i

Preface

The purpose of this document is to meet the need for National Park

Service road design standards that will accommodate current or planned

park road use, while continuing to preserve the natural and cultural

values of National Park System areas; to address the requirements of

Standard 12 of the Federal Highway Safety Program Standards (23 CFR

1230; 23 U.S.C. 402); and to provide design guidance for projects

under the Federal Lands Highways Program for Park Roads and Parkways

(P.L. 97-424; 23 U.S.C. 204) compatible with Chapter 3(C)(1) of the

Federal Highway Administration (FHWA) Direct Federal Manual. This

document is also intended as a definitive guide for managers, planners

and designers involved in the planning, design and construction of

park roads.

As the Senate report accompanying the Federal-Aid Highway Improvement

Act of 1982 states, "Roads through areas administered by Federal land

managing agencies must be carefully designed to protect important

natural and cultural resources under the jurisdiction of those

agencies. Such roads must be designed to blend in with the natural

landscape. Because of the resources preserved in the Federal land

management areas, and the type of tourist use in such areas, the roads

in certain instances do not have to be constructed to normal highway

standards."

The standards contained herein provide flexibility in the planning and

design processes to allow for consideration of variations in types and

intensities of park use, for wide differences in terrain and climatic

conditions, and for protection of natural and cultural resources in

National Park System areas.

It is important to note that the standards vary considerably with the

type of use to be accommodated. Basic decisions will have to be made

by park management in the application of these standards based on

careful examinations of the desired use levels to be allowed

considering impacts on visitor use and resource protection in

conformance with legislative mandates.

The criteria presented have been adapted from available design

standards to meet the unique requirements of park roads. This will

provide a framework within which design and construction of park roads

should be conducted; however, this document is not intended to

encompass a level of detail comparable to that normally found in

design manuals.

These standards will supercede those adopted by the Service in 1968 as

existing park roads are reconstructed or when new roads are

constructed. On resurfacing, restoration and rehabilitation (3-R)

projects they will be utilized to the extent practicable and feasible.
ii

Acknowledgments

The 1983 Park Road Standards Task Force acknowledges its indebtedness

to the Park Roads Task Force appointed by National Park Service

Director George B. Hartzog, Jr., in 1967. Its members were Ansel

Adams, Ira Gabrielson, Joseph Penfold, Charles E. Krueger, Robert

Linn, and William C. Everhart, Chairman.

The Park Road Standards they compiled and published in 1968 set down

the basic philosophy of park roads which had evolved over many years

since the inception of National Parks. Their work provided the

foundation of our efforts, and significant portions of that earlier

document, in many instances verbatim, have been retained in this

document.

We also acknowledge our gratitude to the Federal Highway

Administration, U.S. Department of Transportation, whose technical

assistance and advice have been invaluab1e--particu1ar1y that of

Messrs. Thomas 0. Edick, Otto Mayr, Norman W. Loeffler, Robert Warren

and W. Larry Klockenteger.

Finally, we are indebted to the many NPS and FHWA employees who, over

the years, pioneered an internationally recognized tradition of

excellence in the design and construction of park roads and parkways.

* * *

Participants in 1983 Park Road Standards Task Force:

Jim J. Straughan, Civil Engineer, Denver Service Center,

Chairman

Robert R. Jacobsen, Superintendent, Shenandoah National Park

Merrick Smith, Landscape Architect, Denver Service Center

Donald A. Falvey, Civil Engineer, Rocky Mountain Region

Jerald M. Lorenz, Landscape Architect, Denver Service Center

George Walvoort, Civil Engineer, National Capital Region

John Singles, WASO Maintenance Division
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Table of Contents

Page

Preface .............................. i

Acknowledgments .......................... ii

- - - - - - - - - -

THE PURPOSE OF PARK ROADS ..................... 1

FUNCTIONAL CLASSIFICATIONS OF PARK ROADS ............. 3

Park Road System ........................ 3

Public Use Park Road .................... 3

Principal Park Road/Rural Parkway ............ 3

Connector Park Road ................... 3

Special Purpose Park Road ................ 3

Primitive Park Road ................... 4

Administrative Park Roads .................. 4

Administrative Access Road ................ 4

Restricted Road ..................... 4

Urban Parkways & City Streets ................ 4

Urban Parkway ...................... 4

City Street ....................... 4

FUNDAMENTAL CONSIDERATIONS IN PARK ROAD DESIGN .......... 7

PARK ROAD DESIGN STANDARDS .................... 11

Design Controls & Criteria ................... 12

Design Volume ........................ 12

Design Speed ........................ 12

Design Vehicle ....................... 13

Inter-Relationship of Design Controls ............ 14

Design Elements ........................ 18

Vertical Alignment ..................... 18

Grades .......................... 18

Vertical Curves ..................... 19

General Considerations .................. 21

Horizontal Alignment .................... 21

Maximum Degree of Curvature ............... 22

Superelevation Runoff & Transition Curves ........ 23

General Controls ..................... 23

Sight Distance ....................... 24

Stopping Sight Distance ................. 24

Passing Sight Distance .................. 26

Intersection Sight Distance ............... 26

Intersections ........................ 27

Number of Lanes ....................... 28

Cross-Section ........................ 28

Recreation Areas ...................... 31

Surface Crown ........................ 31
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Roadside Slopes & Drainage ................. 32

Guardrail/Guiderail ..................... 32

Curbs ............................ 33

3-R Projects .......................... 34

Traffic Volume ....................... 34

Design Speed ........................ 34

Pavement & Shoulder Widths ................. 34

Grades, Curvature & Sight Distance ............. 35

Bridges ........................... 35

Historic Structures ..................... 35

Structures, Signing & Maintenance ............... 36

Structures ......................... 36

Signing & Marking ...................... 36

Maintenance ......................... 36

GLOSSARY ............................. 39

* * *

List of Tables

Table Page

1 - Typical Design Speeds for Park Roads ............ 15

2 - Design Vehicle Dimensions ................. 16

3 - Maximum Grades ....................... 18

4 - K-Values for Vertical Curves, 

Stopping & Passing Sight Distance ........... 20

5 - Minimum Radius & Maximum Degree of Curve .......... 22

6 - Stopping Sight Distance .................. 24

7 - Stopping Sight Distance, One-Lane Roads .......... 25

8 - Minimum Passing Sight Distance ............... 26

9 - Sight Distance at Intersections .............. 27

10 - Minimum Roadway Cross-Section Requirements ......... 29

* * *

List of Figures

Table Page

1 - Typical Park Road System--Functional Classifications .... 5

2 - Typical Design Vehicle--Turning Path ............ 17

3 - Typical Park Road Cross-Section .............. 30

* * *
1

THE PURPOSE OF PARK ROADS

Among all public resources, those of the National Park System are

distinguished by their unique natural, historical, cultural and

recreational qualities–values dedicated and set aside by law to be

preserved for the benefit and enjoyment of people in such a manner as

will leave them unimpaired for future generations.

