table below. 

43.1 – Exhibit 01

Side Friction Factors, fs, for Traction Coefficients, Tf

fs = Tf *0.5, where Tf = the traction coefficient of the traveled way

for various road surfaces in a forward direction.

M aterial Dry Surface Wet Surface

Tf f5 = Tf *0.5 Tf Fs = Tf *0.5

Concrete 0.75-0.90 0.35-0.45 0.55-0.70 0.27-0.35

Asphalt 0.55-0.70 0.27-0.35 0.40-0.70 0.20-0.35

Packed and oiled

gravel

0.50-0.85 0.25-0.42 0.40-0.80 0.20-0.40

Loose gravel 0.40-0.70 0.20-0.35 0.36-0.75 0.18-0.37

Crushed rock 0.55-0.75 0.27-0.37 0.55-0.75 0.27-0.37

Wet earth 0.55-0.65 0.27-0.32 0.40-0.50 0.20-0.25

Dry-packed snow 0.20-0.55 0.10-0.27

Loosely packed snow 0.10-0.60 0.05-0.30

Lightly sanded snow 0.29-0.31 0.14-0.15

Lightly sanded snow, 

with chains in use

0.34 0.17

Reference: Stryker, E. R. “Gradeability of Log Trucks.” Oregon State University, Table 

3, Appendix II, from Taborek (1957) and Western Highway Institute (1976).

2. Radius of Curvature and Speeds. The design speed for each curve is affected by the 

radius of the curve, superelevation rate, and the traction coefficient of the surface 

material. Use the values of Tfto determine the side friction factor (fs) in the equation 

above. The reduced value of fs provides a reasonable margin of safety for the average 

driver. It is not always necessary to select a traction coefficient value for inclement 
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weather conditions because the average driver reduces speed under these conditions. 

Inclement weather coefficients should be selected when the road is specifically intended 

for use during inclement weather conditions or when there is a significant volume of 

traffic during those conditions.

Exhibit 02 shows the relationship of design speed to radius of curvature for various 

traction coefficients where there is no superelevation. Exhibit 02 is based on rider 

comfort and safety and the traction coefficients in exhibit 01 of this section. Exhibit 02

shows that speed can be increased by improving the quality of the road surfacing. 

Surface improvements for increased speed is normally justifiable only for flatter grades

where the gradient controls the speed.

43.1 – Exhibit 02

Relationship of Design Speed to Radius of Curvature
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3. Superelevation. Superelevation is an important consideration on level of service G 

and H roads. Superelevation is usually not important on level of service I and J roads, 

where user comfort, convenience, and speed of travel are generally not management 

considerations and road use can be restricted.

To the extent practicable, avoid using superelevation for design speeds of less than 20 

miles per hour. Where there is a possibility of encountering snow and ice, the 

superelevation rate should not exceed 6 percent and should be reduced further on grades 

to accommodate slow truck traffic. In warmer climates where snow and ice are not 

factors, the superelevation rate may be increased to 12 percent. For more information on 

superelevation, refer to AASHTO’s “A Policy on Geometric Design of Highways and 

Streets.”

Achieve superelevation through the rotation around the elevation of the center or a side of 

the traveled way. Design the rotation point and the superelevated segment to protect 

environmentally sensitive areas adjacent to the segment. This includes the:

a. Prevention of ponding near the lower edge of the road, on flatter slopes;

b. Provision for surface drainage prior to the superelevated curve;

c. Provision for ditches through the superelevated segment to carry surface drainage 

to an appropriate termination; and

d. Placement of a designed surfacing course on the superelevated curve. 

4. Superelevation Transitions. Provide transitions into and out of superelevated sections 

to avoid abrupt changes in the roadway template. Transitions from tangents to

superelevated curve sections are covered in detail in AASHTO’s “A Policy on Geometric 

Design of Highways and Streets.”

5. Minimum Radius of Curvature. National Forest System roads should have a

minimum centerline radius of curvature of 50 feet, except for some recreation and 

administrative National Forest System roads where the design and critical vehicles do not 

include heavy commercial trucks. Check the configuration of the design vehicle and the 

critical vehicle to validate that they can negotiate the road when designing roads with 

minimum radius curves. See AASHTO’s “A Policy on Geometric Design of Highways 

and Streets,” chapter 2, for design vehicle dimensions. Vehicle overhang and swing may 

affect cut bank slopes as well as lane width. The effects of sharp curves and vehicle 

configuration in critical locations can be determined by the use of simulators or computer 

applications. See AASHTO’s “A Policy on Geometric Design of Highways and Streets,” 

chapter 2, for the minimum turning path for most vehicles, including cars, buses, large 

recreational vehicles, and trucks.
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43.2 - Vertical Alignment

1. Grade. Vertical alignment is a very important design element. Road gradient has a 

major effect on the environmental impact of a road, particularly in terms of erosion. 

Gradient is the predominant factor affecting speed in many cases, and grade also has a 

significant effect on maintenance and user costs.

It is advisable to establish a minimum grade of 2 percent on native soil or on aggregatesurfaced roads to facilitate surface drainage where possible. Flat grades do not shed 

water sufficiently and tend to develop potholes. Where flat grades are used on hillsides, 

outsloping the road surface can be an effective way to provide surface drainage. Where 

flat grades occur on level ground, crowning the road surface is recommended (sec. 44.1).

The adverse environmental effects of road gradient become increasingly apparent as the 

grade increases. For example, erosion potential increases as a function of the square of 

the slope and of the third power of the water velocity. The intensity of erosion changes 

with the type of soil and/or aggregate of the roadbed, ditch, cut bank, and fill slope

Safety, State laws, and economic and environmental constraints govern selection of the 

maximum grade or at least require mitigating measures to lessen the impacts of steep 

grades. The maximum grade varies with the ability to resist erosion of each type of road 

surfacing material. Steeper grades normally require additional costs for drainage, surface 

stabilization, maintenance, and use. Use caution when considering gradients steeper than 

8 percent.

Strive to fit the best combination of design elements to the range of grades available for 

the road. Determine the most desirable combination of grades and other design elements 

before reaching the design phase. By the time the project progresses to the design phase, 

little opportunity exists to make significant grade changes within the reconnaissance and 

survey limits. 

Vertical alignment normally governs the speed of empty trucks and light vehicles for 

grades of 15 percent in the favorable (downhill) direction and 11 percent in the adverse 

(uphill) direction. The vertical alignment normally governs the speed of loaded trucks for 

grades exceeding 8 percent in the downhill direction and 3 percent in the uphill direction. 

The ability of a vehicle to traverse a particular grade depends on vehicle weight,

horsepower, and the traction coefficient of the driving surface. If steep grades are 

necessary, a higher-quality surfacing may be needed to provide the necessary traction and 

to resist erosion (sec. 44).

Maximum grades should be as follows:

a. Passenger car traffic: 12 percent.
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b. Motor homes or vehicles pulling trailers: 12 percent for pitches up to 300 feet.

c. High clearance and four-wheel drive vehicles: 18 percent or as required by State 

safety regulations.

2. Grade Reduction for Horizontal Curves. It is often advisable to reduce the gradient 

through a horizontal curve, particularly on steeper grades at shorter radius curves. These

curves result in a steeper grade because of the shorter distance traveled along the inside 

track of the curve. For example, if the centerline grade is 10 percent on a 75-foot radius 

curve with a centerline length of 100 feet, the grade along the inside track is 

approximately 12 percent.

Grade reduction is important for switchbacks. When determining road gradient, it is 

important to consider loaded trucks traveling in both directions. Use the following 

guidelines for maximum gradients and gradient reductions.

43.2 – Exhibit 01

Grades on Switchbacks

Hauling

Direction

Above the

Switchback Curve

At the Switchback

Curve

Below the

Switchback Curve

Favorable 5 percent or less for 150 

to 200 feet.

6 percent preferred; 8 

percent maximum.

See section 43.2, 

paragraph 2a.

Adverse See section 43.2, 

paragraph 2a.

6 percent maximum. 1 to 2 percent less than 

the switchback curve 

grade for 100 feet.

a. Strive to maintain a constant grade through the switchback. To the extent 

practicable, avoid locating vertical points of curvature within a switchback. 

b. To minimize gear shifting and to equalize curve resistance to the vehicle, reduce 

the switchback curve grade by 0.04 percent per degree of curvature in the 

approaching grade. For example:

Degree of Curve Radius of Curve (in feet) Percentage Reduction

50 115 2

75 76 3

115 50 4.5

Trucks may have difficulty hauling in the adverse direction when they travel on level of 

service J roads that have a sharp horizontal curve in combination with a steep grade. The 

position of the trailer in relation to the tractor may cause the outside drivers on the truck 
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to lift, resulting in a loss of traction on the curve. A flat grade or reverse superelevation 

will aid in providing traction, but should be used only in special situations when advised 

by engineering judgment. Reverse superelevation should not be used where speed is a 

factor or where downhill haul will occur.