Ideally, perhaps no road would be permitted to violate or despoil the

sanctity of park resources. Pragmatically, the protection, use and

enjoyment of these values in a world of modern technology has

necessitated the encroachment of a system of public park roads. 

Today’s visitors are no longer required, as early motorist to

Yellowstone once were, to chain their cars to logs and turn over the

keys to the superintendent. In most parks today, the basic means of

providing for visitor access is the park road system. It is both a

means and an end. It enables one visitor to reach his goal; for

another, it is the goal.

The marked increase of park visitors in the latter half of the 20th

century represents both a profound threat to park values and an

extraordinary opportunity for those values to become more tangibly

significant in each individual’s recognition of our natural and

cultural heritage.

The fundamental purpose of national parks–-bringing humankind and the

environment into closer harmony–-dictates that the quality of the park

experience must be our primary concern. Full enjoyment of a national

park visit depends on its being a safe and leisurely experience. The

distinctive character of park roads plays a basic role in setting this

essential unhurried pace. Consequently, park roads are designed with

extreme care and sensitivity with respect to the terrain and

environment through which they pass–-they are laid lightly onto the

land.

Each segment of every park road should relate to the resource it

traverses in a meaningful way and should constitute an enjoyable and

informative experience in itself while providing the visitor the

utmost in visual quality. Long tangents that encourage high speeds–-

and only fleeting views of “kinetic scenery”–-should be avoided. The

horizontal and vertical alignment and cross-section should respect the

terrain, blending into the environs. A park road should be

fundamentally designed to maintain an overall continuing sense of

intimacy with the countryside or area through which it passes.

The purpose of park roads remains in sharp contrast to that of the

Federal and State highway systems. Park roads are not intended to

provide fast and convenient transportation; they are intended to
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enhance visitor experience while providing safe and efficient

accommodation of park visitors and to serve essential management

access needs. They are not, therefore, intended nor designed as

continuations of the State and Federal-aid network. Nor should they

be designed or designated to serve as connecting links to those

systems. And within parks, no road or other circulation system should

be planned or designed merely as a device to link points of interest.

As stated on a brochure that was once given to visitors when they

entered National Parks:

Park roads are for leisurely driving only. If you are in a

hurry, you might do well to take another route now, and come

back when you have more time.

* * *
3

FUNCTIONAL CLASSIFICATIONS OF PARK ROADS

PARK ROAD SYSTEM

A park road system includes those roads within or giving access to a

park or other unit of the National Park System which are administered

by the National Park Service, or by the Service in cooperation with

other agencies (16 U.S.C. 8-8f).

The National Park System encompasses many types of environments--

mountains, forests, coastal zones, deserts, and urban areas. Within

each park, there often exists a variety of terrains which offer

potential visitor experiences. Consequently, park road systems must

be appropriately designed to serve a wide range of functions in accord

with the broad statement of purpose in the foregoing chapter.

For purposes of functional classification, the routes which make up a

park road system are grouped based on use into three categories: 

Public Use Park Roads, Administrative Park Roads, and Urban Parkways

and City Streets.

The assignment of a functional classification to a park road is not

based on traffic volumes or design speed, but on the intended use or

function of that particular road or route.

Public Use Park Roads

All park roads that are intended principally for the use of visitors

for access into and within a park or other National Park System area

are included. This includes all roads that provide vehicular passage

for visitors, or access to such representative park areas as points of

scenic or historic interest, campgrounds, picnic areas, lodge areas,

etc. County, State, and U.S. numbered highways maintained by the

Service are included in this category for purposes of functional

classification.

Public Use Park Roads are subdivided into the following four classes:

Class I: Principal Park Road/Rural Parkway. Roads which constitute

the main access route, circulatory tour, or thoroughfare for park

visitors.

Class II: Connector Park Road. Roads which provide access within a

park to areas of scenic, scientific, recreational or cultural

interest, such as overlooks, campgrounds, etc.

Class III: Special Purpose Park Road. Roads which provide

circulation within public use areas, such as campgrounds, picnic

areas, visitor center complexes, concessionaire facilities, etc. 

These roads generally serve low-speed traffic and are often designed

for one-way circulation.
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Class IV: Primitive Park Road

Roads which provide circulation through remote areas and/or access to

primitive campgrounds and undeveloped areas. These roads frequently

have no minimum design standards and their use may be limited to

specially equipped vehicles.

Administrative Park Roads

The Administrative Park Road category consists of all public and nonpublic roads intended to be used principally for administrative

purposes. It includes roads servicing employee residential areas,

maintenance areas and other administrative developments, as well as

restricted patrol roads, truck trails, and similar service roads.

Administrative Park Roads are subdivided into two classes:

Class V: Administrative Access Road. All public roads intended for

access to administrative developments or structures such as park

offices, employee quarters, or utility areas.

Class VI: Restricted Road. All roads normally closed to the public,

including patrol roads, truck trails, and other similar roads.

Figure 1 illustrates the application of these functional

classifications to a hypothetical park road system.

Urban Parkways and City Streets

Urban parkways and city streets are generally dual-use facilities in

that they serve both park and non-park related purposes. In addition

to providing access to park areas, they also serve as extensions of

the local transportation network carrying high volumes of non-park

related traffic.

Class VII: Urban Parkway. These facilities serve high volumes of

park and non-park related traffic and are restricted, limited-access

facilities in an urban area. This category of roads primarily

encompasses the major parkways which serve as gateways to our nation's

capital. Other park roads or portions thereof, however, may be

included in this category.

Class VIII: City Street. City streets are usually extensions of the

adjoining street system that are owned and maintained by the National

Park Service. The construction and/or reconstruction should conform

with accepted engineering practice and local conditions.

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* In this document, the terms "park road", "park" and "National

Park" are used generically in reference to all units of the National

Park System.

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FUNDAMENTAL CONSIDERATIONS IN PARK ROAD DESIGN

National parks are unique. In that park roads serve a distinctly

different purpose from most other roads and highways, National Park

System road standards must also be unique.

Park roads* are constructed only where necessary, and only as

necessary, to provide access for the protection, use and enjoyment of

the natural, historical, cultural and recreational resources which

constitute our National Park System. National park roadways, where

they exist, are planned for leisurely sightseeing and are located with

sensitive concern for the environment and designed with extreme care. 