3. Vertical Curves. Vertical curves are the transition zones between different gradients. 

Design vertical curves with consideration for environmental effects. Do not locate the 

low point of a sag vertical curve directly over a culvert or fill where fill slope erosion 

may be a concern. Instead, locate the low point of a sag vertical curve at a transition 

point between cut and fill.

Sight distance is the primary consideration in designing vertical curves because changes 

in grade result in sight restrictions. Attain the desired sight distance by making 

adjustments in the length of vertical curves. Crest vertical curves present sight distance 

problems under all conditions. Sag vertical curves normally present a sight distance

concern only in terms of providing headlight sight distance during periods of darkness or 

reduced visibility. 

Avoid placing a sharp crest vertical curve at the same location as a horizontal curve to the 

extent practicable. Avoid minimum length vertical curves to the extent practicable. The 

following chart shows the guidelines for the minimum length of vertical curves:

Minimum Lengths for Vertical Curves

For crest vertical curves, or 50 feet, whichever is greater,

For sag vertical curves, L = AV2 or 50 feet, whichever is greater, where:

 46.5

L = length of curve in feet,

V = design speed in miles per hour, and

A = algebraic difference in grades as a percentage.

Check crest vertical curves against the ground clearance of the design vehicle. To the 

extent practicable, avoid reducing the ground clearance of the design vehicle by more 

than 50 percent.

For roads with a design speed of 15 miles per hour or less, it is acceptable to use fielddesigned vertical alignment that approximates the geometric requirements of vertical 

curves. Check the vertical alignment to ensure that it allows other design elements, such 

as stopping sight distance, to be addressed appropriately.
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4. Stopping Sight Distance. Stopping site distance is covered in detail in the AASHTO 

Guidelines. The stopping sight distance for single-lane roads is twice that required for

double-lane roads. If adequate stopping sight distance cannot be achieved, construct 

turnouts or use appropriate signing, to the extent practicable.

43.3 - Intersections

A substantial number of crashes on all roads under all jurisdictions nation-wide occur at 

intersections. The preponderance of these crashes are the result of driver errors. One type of 

error is failure to prudently accomplish the task of making decisions about route following. That

can distract from the basic driver responsibility of recognizing hazards and taking appropriate 

action to avoid them. Drivers make navigational mistakes; some drivers make mistakes stop in 

the roadway; sometimes backing up, instead of proceeding to a location where it is safe to pull 

off the road and turn around. Another cause of intersection crashes is lack of attention to 

driving. When drivers are not expecting intersecting traffic; they are unprepared to take 

appropriate action when surprised by a conflict such as congestion. 

While responsibility for driving safely and prudently for conditions is always that of the driver,

the design of alignment, grade, cross section, signs, and traffic control devices at or near

intersections is important. Doing it well can minimize opportunities for driver error. The 

following guidelines apply to the design of intersections on National Forest System roads:

a. The sight distance along the main road, as viewed from the main road or from the 

intersecting road, should be at least equal to the stopping sight distance for the design 

speed of the main road.

b. Roads intersecting at acute angles tend to limit visibility and restrict the flow of 

entering traffic to one direction. Thus, the smallest angle formed by intersecting 

roads should not be less than 60 degrees. Right angle intersections are desirable.

c. To the extent practicable, avoid designing intersections on sharp curves or with 

grade combinations that make vehicle control difficult. 

d. To the extent practicable, avoid intersections that are slightly offset on opposite 

sides of the main road. More than two roads intersecting at one location may cause 

traffic management problems.

e. Carry the grade of the main road through the intersection, and adjust the grade of 

the intersecting road to the grade of the main road. The grade of the intersecting road,

as it approaches the main road, should be 6 percent or less. On the intersecting road, 

at the main road, it is advisable to provide sufficient width for another vehicle to pass 

a stopped design vehicle and to provide one design vehicle length at a grade of 

approximately 1 percent for stopping. 
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f. The width of the main road combined with the radius of the taper of the 

intersecting road must be wide enough for off-tracking of vehicles entering or leaving 

the main road.

g. On level of service G and H roads, provide warning, regulatory, and guidance 

signs and traffic control devices at intersections, as advised by the Manual on 

Uniform Traffic Control Devices (MUTCD) and engineering judgment.

h. When use of an intersecting road is restricted by a gate, provide for the design 

vehicle to turn around safely when the gate is closed. In addition, check for 

consequences for critical vehicles, particularly when critical vehicles may be driven 

by recreation visitors.

43.4 - Railroad Grade Crossings

Design railroad grade crossings in accordance with applicable laws and regulations. Obtain 

consultation and review by railroad officials, and obtain their approval when necessary.

Legal and real property rights can differ greatly from crossing to crossing. The situation where 

the railroad existed before reservation or acquisition of the underlying lands for the NFS is much 

different from the situation where the railroad was constructed under a special use permit after 

reservation or acquisition of the underlying lands. Consult with the local Lands staff and the 

local Office of the General Counsel, as appropriate, regarding rights, permits, and ownership 

issues at railroad grade crossings. 

The geometric design of a road at a railroad grade crossing involves the elements of alignment, 

profile, sight distance, and cross section. Verify that the critical vehicle can negotiate the vertical 

curve at the crossing. The requirements may vary somewhat with the type of protective or signal 

devices used. The traveled way width should be the same on either side of railroad grade 

crossings and should have all-weather surfacing for a suitable length. It may be advisable to 

provide sufficient width for vehicles to pass on each side of a railroad grade crossing. The 

following provide additional guidance on railroad grade crossings:

1. FHWA’s The Railroad-Highway Grade Handbook, Publication TS-86-215. 

2. The MUTCD.

3. AASHTO’s “A Policy on Geometric Design of Highways and Streets.”

43.5 - Roadside Design

National Forest System (NFS) roads vary from high-standard, high-speed roads in level of 

service G to single-lane, native-surface roads built for a specific purpose in level of service J. 

Engineering judgment should be used to determine appropriate roadside design standards. For 
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roads with more than 400 ADT, use AASHTO’s “A Policy on Geometric Design of Highways 

and Streets” and AASHTO’s “Roadside Design Guide” for specific roadside design standards. 

For roads with less than 400 ADT, use the guidelines in chapter 4 of AASHTO’s “Guidelines for 

Geometric Design of Very Low-Volume Local Roads (ADT <400).” These guidelines state that 

roadside barriers and clear zones are not cost-effective and should be provided only case by case

where advised by engineering judgment. Document design decisions, and maintain the 

documentation in the project file.

Roadside design can affect safety on low-volume roads. Identify potential hazards based on 

local conditions, road geometry, and traffic characteristics. Mitigation of potential hazards 

should include consideration of probability and severity of accidents, environmental impacts, 

economic factors, and management objectives. 

In roadside design, consider the following factors when assessing the probability and severity of 

crashes, forecasting their effects, and recommending mitigating roadside design measures:

1. Roadside Design Factors:

a. Crash history (frequency and severity).

b. Traffic volume and type.

c. User speed.

d. Topography and road template.

e. Visibility.

f. Horizontal and vertical alignment.

g. Climatic and surface conditions.

h. Roadside conditions.

2. Factors Decreasing Crash Probability:

a. No known crashes or visible evidence of crashes.

b. ADT of 400 or less.

c. Drivers who are familiar with the road.

d. Low speeds on roads with low levels of service (G, H, and I).

e. Driver expectations of road conditions ahead are met.
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f. Consistent cross-section.

g. Good visibility.

h. Changes in alignment and grade do not require rapid deceleration.

i. Traveled way is clear of snow and ice.

j. Recoverable shoulders.

3. Factors Increasing Crash Probability:

a. History of crashes.

b. ADT exceeding 400.

c. Drivers who are unfamiliar with the road.

d. Speeds exceeding 40 miles per hour.

e. Driver expectations regarding the road ahead are not met.

f. Abrupt changes from the expected road width, shoulder, or surface type.

g. Abrupt changes in vertical or horizontal alignment.

h. Fog, snow, or ice is present during normal seasons of use.

4. Factors Decreasing Crash Severity:

a. Speeds of 15 miles per hour or less.

b. Adequate clearance from potential roadside hazards.

c. Flatter fill slopes.

d. Good alignment and visibility.

e. Segregated use (for example, commercial and noncommercial are not mixed).