They are often narrow, winding, and hilly--but therein may lie their

appeal.

In some cases, park roads provide such unique experiences that they

have themselves become internationally recognized cultural resources--

Skyline Drive in Shenandoah National Park, Trail Ridge Road in Rocky

Mountain National Park, Going-to-the-Sun Road in Glacier National

Park.

Thus, park roads are often an end in themselves, rather than just a

means to an end, in contrast to more conventional highway systems. 

For some, such as the handicapped, roads may provide the only means of

park use, thereby reinforcing the case for their being intimately

blended with the resource. Where terrain and safety conditions permit

and where such uses are advocated by the general management plan,

opportunities should be provided for random stopping to enable park

visitors to more completely experience the park resources.

Urban Parkways in the Washington, DC area provide a broader service

than other park roads. They serve as attractive, landscaped gateways

to our nation's capital, and at the same time, share many of the highspeed, high-volume traffic characteristics of expressways of the state

and Federal highway network. Due to the nature of traffic use on an

Urban Parkway, a higher standard for design is necessary than is

required for most other park roads. The basic criteria for urban

parkways must be consistent with the intent of the Legislation which

authorized them. Planning, design, operation and maintenance of urban

parkways must consider traffic safety and protect and enhance the

landscape, aesthetic, environmental and cultural characteristics and

values which significantly distinguish urban parkways from

expressways. Urban parkways must be considered as a whole system and

all design elements such as landscaping, signs, lighting, guardrails,
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shoulder design, curbing, ramps, etc., must be considered as elements

of the system. Through careful planning and design, these parkways

can effectively blend high-volume-traffic safety with the aesthetic

and other values of park roads.

The location and design of park roads must continue to be in accord

with the philosophy that how a person views a park is as significant

as what he sees, thereby ensuring that national parks remain places

where people go for a unique and rewarding experience.

In line with this philosophy and Service policies, provision of roads

in a park must be consistent with approved management plans and be

limited to those necessary to carry out management objectives for the

particular area. Based on the Service's policy of minimizing resource

impacts, and where visibly compatible, park roads are combined with

other services (pipe lines, power lines, etc.) into common corridors. 

Their provision, location, design, and construction and the

construction materials used must be consistent with the perpetuation

and protection of the resources, aesthetic values of the area, and

maintainability of the roadway and roadside. While park roads are

designed differently from other roads, they are designed, constructed,

and maintained within the norms of sound geometric standards for

safety and structural sufficiency.

In most parks, a road system is already in place, having been

constructed in accordance with the National Park Service policies. In

updating plans for these parks, the Service will evaluate the existing

road system and determine whether it needs to be curtailed, expanded,

or supplemented by other circulatory modes. Where roads are

chronically being used to capacity, the use of alternate

transportation systems, limitations on use and other traffic

management techniques should be considered as an alternative to road

improvements.

When the Service is faced with a choice between creating a severe road

scar to bring visitors to a destination point, or requiring visitors

to walk a considerable distance or to utilize an alternate

transportation system--the decision should be against the scar. Those

who visit national parks do so for a unique experience, and should be

willing to accept necessary restrictions.

Park roads cannot accommodate all types of vehicles, nor can they

accommodate all levels of speed, without violating these principles. 

While the travel industry continues to develop new kinds of vehicles,

the Service is not obligated to construct roads or to manage traffic

so that all forms of modern transportation technology can be

accommodated within the park.
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Recent transportation trends have significantly affected the use of

NPS roads. There have been substantial increases in the numbers of

recreational vehicles (RV's), bicycles, tour buses and smaller less

powerful automobiles using park roads since 1968, when the previous

road design standards were adopted.

The growth in popularity of RV's (such as motor-homes, pickup campers,

and passenger cars with travel trailers, which are characterized by

greater dimensions, slower operation, and, frequently, inexperienced

operators) is a relatively recent phenomenon. The recreation vehicle

represents a significant element in the traffic service requirements

on park roads. Design of park roads should reflect, to the extent

possible where such vehicles are permitted, the fact that RV's have

different operational and safety characteristics than automobiles.

Bicycles are also rapidly becoming a significant mode of personal

transportation, particularly for recreational purposes. Scenic

bicycle riding and touring have recently achieved an unprecedented

popularity. Bicycle riding on park roads shared with automobiles and

other vehicular traffic can be hazardous and frightening for both

cyclists and motorists. The narrow pavement sections on many park

roads create significant hazards where bicycles, often laden with

picnicking or camping gear, are mixed with other traffic.

An increase in the number of tour buses on park roads has been largely

caused by two recent travel trends: growth in the domestic market of

older persons travelling via tour buses to National Parks and a

substantial increase in foreign visitors with a cultural preference

for organized bus tours. The growth in absolute numbers of large tour

buses on park roads has serious highway safety implications, resulting

from larger numbers of wide vehicles operating on relatively narrow

roads. The resultant increase in the number of repeated heavy-axle

loadings is also detrimental to the service life of road pavements

that were not designed for that type of vehicle. This has resulted in

greatly increased park road maintenance costs.

When existing park roads are analyzed, a determination of the size and

types of vehicles that can be safely accommodated will be made. It

may be desirable for vehicles exceeding these limits to be excluded,

rather than reconstructing the roads to higher standards. Appropriate

alternatives include: providing parking areas for large vehicles at

park entrances; restricting vehicular traffic in certain portions of a

park; converting two-way roads into one-way systems; reducing speed

limits to protect both visitors and wildlife; and furnishing alternate

transportation systems.
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When new roads are to be constructed, they should be planned to

preserve the integrity of the surroundings, respect ecological

processes, protect park resources, provide the highest visual quality,

and meet modern engineering and safety standards commensurate with the

intended use to ensure a fully rewarding and safe visitor experience. 

These are the principles which dictate the means of visitor access and

the development of road design standards for the National Park System. 

Safeguarding these principles and applying the standards that follow

is, therefore, a multi-disciplinary undertaking--a process combining

the policies and decisions of management with the talents of

engineers, designers, planners, architects, and landscape architects,

and incorporating the contributions and expertise of protective and

interpretive professionals, and cultural and natural resource

specialists.

* * *
11

PARK ROAD DESIGN STANDARDS

Road design in the National Park System is based on the need to

provide reasonable, leisurely and safe visitor access to natural,

scenic, historic and recreational features and on facilitating the

administration and protection of park resources.

Development of the existing system of roads in the National Park

System units took place over a relatively long period of time. 

Roadway designs that were appropriate for the types and magnitude of

vehicular traffic when much of the park road system was constructed

may no longer be adequate.

The road standards developed herein provide criteria based on those

design elements considered necessary to accommodate the various,

potential levels of vehicular and pedestrian use that National Park

Service management may decide to permit. The absolute controlling

factor in application of these standards is the level of actual, or

permitted and controlled use. The basic alternative available, where

application of a particular standard is deemed incompatible with

resource protection considerations, is limitation of levels of use,

types of use, or both.

In light of the foregoing, these standards have been developed to

provide definitive guidelines for those involved in making decisions

affecting traffic service and circulation of park visitors. They are

intended to be applied uniformly to both new construction and

reconstruction of park roads on a Service-wide basis to the extent

practicable, based on projections of actual, or planned and controlled

use.

On rehabilitation, restoration and resurfacing (3-R) projects, the

standards applicable to new construction and reconstruction will in

some instances not be attainable. Each 3-R project must be considered

on a case-by-case basis to determine the feasibility and long term

cumulative effects of any improvements. (See pp.34 ff.)

Where resource preservation issues relating to a construction,

reconstruction, or 3-R projects preclude application of an appropriate

design standard based on existing traffic uses, then an alternative is

restriction of use to a level consistent with the roadway geometries

which can be developed without adversely affecting preservation of the

resource. This may be particularly desirable in unusually sensitive

natural areas or in conjunction with historic sites where the

integrity of the original or restored historic fabric may be

jeopardized.
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Often cited as the most important principle relating to safety in road

design is consistency. Attempting to conform all design elements and

features to the driver's expectations and avoiding abrupt changes in

the application of standards greatly contributes to the provision of a

smooth-flowing, accident-free facility.

DESIGN CONTROLS AND CRITERIA

In road design, various controls and criteria are employed to ensure

that the facility will safely accommodate the expected traffic

requirements and to encourage consistency and uniformity of operation. 

Primary considerations in the design of park roads are the types of

terrain traversed, environmental constraints, and the desired visitor

experience. These considerations are addressed through the selection

and application of appropriate design controls. The major road design

controls for park roads are design volume, design speed, and design

vehicle.

Design Volume

A design volume should be established to represent the anticipated, or

planned and controlled , traffic use of the roadway during the park's

normal visitor season. The design volume describes the traffic load

that the road must be able to accommodate at an appropriate level of

service and determines to a large degree the type of facility and

pavement widths required, as well as other geometric features.

The current average daily traffic (ADT) may be used where present

traffic volumes are below 4,000 vehicles per day and are not expected

to grow significantly over time. For park roads that have a current

ADT above 4,000 vehicles per day and/or that are experiencing rapid

growth of traffic, the selected design traffic volume should be based

on planned use projected to some future year--usually up to 20 years

beyond the year of construction. It may not be practical, however,

(particularly for urban parkways) to design to accommodate projected

traffic.

On major reconstruction projects where resource protection issues

which conflict with the reconstruction are identified, a special study

will be required to thoroughly define and evaluate management

alternatives. Based on the results of that analysis of alternatives,

a decision can be made as to the appropriate design volume.

Design Speed

Design speed is the primary control that correlates with the physical

features of design to achieve a roadway that will safely accommodate

traffic for the planned use. The design speed affects such roadway
13

features as curvature, super elevation, sight distance, and gradient. 

Selection of this speed is primarily influenced by the purpose of the

particular park road, the desired traffic volumes, and the character

of terrain and environmental considerations.

Table 1 shows the relationship of typical design speeds to traffic

volumes for the different classes of park roads based on flat,

rolling, and mountainous terrain conditions and was specifically

developed to provide a range of design speeds appropriate to the park

environment.

By regulation, the maximum posted speed limit for any park road cannot

exceed 45 mph, unless a Superintendent exercises his authority to

establish a higher speed limit where this maximum limit is determined

to be less than is reasonable or safe (36 CFR 4.17). Park roads may

have a design speed up to 10 mph higher than the posted speed to allow

a margin for safety.

Once a design speed is selected, all geometric features should be

related to it. Changes in terrain and other physical controls, or

environmental factors such as roadside wildlife may dictate a change

in design speed for certain segments. Any decrease in design speed

along a road should not be introduced abruptly; this decrease should

be extended over a sufficient distance to allow the driver to adjust

to the transition to a slower speed. Pavement and shoulder widths and

clearances to walls and rails are less directly related to design

speed; however, they can affect capacity, vehicle speeds and safety. 

Consequently, higher standards for those features should be used on

roads with higher design speeds.

Desiqn Vehicle

Another major control in geometric design of park roads is the design

vehicle(s), which is based on the types of vehicles that may be

permitted by park management to use the facility. The physical

dimensions and operating characteristics of the design vehicle are

used to develop sight distance, cross-section, intersection design and

other geometric design criteria. Existing and anticipated types of

vehicles to be using park roads must be examined to establish

representative vehicles for use in the process of designing the

roadway.

Table 2 is provided for informational purposes only and lists

pertinent dimensions for ten vehicle types. Minimum turning paths of

the design vehicles are particularly important. The governing paths

are those of the outer front overhang and the inner rear wheel. The

outer front wheel is assumed to follow a circular arc, which is the

minimum turning radius as determined by the vehicle's steering

mechanism. Figure 2 illustrates these requirements for a typical

design vehicle.
14

Interrelationship of Design Controls

The road design process involves identifying on a segment-by-segment

basis the design speed for a planned design volume and selected design

vehicle.

Design speed for park roads is largely determined by the character of

the terrain, the resource traversed, and the planned visitor

experience as described in the park's General Management Plan. In

cases where these considerations control design speed, the planned

design volume and/or design vehicle may require appropriate

adjustment.

For design purposes, topography is generally classified based on three

types of terrain conditions:

S Level terrain is that condition where highway sight distances, in

relation to both horizontal and vertical restrictions, are generally

long, or can be made so, without construction difficulty or undue

adverse affects.

S Rolling terrain is that condition where the natural slopes

consistently rise above and fall below the road grade, and where

occasional steep slopes offer some restriction to normal horizontal

and vertical roadway alignment.