5. Factors Increasing Crash Severity:

a. Speeds of 40 miles per hour or more.

b. Little clearance for roadside hazards or intrusions, or deep or fast water adjacent to

the traveled way.
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c. Steep grades.

d. Abrupt changes in grade or alignment.

e. Mixed use (for example, commercial and noncommercial).

If an engineering analysis identifies a roadside hazard that should be mitigated, assess the most 

cost-effective method of managing the accident risk. Balance reduction of risk against the 

investment required to reduce the risk, and use engineering judgment in the selection of 

mitigation measures. 

Mitigation measures may include clear zones, shielding, or protecting drivers from roadside 

obstructions with traffic barriers such as guardrails and jersey barriers. The use of guardrails to 

protect drivers from obstructions generally is not cost-effective for very low-volume roads. 

Other methods such as delineators, berms, insloping, or varying alignment may be an appropriate 

and less costly means of reducing hazards. Traffic barriers may be appropriate at sites on lowvolume roads where engineering judgment indicates there is a high probability of crashes and 

barriers are the only practical and feasible means of reducing crash severity. When a guardrail is 

determined to be appropriate, apply the most current Federal or State Department of 

Transportation standards for rails, posts, and terminal sections.

For more information on roadside design, see NCHRP Publication No. 214, "Design and Traffic 

Control for Low-Volume Roads," and AASHTO’s "Roadside Design Guide.”

44 - ROAD DRAINAGE

Drainage is one of the most important elements in road design. All drainage can be classified as 

surface or subsurface. The classification depends on whether the water is on or below the 

surface of the ground at the point where it is first intercepted.

1. Surface Drainage. Surface drainage provides for the interception, collection, and 

removal of water from the surface of roads and slopes. Surface drainage is important, 

because water on the surface may interfere with traffic or cause erosion and, if allowed to 

infiltrate, may cause damage to the subgrade.

There are four principal elements to consider when designing surface drainage systems: 

surface shapes, surface cross drains, ditches, and culverts. The design may need to allow 

for debris passage, mud flows, and water heavily laden with silt, sand, and gravel. For 

projects with small drainages or intermittent streams that might affect fish and other 

aquatic species migration, consult with appropriate resource specialists to ensure that the 

design conforms to resource management objectives for the site and applicable directives. 

FSM 7722 and FSH 7709.56b provide guidance regarding design of bridges and 

structures. 
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2. Subsurface Drainage. Road prisms may intercept subsurface water. Subsurface 

drainage structures collect and remove subsurface water that may weaken the subgrade or 

road embankment. Properly designed and maintained surface drainage systems may 

reduce the need for special subsurface drainage structures and may minimize effects on 

the natural hydrological system. Consider relocating roads whose detrimental effects on 

subsurface flows cause reduction in soil moisture or drainage of sensitive wetland habitat.

The natural equilibrium of slope hydrology is easily upset by the introduction of manmade

systems like roads. The results of poor or improper drainage design can be dramatic and 

destructive. While road drainage systems collect and efficiently move water away from roads, 

the results can have unexpected negative aspects. Examples include drainage of meadows and 

wet areas, reduction of soil moisture, conversion of vegetation, increased runoff rates from the 

interception of surface and subsurface flow, and increased channel scour and head-cutting from

concentration of water flow. Strive to design drainage systems that maximize water dispersion 

and that use natural sediment filters. To the extent practicable, road drainage should mimic 

natural drainage patterns and should not interrupt natural hydrological processes.

There are 3 elements critical to road drainage design: 

1) Disconnecting the road from the stream system. 

2) Preventing storm damage to the road, surrounding cut slopes, fill slopes and ditch lines. 

3) Implementing local best management practices in conjunction with National Core Best 

Management Practices (BMPs) (FS-990a, 2012). 

Consult with appropriate resource specialists such as hydrologists, soil scientists and biologists,

to ensure that all watershed protection measures are met. For specific drainage design guidance 

refer to “Water/Road Interaction Technology Series,” Publication 9777 1806-SDTDC, San 

Dimas Technology Development Center.

Only after watershed protection and storm-proofing requirements are met should minimum 

lifecycle costs be considered. Minimum lifecycle costs should include construction, 

maintenance and operating costs. Cost should also consider the physical attributes of the 

watershed and the risk presented by road generated mass failure. Use the most economical 

drainage measures available that meet RMOs and BMPs. Consult with resource specialists to 

assess the risk of mass failure, debris delivery and aquatic habitat concerns. 

Drainage design should include consideration of rainfall, surface runoff, subsurface flow 

interception, and stream crossings. Use the best combination of various design elements, such as 

surface shape, ditches, ditch relief culverts, surface cross drains, fords, subsurface drains, 

culverts, and bridges. Consult with appropriate resource specialists in ascertaining water and 

aquatic management objectives for the site.
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44.1 - Traveled Way Surface Shapes

Traveled way surfaces may be insloped (toward the cut slope), outsloped (toward the 

embankment), or crowned (highest point near or at the road center sloping to both sides). 

Crowned traveled way surfaces are typically used for level of service G and H roads, while 

insloped or outsloped traveled way surfaces are typically used for level of service I and J roads. 

The amount of crown or slope varies with the type of surface and is generally less for impervious 

surfaces, such as asphalt, and more for relatively pervious surfaces, such as gravel or native soils. 

If the cross slope is too flat, water remains on the traveled way surface for a longer period and 

may penetrate into the base course and subgrade. A large buildup of moisture below the surface 

may cause instability and severely reduce the road's load-carrying capabilities. Drainage designs 

that result in water concentrations usually require more maintenance and present more risk to 

watersheds if maintenance is deferred than drainage designs that disperse water.

Decisions on traveled way surface shape should consider the resistance of the soil to erosion and 

the benefits of dispersing water gradually (outslope) or concentrating it at a specific location 

(inslope). If the road surface is unstable soil and subject to major erosion, then design 

considerations should include removal, surfacing, hardening, restricted use, or relocation. It may 

be necessary to stabilize ditches or the toe of the cut slope on insloped or crowned roads to 

reduce erosion. Removal of water from the traveled way surface becomes more difficult as 

grade increases.

The decision to inslope or outslope also depends, in part, on the natural slope hydrology; that is, 

how the undisturbed slope handles water. Convex topography tends to disperse water, and 

concave topography tends to concentrate water into defined drainages. Outsloping roads 

complement convex topography, while insloping roads with well-placed cross drainage tend to 

work best with concave topography.

1. Crown. Double-lane roads usually have a crown surface. Single-lane roads may have 

a crown surface. Crown surfaces are more expensive than other traveled way surface 

shapes to maintain on single-lane roads. A crown surface shares some of the 

characteristics of both an outsloped and insloped surface.

2. Outslope. Outsloping maximizes the dispersion of surface runoff and, when properly 

applied, minimizes hydrological impacts. Outsloped roads have the additional 

advantages of reducing road cut depth, reducing roadway width, and usually lowering

construction costs. A ditch is usually not necessary, but drainage concentration from 

upslope areas may be handled with short ditch sections, culverts, or surface cross drains. 

Make this determination based on the erosive characteristics of the soil, precipitation, 

runoff ratios, gradients, and the length of run before the water may be removed.

Outsloping is generally not appropriate on levels of service G and H roads intended for 

all weather use. Cross slopes generally between 2 and 4 percent are designed to disperse 

surface runoff but become slippery when wet or frozen. Consider vehicle stability on the 
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road surface when design elements of outsloping, steep down grades, and sharp outside 

curves are combined. Consideration to season of use is necessary in choosing the design 

template. 

3. Inslope. Where embankment soil is unstable or on steeper grades where outsloping is 

not recommended, consider applying an inslope road template. Depending on local 

conditions, a ditch may be required. Inslope sections collect water and will require cross 

drains.

For additional guidance on traveled way surface shapes, refer to “Traveled Way Surface Shape,”

Publication 9777 1808-SDTDC, San Dimas Technology Development Center.

44.2 - Surface Cross Drains

Surface cross drains intercept and remove surface water from the traveled way, wheel tracks, and 

shoulders before the combination of water volume and velocity begins to displace the surface 

materials. Surface cross drains also can serve as ditch relief structures in lieu of a culvert. 

Surface cross drains are particularly useful for low-volume, low-speed roads during periods of 

storage (maintenance level 1). When properly constructed, the drains can provide a relatively 

maintenance-free traveled way. They are beneficial in areas where culverts may plug and 

overtop resulting in diversion of water down the road surface. Surface cross drains should be 

designed to prevent erosion and maintain functionality in wet conditions.