S Mountainous terrain is that condition where longitudinal and

transverse changes in ground elevation, with respect to the road,

are abrupt, frequently requiring benching and sidehill excavation to

obtain acceptable horizontal and vertical alignment.
15

Table 1

Typical Design Speeds (mph)

FUNCTIONALCLASSIFICATION

DESIGN VOLUME

Vehicles/Day

TERRAIN

FLAT

Design Speed

ROLLING

Design Speed

MOUNTAIN

Design Speed

Preferred Minimum Preferred Minimum Preferred Minimum

I-Principal Park

Road and Rural

Parkway

< 200 40 30 35 25 30 20

200-400 50 40 40 30 30 20

400-1000 50 40 40 30 35 25

1000-4000 55 45 45 35 40 304000-8000 60 50 50 40 40 30

> 8000 60 50 60 50 50 40

II-Connector,

III- Special

Purpose,

V-Administrative,

or

VI-Restricted

< 400 30 15 30 15 30 15

400-1000 35 25 35 25 30 15

1000-4000 40 25 40 25 35 25

> 4000 40 25 40 25 40 25

VII-Urban Parkway

< 4000 55 45 45 40 45 40

4000-8000 60 50 50 40 45 40

> 8000 65 55 60 50 50 40

Note: Posted speeds will normally be 5-10 mph less than

design speed at design speeds above 30 mph.Environmental concerns should be consideredwhen selecting design speed.
16

Table 2

Design Vehicle Dimensions

(Dimensions in Feet)

Design VehicleType Symbol Wheelbase

Front

Overhang

Rear

Overhang

Overall

Length

Overall

Width Height

Min.

Turning

Radius

Min.

InsideRadius

Passenger Car P 11 3 5 19 7 - 24 15.3Passenger Car w/Travel Trailer PT 34 3 10 49 8 - 24 5.5Passenger Car w/BoatTrailer PB 31 3 8 42 8 - 24 10.0Motor Home MH 20 4 6 30 8 13.5 42 28.4Single Unit Truck SU 20 4 6 30 8.5 13.5 42 28.4Single Unit Bus Bus 25 7 8 40 8.5 13.5 42 23.2Articulated Bus A-Bus 18+24=42 8.5 9.5 60 8.5 13.5 38 21.0Semi-TrailerCombination,Intermediate

WB-40 13+27=40 4 6 50 8.5 13.5 40 19.9

Semi-TrailerCombination,Intermediate

WB-50 20+30=50 3 2 55 8.5 13.5 45 19.8

Semi-TrailerCombination,Intermediate

WB-60 9.7+20+9.4+2 0.9=60 2 3 65 8.5 13.5 45 22.5

Compiled From: A Policy on Geometric Design of Highways and Streets.

AASHTO, 1984

18

DESIGN ELEMENTS

The foregoing controls and criteria have a major influence on the

geometries of park roads and other elements of design. The elements

which follow have been tailored to meet the special needs and

limitations of National Park System roads. While all these elements

are closely interrelated, each influencing the others, all must be

properly coordinated in their application to a particular project.

Key among the park road design elements are horizontal and vertical

alignment, which have a subtle, yet very important interrelationship. 

The topography of the land traversed influences both, but is more

evident in vertical alignment. It is the skillful manipulation of

these two elements in a manner compatible with the terrain traversed

which results in the most distinctive traditional characteristic of

park roads--curvilinear alignment.

Vertical Alignment

Vertical alignment consists of ascending and descending grades

connected by parabolic curves.

Grades - Maximum allowable design grades in relation to design speed

and type of topography are shown in Table 3.

Table 3

Maximum Design Grades in Percent

Type of

Topography

Design Speed (mph)

15 20 25 30 35 40 45 50 55 60 65

Flat 8 8 7 7 7 7 6 5 4 4 3

Rolling 11 11 10 10 9 9 8 7 6 5 4

Mountain 17 16 15 14 13 12 10 9 7 6 6

Compiled From: A Policy on Geometric Design of Highways and

Streets. AASHTO, 1984
19

Maximum design grade should be used very infrequently, and should not

be considered a value to be applied in most cases. The grades shown

in Table 3 relate primarily to the operational performance of

vehicles. Other concerns in the selection of a maximum grade are the

capability of the soil for erosion resistance, the type of surface and

cross-section of the roadway, the drainage treatment, and the length

of maximum grade.

Short grades, less than 500 feet in length, and one-way down-grades

may be 1% steeper than those shown in Table 3. In extreme cases

(e.g., at some underpasses and bridge approaches), steeper grades for

relatively short lengths may be considered. For low-volume roads,

grades may be 2% steeper.

Critical length of grade is usually not a major concern for park

roads. However, appropriate consideration should be given to this

element of design for roads constructed at higher design speeds or

roads with large numbers of tour buses or recreational vehicles.

Flat and level grades are not objectionable on uncurbed pavement when

the crown is adequate to drain the surface laterally. On curbed

pavements, a minimum grade of 0.5% is normally required, but a grade

of 0.35% may be used where there is high quality pavement, accurately

crowned and supported by firm subgrade.

Vertical Curves - Vertical curves are used to effect a gradual change

between tangent grades. Vertical curves should be simple in

application and result in a design that is safe, comfortable in

operation, pleasing in appearance, and adequate for drainage.

The major control for safe operation on crest vertical curves is the

provision of ample sight distances for the design speed. Minimum

stopping sight distance should be provided in all cases. Additional

stopping sight distance should be provided at decision points

(intersections, overlooks, etc.) and, where feasible, more liberal

distances should be used.

The rate of grade changes should be kept within tolerable limits. 

This consideration is most important in sag vertical curves, where

gravitational and vertical centrifugal forces act in the same

direction. Appearance should also be considered--a long curve is

generally more pleasing in appearance than a short one, which may give

the appearance of a sudden break in the profile due to the effect of

foreshortening, particularly when combined with horizontal curvature. 

Wherever both horizontal and crest vertical curves are required, the

crest vertical curves should fall within the horizontal curve.
20

For simplicity, a parabolic curve is used to affect a gradual change

between grades. Parabolic curves are identified by their lengths and

the algebraic difference of the grades they connect. The minimum

length of vertical curve may be computed from the following formula:

L = KA, where:

L = the length of the vertical curve in feet;

K = the distance in feet required to affect a one percent change

in gradient; and

A = the algebraic difference in grades in percent.

The selection of a K value for crest vertical curves is based on sight

distance requirements. For sag vertical curves, K is based on

headlight sight distance, using criteria based on a light beam

emanating from a source 2.0 feet above the pavement on a 10 upward

divergence from the longitudinal axis of the vehicle to where the beam

intersects the roadway surface. The formula computes minimum lengths

of vertical curve for either crest or sag curves; however, K values

are different for each design speed and condition. Table 4 gives the

K values to be used.

Table 4

K Values for Vertical Curves

Stopping Sight Distance

Design

Speed

Passing Sight

Distance Crest Curves Sag Curves

mph ft. Minimum Desirable Minimum Desirable

15 149 5 5 8 8

20 210 10 10 20 20

25 300 20 20 30 30

30 400 30 30 40 40

35 550 40 50 50 50

40 730 60 80 60 70

45 890 80 120 70 90

50 1050 110 160 90 110

55 1230 150 220 100 130

60 1430 190 310 120 160

65 1720 230 400 130 180

Compiled From: A policy on Geometric Design of Highways and

Streets. AASHTO, 1984
21

General Considerations in Vertical Alignment - In addition to grade

and vertical controls, applied in concert with the criteria of

providing safety and respecting the terrain, other considerations in

designing vertical alignment include:

1. Smooth gradelines with gradual changes, consistent with the class

of park road and character of terrain, are preferable to a line

with numerous breaks and short lengths of grade.

2. "Rollercoaster" or "hidden-dip" profiles should be avoided. 

These types of profiles are aesthetically unpleasant and

hazardous.

3. Undulating gradelines with relatively long grades should be

evaluated to determine their effect on traffic operation,

particularly where there are significant volumes of recreational

vehicles and tour buses.

4. A broken-back gradeline (two vertical curves in the same

direction separated by short sections of tangent grade) generally

should be avoided, particularly in sags where the full view of

both vertical curves is not pleasing.

5. On long grades, it may be preferable to place the steepest grades

at the bottom and to lighten the grades near the top of the

ascent, or to break the sustained grade with short intervals of

lighter grade, instead of utilizing a uniform sustained grade. 

This is particularly applicable to low-design-speed roads.

6. Grades through at-grade intersections on roads with moderate or

steep grades should be 6% or less whenever possible.

Horizontal Alignment

Horizontal alignment consists of tangents and horizontal curves that

are circular curves with constant radius. Transitional spiral curves

may be used to connect the tangents to the horizontal curve. Criteria

for determining minimum radius (or maximum degree of curvature) are

based on laws of mechanics, with design values depending on practical

limits for superelevation and frictional factors representative of

pavement surfaces.

Maximum superelevation rates are controlled by several factors that

may vary widely: (1) frequency and amount of snow and ice; (2) extent

of development in the area; and (3) frequency of slow-moving vehicles. 

Maximum superelevation (e) values range upward to 0.12 for roadways in

undeveloped areas where there is no snow and ice. Consideration

should be given to limiting maximum superelevation on steep grades. 

Another variable that influences maximum curvature is the side

friction factor (f). The minimum radii and maximum degree of curvature

for various design speeds in relation to superelevation rate are shown

in Table 5.
22

Table 5

Minimum Radius & Maximum Degree of Curve

Design Speed (mph)

15 20 25 30 35 40 45 50 55 60 65

Maximum

Superelev.

(ft/ft)

Minimum Radius (ft)

0.04 74 127 202 302 425 573 749 955 1207 1528 1878

0.06 71 116 184 273 381 509 663 849 1075 1348 1637

0.08 60 107 166 252 349 468 606 764 958 1206 1528

0.10 55 99 15 231 322 432 557 694 869 1091 1348

0.12 52 95 143 215 301 400 513 640 789 960 1146

Maximum Degree of Curve (rounded)

0.04 77 45 28 19 13 10 7.5 6.0 4.5 3.75 3.0

0.06 81 49.25 31 21 15 11.25 8.5 6.75 5.0 4.25 3.5

0.08 95 53.5 34.5 22.75 16 12.25 9.0 7.5 6.0 4.75 3.75

0.10 104 58 37 24.75 17.5 13.25 10 8.25 6.5 5.25 4.25

0.12 110 62.5 40 26.5 19 14.5 11 9.0 7.5 6.0 5.0

Compiled From: A Policy on Geometric Design of Highways and

Streets. AASHTO, 1984

Note: The values shown are for wet, bituminous or concrete paved

surfaces with the friction factor varying from 0.11 at 65 mph to

0.17 at 20 mph. For aggregate loose-surface roads, use a

friction factor of 0.12 maximum at 10 mph to 0.10 at 30 mph. 

Where snow and ice conditions are prevalent, maximum

superelevation should not exceed 0.06.

A maximum superelevation rate of 0.06 is suggested at locations on

Class II-V roads where there is a tendency to drive slowly. On roads

with a design speed of 20 mph or less, superelevation may not be

warranted. On low-volume park roads with gravel or dirt surfaces,

different relationships between minimum radius and superelevation

exist due to lower side-friction values. In general, longer radius

curves are required for a given design speed and rate of

superelevation where unpaved road surfaces exist.
23

In addition to the controls on maximum curvature, there is a need to

provide minimum stopping sight distance around curves which may

control the minimum radius of curve when sight distance cannot

otherwise be provided by removing the sight obstruction.

Superelevation Runoff and Transition Curves - Spiral curves used as a

transition between curves and tangent sections provide a natural,

easy-to-follow driving path, can enhance roadway appearance, and

provide a desirable arrangement for superelevation runoff.

General Controls for Horizontal Alignment - In addition to the

specific design elements for horizontal alignment, there are the

following other general controls:

1. Alignment should be as directional as possible consistent with

topography and resource protection considerations; long flowing

curves fit to the topography are preferable to long tangents

which slash artificially across the land.

2. Two lane road alignments should provide as many safe passing

sections as possible. Turn-outs may be provided as an

alternative when passing sections are not feasible.

3. Maximum allowable degree of curvature should be avoided whenever

possible.

4. Consistent alignment should be sought; sharp curves at the end of

long tangents or at the end of long, flat curves should be

avoided.

5. Short lengths of curves should be avoided on flat curves to avoid

the appearance of a kink.

6. Compound circular curves, should generally be avoided, but if

used differences in radii should not exceed a ratio of 2 to 1 at

the lower design speeds, and should not exceed a ratio of l 1⁄2 to

1 at design speeds above 50 mph.

7. In general, direct reverse curves should be avoided, and a

tangent length should be used between them.

8. "Broken-back curves" (two curves in the same direction on either

side of a short tangent or large radius curve) should be avoided.

9. The horizontal alignment should be coordinated carefully with the

vertical alignment. The beginning and ending of vertical and

horizontal curves should not occur at the same station.
24

Sight Distance

Sight distance is the length of roadway on which another vehicle or

obstruction is continuously visible to the driver. Minimum stopping

sight distance and passing sight distance are directly related to the

design speed of the road. Intersection corner sight distance is a

direct function of the design speed of the through road only when the

minor road is stop controlled.