There are a variety of surface cross drains, including broad-based (drivable) dips, drain dips and 

waterbars, open-top culverts, slotted metal pipe, and flexible rubber stripping. For design 

guidance on surface cross drains, refer to “Water/Road Interaction: Introduction to Surface 

Cross Drains,” Publication 9877 1806-SDTDC, San Dimas Technology Development Center.

The initial cost of constructing dips and water bars is less than the cost of constructing roadside 

drainage ditches and purchasing and installing culvert pipe. If dips and waterbars are properly 

located, designed, and constructed, their maintenance cost is also less than the cost of 

maintaining roadside ditches and culverts. The costs of other types of surface cross drains may 

not be less than the cost of culverts.

The disadvantages of surface cross drains are lower travel speeds, poor riding comfort, and 

discontinuity of blading operations for all drain types except broad-based dips.

To the extent practicable, avoid constructing dips on level of service G and H roads on grades 

greater than 10 percent, unless specifically designed for the critical vehicle and constructed to 

appropriate tolerances. On these roads, consider other surface drainage methods such as opentop drains or broad-based (drivable) dips designed to accommodate the critical vehicle. 
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Dips should discharge runoff in small amounts before runoff can significantly accumulate. For

recommended spacing for surface cross drains on insloped, crowned, or unsurfaced roads with 

no ditch, see section 44.4, exhibit 01. The location of surface cross drains should be reviewed 

and adjusted as necessary in the field to account for geomorphic features such as stream 

locations and suitable release areas. Dips skewed from a line perpendicular to the centerline of 

the road may drain and self-maintain better than dips that are not skewed. However, an 

unskewed dip normally results in better driving characteristics, minimizes twisting of truck

frames, and is easier to maintain with a motor grader. 

On roads which are not likely to receive regular surface maintenance, dips and waterbars provide 

a safety valve for removing water from any wheel ruts that may develop. The design of dips and 

waterbars on these roads must be done with rutting in mind. The down-grade end of dips (crest) 

should be designed so that ruts do not readily cut through the crest and fail the dip. The height of 

the rise at the crest should be at least as high as the depth of potential ruts. The transition back to 

grade on either side of the crest is a designed length. It may be desirable to stabilize the crest 

and trough portions of the dips with aggregates or in-place soil treatments to reduce deformation 

and to maintain stability.

Where a low-clearance truck or tractor-trailer is the design vehicle, the following guidelines 

apply:

1. Dips should not be located on gradients steeper than 8 percent.

2. Do not locate drainage dips within curves that have radii of less than 100 feet.

3. Maintain constant inslope or outslope throughout the length of dips to avoid the 

twisting of truck frames. 

4. Do not deepen the outlet dips unless the dips are perpendicular to the road.

5. Construct transitions at least 60 feet long in both directions from the low point and the 

crest to avoid abrupt changes in grade. 

6. When appropriate, design to accommodate surfacing or armoring placed through the 

dip.

Long continuous grades have a tendency to transport large volumes of water and sediment and 

therefore should be avoided. The use of a grade breaks allow long continuous grades to be 

broken into smaller segments. A reduction in grade combined with outsloping at convex 

topography can have the same effect as a surface drain dip and may be easier to maintain than a 

surface dip. 
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44.3 - Ditches

When developing the geometric design of ditches, consider the resource objectives for soil, 

water, and visual quality, maintenance capabilities and associated costs, and construction costs. 

Ditches can be a major source of sediment if left unarmored or if frequently bladed. Designers 

should consider larger ditches and erosion control measures to reduce the frequency of required 

maintenance. Ditch grades should be no less than 0.5 percent to provide adequate drainage and 

to avoid siltation. Consider outsloping the road and eliminating the ditch when the grade of the 

road is less than 2 percent.

The following lists typical types of ditches and describes their use.

1. Drainage Ditch. A drainage ditch transports water collected from the road surface 

or cut slope to the nearest ditch relief culvert or outlet ditch and also drains subsurface 

water from the roadbed. The ditch is constructed between the traveled way and the 

adjacent terrain in subgrade material. It should not have slopes steeper than 1V:1.5H. 

2. Trap Ditch. A trap ditch is a large drainage ditch that is designed to catch and hold 

cut bank slough and snow as well as provide effective drainage. Future maintenance 

requirements need to be considered when designing a trap ditch. Because it is a type 

of drainage ditch, a trap ditch can perform all the functions of a drainage ditch.

3. Intercepting Ditch. An intercepting ditch protects cut and fill slopes. Consider the 

use of intercepting ditches in areas with extremely high cut slopes or high 

embankments in erosive soils or areas that have experienced high-intensity fire with 

significantly increased surface runoff. To the extent practicable, locate and design 

intercepting ditches so that erosion caused by intercepted water is no greater than what 

would have occurred had the water sheeted down the cut slope. Use energy dissipation 

and erosion control measures to prevent erosion, gullying, and connectivity with 

streams.

4. Outlet Ditch. An outlet ditch carries water away from a road to prevent the road 

subgrade from being saturated or eroded. An outlet ditch is typically used in fairly flat 

ground when the topography does not allow water to run off the road. Locate an outlet

ditch at the lower end of a culvert or drain dip or at the point where a roadside ditch 

daylights onto natural ground. Consider use of energy dissipation measures as needed.

44.4 - Culverts

Low-volume roads are often steep due to economic factors, the need to avoid unstable areas, and 

a desire to disturb the least amount of land. Ditch relief culverts installed in roads with steep 

grades, particularly in steep, mountainous terrain with high-intensity storms, have an increased 

potential for failure. Failure of ditch relief culverts may result in increased ditch scour, extensive 
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erosion of road surfacing, and broad-scale failure of roadway fills. The alternative combination 

of outsloped roads and surface cross drains to help shed and disperse water quickly from the road 

surface should be considered. To the extent practicable, avoid draining water onto steep slopes.

Drainage failures may also have a detrimental effect on land below the road. Siltation may 

degrade water quality and may damage aquatic habitat. Runoff concentration increases surface 

erosion, mass soil movement, and stream channel scour. Consult with appropriate resource 

specialists, such as a hydrologist, fluvial geomorphologist, soil scientist, and biologist, as 

appropriate to address site-specific issues regarding culverts. Follow agency guidelines for 

designing structures for fish and aquatic organism passage. FSM 7722 and FSH 7709.56b 

include design data and guidance regarding design and risk analysis of bridges and structures.

Some culverts are considered “bridges” under the requirements of the National Bridge 

Inspections Standards (NBIS). The classification of and the authority for approval of design of 

culverts is shown in exhibit 01 of this section.

44.4 - Exhibit 01

Classification and Responsibility for the Design of Culverts and Bridges

Classification and Responsibility for the Design of Culverts and Bridges

Any Single Opening Structure 

with a bottom, regardless of fill 

depth or depth of invert burial 

(fn 2)

Any Single Opening Bottomless 

(Natural channel) Structures 

with footings that will not have 

wheel loads in direct contact 

with the top of the structure 2

Any Single or Multiple 

Opening Bottomless 

(Natural channel) Structures 

with footings that has wheel 

loads in direct contact with 

the top of the structure 2

(CS) 1 < 20 

feet CS > 20 feet CS < 20 feet CS > 20 feet All spans

Design3 Classification Culvert Culvert Bridge

Responsibility

Forest Design 

with 

standards or 

by qualified 

engineer. 

Design by a 

certified Bridge 

Design 

Engineer (FSM 

7722.1 and .2).

Forest 

Design with 

standards or 

by qualified 

engineer;

Design by a 

certified Bridge 

Design 

Engineer (FSM 

7722.1 and .2).

Design by a certified Bridge 

Design Engineer (FSM 

7722.1 and .2).

1. Clear span (CS) is defined as the NBIS opening for a single or multiple opening structures.

2. Design all structures to have a clear span ≥ the design bankfull width.

3. For information concerning multiple opening culverts see FSH 7709.56b, sec. 76 for design, and sec. 91.6 for operation.

All culvert designs should consider storm flows, sediment, and debris. Consider debris loading,

access for debris removal, and potential effects from upstream blockages. 

1. Ditch Relief Culverts. Ditch relief culverts periodically relieve ditch flow by directing 

water to the opposite side of the road where the flow can disperse away from the traveled 

way. Use energy dissipation and erosion control measures to prevent erosion, gullying,

and connectivity with streams.
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The spacing of ditch relief culverts depends on the road gradient, road surface, ditch soil 

types, runoff characteristics, and the effect of water concentration on slopes below the 

road. See section 44.4, exhibit 02, for recommended spacing of relief culverts based on 

ditch or road surface soil type. The location of culverts should be reviewed and adjusted

as necessary in the field based on geomorphic features such as stream locations and 

suitable release areas.