Stopping-Sight Distance - Stopping sight distance used for road design

is the sum of two distances: (1) the distance a vehicle travels after

the driver sights an immobile vehicle or object in the roadway and

before braking occurs; and (2) the distance traveled during braking. 

Criteria for measuring stopping sight distance assume an eye height of

3.5 feet and an object height of 6 inches.

The minimum and desirable stopping sight distances for roads at

various design speeds are shown in Table 6. Minimum distances assume

that the vehicle is traveling at less than the design speed, while the

desirable distances assume that the vehicle is traveling at the design

speed.

Table 6

Stopping Sight Distance

(Wet Pavements)

Stopping Sight Distance *

Design Speed

(mph)

Minimum

 (ft)

Desirable

(ft)

15 80 80

20 125 125

25 150 150

30 200 200

35 225 250

40 275 325

45 325 400

50 400 475

55 450 550

60 525 650

65 550 725

* Distances rounded for design.

Compiled From: A Policy on Geometric Design of Highways and Streets. 

AASHTO, 1984
25

On two-directional, one-lane roads, enough sight distance must be

allowed to enable one of two vehicles approaching from opposite

directions to reach a turnout or for both to stop before colliding. 

Criteria for measuring stopping sight distance for one-lane roads

assume an eye height of 3.5 feet and an opposing-vehicle height of

4.25 feet. The stopping sight distance for a two-way, one-lane road

must be approximately twice the stopping sight distance for a two-lane

road. Suggested stopping sight distances and "K" values for one- and

two-directional, one-lane roads are given in Table 7.

A "K" value is a coefficient by which the algebraic difference in

grade is multiplied to determine the length in feet of the vertical

curve which will provide minimum sight distance. The length in feet

should not be less than three times the design speed in mph for any

vertical curve.

Table 7

Stopping Sight Distance

One-Lane Roads

Design Speed (mph) 15 20 25 30 35 40 45 50 55

One-Directional

One-Lane Roads

Stopping Sight - minimum

Distance (ft) - desirable

80 125 150 200 225

250

275

325

325

400

400

475

450

550

K Values for:

 Crest Vertical - minimum

 Curve - desirable

5 10 20 30 40

50

60

8

80

120

110

160

150

220

 Sag Vertical - minimum

 Curve

8 20 30 40 50

50

60

70

70

90

90

110

100

130

Two-Directional

One-Lane Roads

(These design speeds

are not recommended for

two-directional, onelane roads.)

Stopping Sight

 Distance (ft) 175 250 400

K Values for:

 Crest Vertical Curve

 Sag Vertical Curve

10

11

20

20

52

40

Compiled From: A Policy on Geometric Design of Highways and

Streets. AASHTO, 1984
26

Passing Sight Distance - Safe passing sight distance applies only to

two-lane, two-way roads. Passing sight distance is the length of

highway ahead necessary for one vehicle to pass another before meeting

an on-coming vehicle that appears after the passing maneuver has

begun. Passing sight distances used for design are given in Table 8.

These distances for design should not be confused with the values for

no-passing zone pavement markings on completed roads shown in the

MUTCD. For multi-lane roadways, stopping sight-distance criteria

control.

Table 8

Minimum Passinq Siqht Distance

for Design of Two-Lane Roads

Design Speed

(mph)

Minimum Passing

Sight Distance

(ft - rounded)*

15 700

20 800

25 1000

30 1100

35 1300

40 1500

45 1700

50 1800

55 2000

60 2100

65 2300

* These distances assume that the

passed vehicle is traveling 10 mph

slower than the passing vehicle.

Compiled From: A Policy on Geometric Design of Highways and

Streets. AASHTO, 1984

Minimum passing sight distance should be provided as frequently as

possible, particularly on Class I roads that require visitors to

travel considerable distances before reaching activity sites.

Intersection Sight Distance - Intersections should be planned and

located to provide as much sight distance as possible. To achieve a

safe design, sufficient sight distance should be provided as a minimum

to allow a driver to cross the through road without requiring

approaching traffic to reduce speed. Minimum intersection sight

distances for different design speeds are shown on Table 9.
27

Table 9

Sight Distance at Intersections

Design Speed

(mph) 15 20 25 30 35 40 45 50 55 60

Minimum Corner

Sight Distance*

(ft)

150 200 250 300 350 400 450 500 550 600

* Corner sight distance is measured from a point on the minor

road at least 15 feet from the edge of the major road traveled

way and measured from an eye height of 3.5 feet on the minor

road to an object height of 4.25 feet on the major road.

Compiled From: A Policy of Geometric Design of Highways and

Streets. AASHTO, 1984

Intersections

As a general rule, the alignment and grade at or near intersections

are more critical than on the open road. The sight distance should

exceed the minimum. 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. Sight distance considerations also apply where roads intersect

with pedestrian, equestrian, and bicycle facilities.

Roads intersecting at acute angles tend to restrict visibility and

traffic flow to one direction. The smallest angle formed by the

intersecting roads should not be less than 60o. Right angle

intersections are desirable.

Intersections that are slightly offset from each other on opposite

sides of the main road should be avoided. More than two roads

intersecting at one location tend to cause traffic management problems

and should also be avoided.

Intersections on sharp curves and grade combinations that make vehicle

control difficult should be avoided. The gradeline of the main road

should be carried through the intersection and that of the

intersecting road should be adjusted to it. The grade of the

intersecting road approaching the main road should be 6% or less, and

when practical, should be flattened to approximately 1% for a distance

sufficient to accommodate stopping and storage of the design vehicle.
28

It is advisable to provide sufficient width for a vehicle to pass

another vehicle stopped at the intersection. The combination of the

width of the main road and the radius of the taper of the intersecting

road should provide adequate width for vehicles entering or leaving

the main road. For intersections where significant volumes of turning

maneuvers occur, consideration should be given to providing turning

lanes. Where turning roadways are used, adequate lengths for

merge/diverge lanes should be provided.

Number of Lanes

The number of lanes should be sufficient to accommodate the design

traffic volume. For low-volume park roads, capacity conditions do not

normally govern design, and two travel lanes are appropriate. 

However, on extremely low-volume access roads to remote areas, onelane, two-way roads with frequent turnouts may be considered. Where

passing opportunities are significantly limited, auxiliary passing

lanes or turnouts should be considered. When the design volume

exceeds 8,000, a four-lane divided roadway section may be required to

accommodate traffic. For multi-lane roads, the design hourly volume

controlling lane capacity is 1,500 vehicles/hour/lane.