Ditch relief culvert maintenance can be minimized by proper design. Culverts should be 

no smaller than 18" diameter or equivalent size pipe arch. When plugging by sediment 

and debris is a concern larger diameter culverts should be used. A decision to use a 

culvert smaller than 18" should be supported by a written statement in the project file 

signed by a qualified engineer that there is no possibility of the culvert plugging. 

The following table is intended for reference, actual culvert location depends on the 

topography as site specific conditions. Consult with appropriate resource specialists to 

determine site specific BMP’s with respect to culvert spacing on the road segment being 

designed.

44.4 - Exhibit 02

Recommended Spacing of Relief Culverts or

Surface Cross Drains Based on Ditch Soil or

Aggregate Surface Type
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Ensure pipes are buried at an appropriate depth to prevent damage to the culvert. Provide 

adequate depth of cover to account for expected surface material losses from traffic, 

maintenance, and erosion over the design life of the culvert. The minimum depth should 

equal or exceed the manufacturer’s specifications for the particular installation.

Skewing ditch relief culverts from a line perpendicular to the centerline of the road may 

increase culvert grade and flow velocity, reducing siltation and the possibility of debris 

plugging the culvert inlet. Do not use skewing to increase the distance between ditch 

relief culverts. Do not use skewing when water is flowing toward the culvert inlet from 

both directions, except to reach or fit a natural channel. Skewing may increase the length 

of culvert necessary for the site. When determining whether to use skewing, consider: 

a. Improved bedload and debris flow.

b. Proper dispersion of water below the road surface.

c. Improved flow through the pipe.

d. The additional cost caused by the additional culvert length.

It is also possible to use culverts placed in natural drainages for ditch relief. However, 

consider the effect of possible sedimentation or increased flows on the natural drainage. 

Do not drain ditches into live streams or streams with aquatic life. For more information 

on culverts, refer to “Relief Culverts,” Publication 9777 1812-SDTDC, San Dimas 

Technology Development Center. 

2. Inlet Structures. Inlet structures may consist of hand-laid rock headwalls, ditch dams, 

inlet basin liners, drop inlets, metal end sections, or other structures designed for specific 

conditions at the site. Determine the location of the inlet of relief culverts during design

to provide for design of inlet structures. The design of inlet basins should include 

adequate width for the culvert entrance and for any structure necessary to prevent erosion 

of the road bed and back slope. Design inlet basin back slopes at a stable slope to 

minimize the possibility of the culvert’s plugging from ravel or slumping. 

Inlet basins trap sediment due to a decrease in water velocity and are a maintenance 

commitment once installed. Strive to prevent the velocity of ditch water from decreasing 

in the culvert inlet basin. This results in transported sediment settling and accumulating 

in the basin, eventually blocking the culvert inlet. The slope of the culvert invert should 

be greater than the slope of the road ditch to avoid a decrease in ditch water velocity. 
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Catch basins should be designed for mechanized maintenance. Mechanized maintenance 

is usually cheaper than hand methods, and it is often necessary to use machinery to load 

sediment on trucks for hauling to waste disposal areas. An alternative to an inlet basin is 

to increase the size of the pipe and ensure that skew of the pipe captures and transports 

the water and sediment delivered to that pipe.

3. Wetland and Meadow Crossings. Care should be exercised in designing wetland and 

meadow crossings, as they are sensitive to unnatural fluctuations in water level. Since 

marshy and swampy terrain may contain bodies of water with no discernible current, 

designing culverts for roads crossing these locations involves unique considerations. 

Consult with resource specialists, and refer to the “Road/Water Interaction: Introduction 

to Surface Cross Drains,” Publication 9877 1806-SDTDC, San Dimas Technology 

Development Center, in connection with design of wetland and meadow crossings.

Design wetland culverts with a nearly flat grade so that water can flow either way and so 

that the natural water level can be maintained on both sides. The culvert may be partially 

blocked by aquatic growth and installed with the flow line below the standing water level 

at its lowest elevation. Consider selecting culvert materials that resist corrosion.

For wetlands and wet meadows that have a slight gradient, place the culvert at the stream 

gradient, and dissipate the flow at the culvert outlet. Culverts placed too low at the inlet 

and unchecked flow through the culvert can cause erosion and gullying upslope for 

considerable distances, lowering the water table and reducing the extent of the wet area. 

Permeable dams constructed around inlets and outlets can dissipate erosion. Erosion 

control blankets may be used at outlets, but need to be long and wide enough to dissipate 

the flow fully before release.

Crossing wet areas may require special subgrade treatment due to wet, soft, organic soils. 

Geotextile-reinforced fill or subgrade reinforcement may be used. The design should 

consider the hydraulic effects of the materials used to avoid disrupting wet area drainage. 

When crossing meadows with more than one defined channel, install culverts in all 

identified channels. When crossing meadows without a defined channel, use a permeable 

fill or multiple culverts installed at the same elevation of the meadow surface to allow the 

natural sheet flow to continue. Head-cuts and erosion in meadows can be restored by 

employing this strategy and filling in the eroded sections. Always consult resource 

specialists before undertaking watershed restoration such as repairing head cuts or wet 

meadow restoration. 

44.5 - Subdrainage Systems

1. The design of subdrainage systems should remove water from the subgrade or 

pavement structure to improve stability and load-bearing capacity, decrease the adverse 

effects of frost, and reduce safety hazards caused by freezing on the traveled way.
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2. Design subsurface drainage systems to:

a. Intercept groundwater that cannot be intercepted by side ditches before entering 

the traveled way.

b. Reduce the hydrostatic pressure behind structures.

c. Release collected groundwater without causing erosion or silting.

d. Last as long as the roadway or structure.

3. Because each site is different, conduct a field investigation to determine the best 

solution. The field investigation may necessitate the:

a. Review of available soil and geological studies or gathering new data.

b. Borings or digging test holes to locate groundwater.

c. Inspection of natural lakes and slopes in the area and studying the natural drainage 

patterns.

d. Measurement of discharge when possible.

e. Test of slope stability.

4. Perforated pipe drains are a common solution, but they do not function properly unless 

some method is used to prevent the perforations from plugging. The following 

alternatives prevent plugging, depending upon the characteristics of the soil:

a. Use of a prefabricated drain, which consists of a geotextile covering one or both 

sides of a drain core. The core provides open channels for water flow.

b. Surround the pipe with an open-graded aggregate material, which, in turn, is 

surrounded by a geotextile. The use of fabric eliminates the need for an inverted filter 

consisting of various-sized gravel and sand layers.

c. Use of a graded aggregate filter. Use of this filter has diminished with the advent 

of geotextiles.

5. Other types of subsurface systems include:

a. Drilled Drains. For this system, place perforated pipes in holes drilled into cut or 

fill slopes to intercept the groundwater flow.
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b. French or Trench Drains. This system is identical to the drilled drain system, 

except a perforated pipe is not used. Use open-graded drain rock for the drainage 

path.

c. Engineered Drainage. This system usually consists of a porous, chemically inert 

medium covered on one or both faces with a geotextile. Place the system directly in a 

trench or against a structure and backfill it with excavated material. This system can 

eliminate the need for special backfill necessary when the drilled drain and French 

drain systems are used.

6. Select a system that best meets the structural requirements and the corrosive 

conditions of the soil and water at the site. Because of the complexity of soils in many 

areas, it is advisable to consult geotechnical engineering specialists about the use and 

performance of various types of geotextiles and graded aggregate filters.

7. Subdrainage systems may effectively reduce final road costs by decreasing the depth 

of base rock and subgrade widths, resulting in less clearing and excavation. Lower 

maintenance costs also may result from a more stable subgrade.

8. The solutions to subdrainage problems can be expensive. Consider as alternatives 

road use restrictions, such as seasonally reducing or prohibiting traffic until a subgrade 

dries out. Also consider incorporation of new materials and technology specifically 

developed for roads in wet regions.

44.6 - Drainage Systems on Stored Roads

Road surface drainage should also be considered if the road is in storage or going to be placed in 

storage. Installing waterbars and outsloping should be a preferred surface drainage treatment on 

stored roads. Serious consideration should also be given to the removal of stream crossing 

structures that are near the end of their service life, smaller than bankfull width, or are a high risk 

to the watershed and could fail during storage. High risk structures may be structures in very 

sensitive habitat that have a plugging or overtopping flow risk. Watershed specialists should be 

consulted on whether stored structures are high risk structures. 

45 - EROSION CONTROL AND WATERSHED PROTECTION

Minimize soil displacement from roads. Factors that influence road impacts on soil and water 

resources include route location, geometric standards, drainage design, and long-term erosion 

control features. Include measures to avoid or mitigate erosion and to provide structural or 

vegetative treatments.
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45.1 - Permanent Erosion Control and Watershed Protection

Mitigate long-term impacts on soil and water quality by incorporating cost-effective measures 

into the design. Specific requirements may be found in applicable Forest Service directives, 

regulations, and statutes, including Section 10 of the National Forest Management Act 

(16 U.S.C. 1600), the Clean Water Act, and their implementing regulations. Additional 

requirements may be outlined in BMPs at the regional or forest level.

Consider the following erosion control and mitigation strategies during road design:

1. Design, treatment, and revegetation of cut and fill slopes to control surface erosion. 

This includes conservation of top soil during initial excavation, temporarily placing it in 

stockpiles, and using it for revegetation of slopes once the road is constructed.

2. Inclusion of mitigation measures such as berms, dips, oversized drainage features, and 

debris control to minimize runoff. Additional slope protection measures that may be 

beneficial when significant amounts of water are anticipated on slopes include but are not 

limited to rock blankets, gabion blankets, down spouts, flumes, mulch netting, and 

plantings.

3. Use of smaller cuts and fills pose less significant erosion problems. Erosion potential 

is reduced because the area exposed or disturbed is smaller, and if a failure does occur, 

much less material is involved.

4. Design of flatter slope ratios to reduce soil loss from slides and slumps. One way to

determine mass stability is to analyze successful practices on similar land types. 

Geotechnical assistance may be necessary where bedrock characteristics or soil stability 

is unknown.

5. Control of erosion by placing sedimentation basins between large cuts or fills and 

critical waterways.

6. Use of full bench construction when the natural side slopes exceed 55 percent. This 

method usually results in excavating and end-hauling throughout the steep area.

Where problem cut and fill slopes are encountered in the vicinity of perennial streams, consider 

using more extensive erosion control measures. Use erosion control references specific to local 

conditions and the geographic location. Federal agencies such as the Natural Resources 

Conservation Service, FHWA, and the U.S. Environmental Protection Agency as well as many 

State agencies publish erosion control references. The most current erosion control strategies are 

available on the internet.
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45.2 - Temporary Erosion Control and Watershed Protection

Temporary erosion control measures are incorporated during construction. Ensure that road 

construction contracts specify contractor responsibility for mitigation of erosion from 

construction activities. Develop erosion control plans for road construction projects that address 

implementation as well as removal of temporary erosion control measures. For example, the 

requirements may include constraints on how much pioneer road or excavated area may be open 

at any given time or the amount of work that may be done without requiring seeding or 

revegetation. Include winterization if construction is likely to extend beyond one construction 

season. Specify that the contractor’s operations must meet all applicable legal requirements. 

Water quality standards may vary State to State. Therefore, consult with local resource 

specialists to identify applicable Clean Water Act and other water quality requirements.

45.3 - Aquatic Habitat Protection

As appropriate and applicable, include design measures to protect aquatic habitat. Consult local 

resource specialists regarding site-specific issues and appropriate potential mitigation measures. 

As appropriate and applicable, conduct field reviews to verify that the design provides adequate 

protection for aquatic resources. To the extent practicable, avoid channelizing streams.

45.4 - Disposal of Waste

Deposit waste from road construction at locations that are not susceptible to erosion and mass 

failure. In some cases, it may be necessary to designate separate disposal areas for specific types 

of waste, such as rock and common excavation.

If it is not possible to find flat sections of ground to deposit the excavated fine material in fills or 

to haul the waste to a designated disposal area, consider mitigating measures, such as relocating 

the road, installing retaining structures, or constructing silt basins.

Coordinate the location of waste areas with the appropriate resource managers or specialists. If 

waste may be used as construction material incorporated in another project, consider stockpiling 

the waste. In some areas, construction of low earth mounds can reduce landform contrasts 

associated with road construction. For further discussion of aesthetic treatment of waste, see 

“National Forest Landscape Management,” Volume 2, chapter 4, Roads.

45.5 - Retaining Structures

Consider using retaining structures to reduce disturbance to the landscape and environment by 

decreasing the quantities of excavation and fill material for construction. In steep terrain where 

there are slope stability problems or environmental constraints, retaining walls may be a practical 
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and economical solution. When visual quality is a concern, design should strive to conform to 

natural topography and the surrounding terrain. Visual quality concerns may also require careful 

selection of materials or modification of the form and line of finished structures.

Retaining structures should be designed by qualified engineers (FSM 7722). Specific 

information on types and use of retaining structures can be found in the Retaining Wall Design 

Guide, EM-7170-14, and the Slope Stability Guide, EM-7170-13.

46 - UTILITIES AND OTHER EXISTING USES, RIGHTS-OF-WAY, AND 

CONSTRUCTION EASEMENTS

1. Utilities and Other Existing Uses. Utilities and other uses operated by the Forest 

Service, other governmental agencies, utility companies, or individuals may be located in 

the project area. Before beginning the preliminary survey for road construction projects, 

obtain information about the exact location, type, and size of existing utilities and other 

uses in the project area. Design documents should include a list of the holders of special 

use authorizations and their authorized uses in the project area.

Before beginning the design, become familiar with Forest Service utilities, other utilities,

and other uses authorized by a special use authorization in the project area. Furnish the 

local Lands staff with the inventory of utilities and other uses in the proposed route, an 

assessment of potential conflicts, and a list of the resource specialists required for their

resolution. Work closely with the local Lands staff to coordinate issues involving other 

authorized uses in the project area. Special use authorizations typically reserve the right 

of the Forest Service to use or allow others to use any part of the permit area for any 

purpose, including road construction. Authorizations also typically provide that permit 

holders must move permitted utilities at the holder’s expense, should those utilities 

interfere with future Government needs for construction of infrastructure.

As necessary, develop criteria to address requirements for specific types of utilities to be 

accommodated. Utilities such as above-ground power and telephone lines, underground 

power lines, water mains, sewers, fiber optic cables, telephone lines, irrigation pipes, and 

oil and gas lines have specific requirements that must be addressed. Refer to AASHTO’s 

“A Guide for Accommodating Utilities within Highway Rights-of-Way,” for general 

criteria in this context. Utility owners may wish to prepare the designs, drawings, and 

specifications for any revision or relocation of their utilities needed for road construction. 

Their road drawings and specifications must include criteria to address applicable 

requirements and must provide for coordination of any adjustment in their utilities with 

appropriate phases of the road design. An alternate road location may be necessary to 

avoid existing utilities that cannot be relocated or crossed economically. Coordinate with 

affected utility owners during the design phase. Strive to resolve conflicts during the 

early stages of design. To avoid lengthy delays, submit preliminary drawings to the 

utility owners as early as possible for their comments and recommendations.
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There are hazards associated with constructing roads near gas and oil pipelines and high 

tension power lines. It may be necessary to use special safety measures and features to 

protect both the utility installation and road users. Where possible, ensure that utilities 

are not situated under the traveled way, except where a utility must cross a road. Utilities

should be placed through larger conduits when they cross roads. These measures 

minimize road damage and disturbance to traffic during maintenance and reconstruction 

of utilities. 

In the road construction contract, identify the location of affected utilities, and enumerate

the contractor’s responsibilities for them. Identify who is responsible for any work, 

necessary permits, coordination, and relocation costs involving the utilities.

2. Rights-of-Way. The Forest Service must obtain a right-of-way for road construction 

projects that will cross private land or other lands not under the administration of the 

Forest Service. Consult with the local Lands staff and, if appropriate, the local Office of 

the General Counsel, during project planning to coordinate right-of-way acquisition. 

Failure to do so may delay the project, create complications during construction and 

operation of the road, or result in project cancellation. See FSM 5460 and FSH 5409.17

for further guidance on obtaining rights-of-way.

To establish a right-of-way on the ground, it may be necessary to have a right-of-way plat 

showing the road centerline and widths, metes and bounds for the right-of-way, a legal 

description and identification of ownership of the underlying land, ties to survey 

monuments, and other figures and measurements. Show the scope of rights-of-way 

obtained for a project on construction plans.

Determine the approximate right-of-way width during design. Make the right-of-way 

width sufficient to accommodate the roadway and roadside. Often the Forest Service has 

existing agreements with non-Federal landowners that provide for a specified right-ofway width. Uniform widths are easier to describe and locate on the ground. However, 

vary widths as necessary to minimize the acquisition of rights-of-way across high-value 

or intensively used land.

3. Construction Easements. Acquire short-term easements when road construction 

cannot be accomplished within the existing right-of-way. Identify temporary easement 

requirements during project development. Consult with the local Lands staff before 

beginning the design to plan and coordinate easement acquisition and to avoid 

complications during construction.
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47 - MATERIALS

A geotechnical and materials investigation is an integral part of any road project design. The 

investigation should include the delineation, classification, and description of the engineering 

characteristics of the road construction materials and should locate potential drainage, erosion, 

settlement, and stability problems. A layer of topsoil may hide defects or abrupt changes in 

materials. Knowledge of subsurface conditions helps in anticipating many design problems.

Before starting a road design, review any information available about the materials for the 

project (secs. 22.3 and 36). At a minimum, obtain the materials classification notes compiled by 

an experienced person who has walked over the proposed route. For more complex projects, it 

may be necessary to obtain more detailed information, such as:

1. Environmental features and concerns, such as mine tailings, leachate from abandoned 

mines, materials containing heavy metals, or other potentially hazardous materials.

2. Geological features such as bedrock type and characteristics, rock fall areas, existing 

landslides, and earthquake potential in the area.

3. The depth, thickness, and classification of each layer of material that may influence 

the design.

4. The engineering or behavioral properties of the materials along the proposed route and 

in proposed borrow sources.

5. The depth and character of groundwater in the project area.

If problem areas are identified, it may be necessary to conduct an intensive materials 

investigation prior to design.

47.1 - Slopes

Selection of cut and fill slopes may have a significant effect on initial, operating, and 

maintenance costs and environmental disturbance. Therefore, consider:

1. Classification, strength, and variation of materials in the project area.

2. Resource management objectives, including acceptable level of risk.

3. Design criteria.

4. Requirements for revegetation.
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Consideration of these factors may result in selection of slopes that allow ravel and small slumps 

in areas where environmental constraints would not be exceeded, where the road’s level of 

service is low, or the use of the road is intermittent.

When selecting slope ratios in design, balance the additional cost of constructing roads with 

flatter slopes against the additional cost of maintaining roads with steeper slopes. Factors that 

affect slope stability include the material used, their steepness and height, subsurface moisture, 

rainfall, exposure to sunlight, compaction, and vegetative cover. When available, use data from 

geotechnical investigations and testing in designing slopes. Use slope stability analysis to select 

proper slope ratios. The use of compound slopes can be effective when material types change 

within the cross section.

Rounding at the tops and ends of cut slopes may reduce erosion. However, given its cost, 

rounding is usually appropriate only when trying to retain a natural effect by blending the 

disturbed area with the natural ground. 

Use cut ratios appropriate for the rock type and orientation when designing rock cuts to minimize 

raveling or falling rock. A true vertical slope next to a traffic lane cut can create a roadside 

hazard with no room for rock fall. Laying back the slope is an appropriate measure for safety 

even if the rock will stand at a vertical slope. A common slope in solid rock is 4V:1H. Consider 

using benching and trap ditches when designing rock cuts to plan for debris from raveling or 

falling rock.

In steep, mountainous terrain, it is sometimes difficult to establish catch points without 

developing long sliver cuts or fills. Avoid sliver fills by using full bench construction on slopes 

greater than 55 percent. If necessary, employ special drainage and stabilization features, 

construction techniques, or retaining structures. Fill slopes can sometimes be steepened by using 

special compaction, reinforcement methods, or constructing fills composed of rock.

47.2 - Pavement Structure 

The pavement structure consists of one or more layers of base material and a surfacing course. 

The structure supports the traffic and reduces the load on the subgrade by distribution through 

designed layers.

Pavement structures are either rigid or flexible. Rigid pavement includes plain or reinforced 

concrete and soil cements with or without a surface course. All other pavement, such as 

macadam, bituminous concrete, cold road mix asphalt, and bituminous surface treatment, is

flexible. Most pavement structures on NFS roads are surfaced with native soils or aggregates 

such as gravel, stone, slag, or volcanic cinders. Aggregates may be screened, crushed, or hauled 

directly from pits. 
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The pavement structure is used to stabilize the roadbed and support the estimated volume of 

traffic. Additional benefits include dust control and reduced sediment transport. Land 

management plans, sound engineering principles, adequate geotechnical expertise, and economic 

analysis influence selection of the appropriate pavement structure.

Roadbed widths should be constructed to accommodate future pavement structures, including 

widening and surfacing.

1. Surfacing Considerations. Considerations for surfacing include cost, efficient traffic 

management, structural requirements, and resource protection. 

a. The following factors apply to consideration of traffic management and structural 

requirements:

(1) Current and projected road use.

(2) Current and projected traffic type and volume.

(3) Seasons of desired use.

(4) User safety, such as past accidents, near misses, and the current and proposed 

design speed and alignment.

(5) Available resources to construct and maintain the road at the desired level of 

service.

(6) Tire pressure management.

b. The following factors apply to consideration of resource protection:

(1) Requirements of applicable BMPs.

(2) Whether road use restrictions or roadway surface drainage and erosion protection 

can adequately mitigate adverse impacts.

(3) Whether road impacts during periods of use and non-use significantly affect water 

quality.

(4) Whether adverse impacts outweigh repair and maintenance costs.

(5) Availability of resources to address traffic-generated maintenance needs, such as 

rutting of native-surfaced roads that might result from late fall use by hunters.
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If traffic considerations do not require surfacing, and if resource impacts are not

significant or may otherwise be mitigated, do not use any surfacing. Instead, implement 

other mitigation measures, such as drainage and dust abatement. Use the surfacing 

decision tree found in the Earth and Aggregate Surfacing Design Guide, EM-7170-16, to 

determine the need for surfacing. 

Aggregate Surfacing Considerations. Aggregate surface courses are used to: 

1. Support traffic within acceptable deformation limits, 

2. Resist the abrasive action of traffic, and shed a large portion of precipitation, thereby 

reducing erosion.

The primary purpose of aggregate base courses is to transfer traffic loads to the 

underlying layers within acceptable deformation limits. The base course may also be 

designed to shed water that infiltrates the surface course depending on the type of 

aggregate surface course used. If the surface course is designed to shed surface water, 

the base course would normally have less fines than the surface course.

When advised by engineering judgment, use an aggregate subbase or geosynthetic 

subgrade reinforcement as an additional layer for distributing traffic loads when 

subgrades are extremely weak or when frost action is severe and would seriously damage 

an asphalt surface course.

Select the amount and type of aggregates based upon traffic, cost, road gradient, 

drainage, soil conditions, and available materials. Aggregate may not be necessary for 

level of service J roads. Assess physical conditions in the field and the natural variability 

of soils. For example, designs for wet seepage areas may be different from those for poor 

soils in well-drained areas. The reduction of aggregate thickness from an optimum 

design perspective may be a valid management and technical decision, after assessment 

of possible risks, costs, and other relevant factors. 

a. Seasonal Use. Considerable savings in aggregate surfacing are possible if traffic 

can be restricted during wet periods. If roads are designed for seasonal use, manage 

traffic to conform to that design. Include seasonal haul restrictions in timber sale 

contracts.

b. Specifications. Consider economically available materials and local experience to 

write specifications for aggregate surfacing based on the intended use. Marginal 

aggregates may be acceptable where high-quality, commonly specified materials are 
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not economically available. Be specific as you draft the specifications for design 

conditions. Assess whether the requirements in the specifications are economically 

justified, and assess whether standard requirements in the specifications need to be 

modified or removed. 

For example, naturally occurring materials which meet the plasticity index (PI)

requirement (AASHTO T90) may not be available. However, some rock fines 

display adequate binding characteristics that do not show up in the PI test. If the PI 

requirement does not apply, delete it from the specifications. Another example is the 

use of aggregates with marginally low durability index test results (AASHTO T210)

on low-volume roads. If the rock is crushed to a coarser grade initially, it may 

degrade under traffic and provide satisfactory performance for some time. Under 

these circumstances, it may be appropriate to remove the durability index test 

requirement from the specifications. In addition, marginal materials may be made 

acceptable by blending, scalping certain sizes, wasting certain sizes, or washing if 

these options are recognized in the design phase and provided for in the 

specifications.

When preparing specifications for surfacing of level of service I and J roads, user 

comfort, convenience, and speed of travel are not considerations. If surfacing is 

specified, there is a range of choices, such as pit run, shot rock, screened rock, 

crushed to a maximum size, and crushed to a specified gradation.

c. Correlation of Soil Strength to Soil Type. In the preliminary stage of road base 

and road surfacing design, correlate soil strength to soil type using the results of all 

soil strength tests, such as CBR and R-value, performed locally. Use the correlation 

to develop alternative designs with the minimum amount of new testing. If deemed 

appropriate based on an assessment of risks, costs, and other relevant factors, conduct 

a verification strength test on the selected design.

d. Variation of Surfacing Thickness. Road surfacing thickness is often designed to 

address worst-case soils and subgrade conditions. If worst-case conditions do not 

exist over a large portion of a project, it may be economical to vary surface thickness

along the road. In addition, more aggregate wear may occur on steep grades and 

horizontal curves than on flat segments. It may be appropriate to vary surfacing 

thickness depending on site-specific needs for aggregate replacement. 

e. Spot Surfacing. Roads often include sections of structurally inadequate native 

soils. It may be difficult to locate the extent of these sections without an extensive 

subsurface soils investigation. The amount of spot surfacing may be adjusted during 

construction.
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f. Traction or Erosion Control. Aggregate surfacing may be an effective way to 

improve traction or control erosion on steeper grades. In these cases, 1 to 2 inches of 

aggregate is usually adequate for the steeper grades.

g. Limiting Surfacing of Turnouts on Low-Volume Roads. Where traffic and 

subgrade conditions permit, it may be possible to reduce aggregate depth on turnouts

on low-volume roads.

h. Use of Marginal Aggregates. Some aggregates do not meet common standards of 

quality aggregate, that is, they are in poor gradation, have excessive fines or poor 

resistance to traffic or weathering, or have excessively deteriorated to plastic fines. 

The use of marginal-quality aggregate at appropriate depths may be adequate. 

Mixing marginal aggregate with quality aggregate or other additives may be another

cost-effective alternative. American Society of Testing and Materials (ASTM) 

Technical Publication 774 and FHWA Reports RD-81-176 and RD-82-056 include

recommendations with regard to the use of a variety of marginal aggregates. When 

considering marginal aggregates, utilize the expertise of individuals who have 

experience with local materials and conditions.

i. Stabilization. Stabilization methods may be used to improve aggregate used for the 

road base or road surfacing. There are many different types of stabilization methods 

and materials available, including Portland cement, lime, fly ash, bitumens, chlorides, 

and organic cationic compounds. Stabilization may involve mixing a poor material 

with a higher-quality material, such as mixing clean sand in with a silty soil.

The type and amount of stabilizing agent depends on many factors, and each agent 

has its own strengths and limitations. There are many sources of information on the 

selection and use of these agents, including manufacturer's associations such as the 

Portland Cement Association, the Lime Association, and the Asphalt Institute; 

professional publications such as those of ASTM, AASHTO, and Transportation 

Research Board, FHWA and Forest Service reports; and manufacturers' technical 

literature. A good reference is FHWA’s “Soil and Base Stabilization and Associated 

Drainage Considerations Volumes I and II,” Report No. FHWA-SA-93-004. Local 

experience is an additional source of information. 

j. Sand Stabilization With Topsoil or Geocells. Sand can be an excellent road 

building material if stabilized or confined. Mixing topsoil and organic duff during 

construction or maintenance helps to stabilize sand. Plastic geocells filled with sand 

or rock may also provide a stable surface.

k. Geosynthetics (sec. 47.4). Geosythetics may be used in designing aggregatesurfaced roads over poor subgrade soils. Geotextiles are used for separation and 

geogrids are used to reinforce soil structure. The amount of aggregate surfacing 

material required can be reduced by using appropriate geosynthetics.
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l. Weak Subgrades. For weak subgrades, consider various stabilization techniques to 

strengthen the subgrade rather than increasing the pavement structure. In addition to 

the most common method, compaction (sec. 47.3), stabilization using lime, cement, 

asphalt, blends of sands and clays, various chemicals, or the use of fabrics has been 

successful. The references for stabilization (sec. 47.2, para. i) and geosynthetics 

(sec. 47.4) also apply to weak subgrades.

m. Surfacing Thickness. Use the Earth and Aggregate Surfacing Design Guide for 

Low-Volume Roads, EM-7170-16, to design the necessary thickness for aggregatesurfaced roads.

n. Seasonal Frost Conditions. For seasonal frost conditions, use “Revised Procedures 

for Pavement Design Under Seasonal Frost Conditions,” published by the U.S. 

Department of Defense, U.S. Army Corps of Engineers, Cold Regions Research and 

Engineering Laboratory, Special Report 83-27, to determine the appropriate

pavement structure.

o. Dust Abatement. When addressing dust abatement, consult the Dust Palliative 

Selection and Application Guide, Publication 9977 1207, San Dimas Technology and 

Development Center.

3. Other Surfaces. Other surfaces may consist of a road mix or bituminous surface 

treatment, bituminous concrete, Portland cement concrete, or reinforced Portland cement 

concrete. The design of an asphalt or concrete surface course depends upon the volume 

and composition of traffic and available materials and construction funds. Usually, it is 

possible to achieve economy of construction by making full use of local materials. 

Design these pavement surfaces using the latest version of AASHTO’s “Guide For 

Design of Pavement Structures.”

47.3 - Compaction

Compaction should be considered to improve the stability of soil through the increase of soil 

strength and the restriction of water movement. Compaction:

1. Permits the construction of stable fills using local material;

2. Allows steeper fill slopes;

3. Decreases erosion on fill slopes, thereby decreasing the need for fill widening; 

4. Increases the load-carrying capacity of the subgrades and base courses, thereby 

decreasing the need for surface rock (this may require compaction in cut areas as well as 

fill areas); and 
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5. Provides additional lateral support for structures such as culverts and bridge 

abutments.

Compaction can also reduce settlement of fills. Settlement can be problematic for deep fills, 

asphalt paved roads, or bridge approaches. Increasing compaction of most aggregate base and 

surface courses provides longer life with less maintenance. Asphalt concrete surface courses 

placed over well-compacted bases are less prone to cracking and deflection.

47.4 - Geosynthetics

Geosynthetics have various functions. Selection of the proper geosynthetic material is based on 

its properties and limitations. When considering geosynthetics, use FHWA’s “Guidelines For 

Use of Fabrics in Construction and Maintenance of Low-Volume Roads,” Report No. FHWATS-78-205, to determine the necessary geotextile and material depth. Geosynthetics may 

perform the following functions more economically and more effectively than other materials:

1. Separation. Geotextiles are useful for separation because they can keep two unlike 

materials apart. The following are typical applications:

a. Separation of zoned sections of unlike materials within an embankment.

b. Separation of an aggregate base from the subgrade.

c. Separation of temporary and long-term aggregate or other material.

d. Separation of frost-susceptible soils into distinct layers, thereby breaking the 

continuity of the capillary flow zone.

2. Reinforcement. Geosynthetics are useful for subgrade, base course, or pavement

reinforcement because they decrease the level of stress in the foundation soil by 

spreading the load over a large area. Reducing the stress decreases the chances of failure 

and settlement. Geosynthetic reinforcement can be used to:

a. Build roads over marshes, swamps, peat soils, or compressible fine-grained soils.

b. Build roads of almost any type over permafrost, muskeg, and other soils in cold 

weather regions.

c. Construct geogrid walls and reinforcing selected zones. In some instances,

geotextiles may be used depending on the strength requirements.

d. Reduce the need for removing material that may be unsuitable.

e. Contain soils that would spread laterally if left unconfined.
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f. Place bituminous overlays on existing pavements reduces the amount of cracking 

in new pavement caused by upward reflection of cracks in the old pavement

g. Reduce the thickness required in asphalt pavements when used as base course 

reinforcement.

h. Construct reinforced embankments and subgrades (see “Deep Patch Road and 

Embankment Repair,” Publication 0577 1204 — SDTDC, San Dimas Technology 

Development Center, October, 2005).

3. Drainage. Geotextiles are useful in many drainage installations because of their 

controlled permeability. Geotextile drainage applications can be used to:

a. Prevent migration of soil fines into aggregate or pipe underdrain systems, thereby

eliminating the need for an inverted filter.

b. Prevent the migration of coarse material in a filter blanket into the adjacent soil.

c. Provide a flow path for water in an underdrain system and behind retaining walls.

4. Erosion Control. Geotextiles and geogrids can be used for erosion control to:

a. Protect embankments where the geogrid holds the soil in place while allowing 

vegetative growth.

b. Protect against erosion at culvert inlets and outlets.

c. In combination with riprap, acting as erosion control mattresses to protect slopes 

adjacent to flowing water.

d. Serve as silt fencing to block the movement of soil by water or wind.

5. Forms. Geotextiles can act as forms to be filled with other materials. They can 

conform to the shape and topography of the surface on which they are placed. Their

controlled permeability allows the escape of air or water but contains the injected 

permanent material. The following are examples of applications for forming:

a. French drains. 

b. Cellular mats. 

c. Retaining wall construction.

d. Stream bank stabilization and erosion protection systems.