Cross-Section

The roadway cross-section consists of traveled ways, auxiliary lanes,

shoulders, medians, and roadsides. Proper roadway width is selected

on the basis of numerous factors including park resource

considerations, existing and/or planned volumes and types of traffic,

safety, terrain, and design speed. A typical cross-section of park

road is illustrated in Figure 3.

Traveled Way - The traveled way is that portion of the roadway

available for movement of vehicles, exclusive of shoulders and

auxiliary lanes. It is usually comprised of two or more traffic

lanes.

Shoulders - The term shoulder describes that portion of the roadway,

contiguous with the traveled way, intended for the accommodation of

stopped vehicles, for emergency use, and for lateral support of

subbase, base, and surface courses. The shoulder on park roads with

low traffic volumes serves essentially as structural lateral support

for the road surfacing and as an additional emergency width for the

narrower traveled way. Where there is appreciable traffic volume,

roads with narrow surfacing and narrow shoulders often give poor

traffic service, may have high accident experience, and require

frequent and costly maintenance. The low traffic volumes and

relatively low operating speeds on most park roads do not warrant wide

shoulders.
29

In addition, wide shoulders may be environmentally and aesthetically

objectionable, or may encourage undesirable random stopping or

parking. However, as design volumes, design speeds or vehicle sizes

increase, additional shoulder width is required for safety and

structural support. Shoulders may be dirt, gravel, paved, turf,

stabilized turf or a combination of surfaces, depending upon land

surface type, bicycle use, climate, maintainability and aesthetic

objective.

These facts and concerns are reflected in the lane and shoulder widths

shown in Table 10.

Table 10

Minimum Roadway Cross-Section Requirements

Average

Daily

Traffic

(ADT)

Number of

Lanes

Lane Width

 Feet a/

Shoulder

Width

Feet/Side

Lane Surface

Type(s)

<50 2 8 1 Dirt/Gravel/Paved

50-200 2 9 1 Dirt/Gravel/Paved

200-400 2 9 2 Gravel/Paved

400-1000 2 10 3 Paved

1000-4000 2 11 3 Paved

4000-8000 2 11 4 Paved

>8000 4 12 8 b/ Paved

a/ Widening of traffic lanes should be provided on the inside

of sharp curves. Where tour buses are allowed or the

proportion of recreational vehicles exceeds 5% of the design

volume, an additional foot of lane width shall be

considered, not to exceed 12 feet.

b/ Would only apply, as appropriate, to urban parkways.

Where guardrail is used, the graded width of shoulder should be

increased to provide about 2 feet outside the guardrail posts to

provide lateral support.

The total roadway width (including shoulders) for low-volume, onelane, one-way roads should not exceed 14 feet because of the tendency

of drivers to use a wider facility as a two-lane road.

31

Recreation Areas

As a rule, Class I roads in areas used primarily for recreation serve

functions broader than other park roads. Accordingly, where necessary

to accommodate recreational vehicles and boat trailer use, traveledway widths for two-way roads in recreation areas should be 24 feet of

pavement (two 12' lanes) plus shoulders of 4 feet. In those

recreation areas where the road is part of a through highway, it is

not necessary to apply a higher standard within the area than exists

for the roadway outside the area, provided minimum park road standards

have been met.

Bikeways

If bicycling is encouraged, consideration must be given to providing

safe travel ways. Separate bikeways are normally the safest

alternative and should be considered. Where this is not practical,

and where a wider road section can be accommodated, shoulder areas may

be improved to provide reasonable separation of bicycles from higherspeed traffic. Shoulder areas intended for bicycle use should provide

4 feet of width per direction of travel and should be smooth surfaced,

and delineated and marked to indicate the usage. Two-way bicycle

traffic on the same shoulder should not be allowed. On low-volume,

low-speed roads, the sharing of travel lanes may be considered.

Surface Type

The type of roadway surface to be used is determined by the desired

visual appearance, the volume and composition of traffic,

environmental considerations, soil conditions, availability and cost

of materials, and the extent and cost of maintenance. Table 10 shows

the suggested surface types for various design volumes. High volume

traffic justifies high-type pavements with smooth riding qualities and

good non-skid properties in all weather. Most park roads should have

a surface which will retain the cross-section, which will adequately

support the planned volume and weights of vehicles without failure to

keep non-routine maintenance to a minimum, and which will be

harmonious with the park environment.

Surface Crown

Surface cross-slope must be provided to ensure adequate drainage. 

However, excessive surface sloping can cause steering difficulties. 

Cross-slope for paved surfaces should range from 0.01 to 0.03 ft/ft. 

For unsurfaced dirt or gravel roads, a cross-slope ranging from 0.04

to 0.06 ft/ft should be provided to allow adequate drainage. On onelane roads with low-type surfaces, a crown would not usually be

provided. Roads of this type should be slope-graded to provide for

proper drainage.
32

Roadside Slopes and Drainage

Where terrain conditions and park resources permit, backslopes,

foreslopes, and roadside drainage channels should have gentle, wellrounded transitions which will be achieved through slope rounding. 

Generous rounding at the top of backslopes is especially important to

minimize erosion and ensure long term stability and revegetation of

cut slopes. Each end of a cut-or-fill-slope should be warped to

provide a pleasing appearance. Flatter foreslopes are generally more

stable and facilitate revegetation. The maximum rate of foreslope

depends on terrain conditions and the stability of soils as determined

by local experience. In desert country where fill-slopes do not

vegetate readily, embankments or slopes should be overlaid or

"painted" with rock or soil of matching color.

The ditch foreslope used in design should be related to design speed,

type of terrain, soil types, and resource management considerations. 

Preferably the ditch foreslope should be 6:1 for urban parkways and

4:1 for Class I Park Roads. The first 10 feet of the fill slope

should also be 4:1 where practicable to provide emergency pull off,

yet discourage random parking.

Cut sections should be designed to provide for adequate ditches or

other drainage features to ensure positive drainage. The ditch must

be large enough to accommodate the design flows and deep enough to

provide for satisfactory drainage of the pavement base. In

appropriate cases, underdrains should be used to lessen impacts. In

the case of rock cuts, drill holes or tool marks should not be visible

from the roadway.

Drainage structures, channels, and ditches must be hydraulically

designed based on sound principles of hydrology and based on the

required considerations of the effect on floodplains.

Guardrail/Guiderail

Guardrails or guardwalls are barriers intended to redirect an errant

vehicle, thus preventing it from hitting a roadside or median hazard. 

Guiderails, quidewalls or guide posts are intended only to delineate

the roadway, or to warn of roadside hazards.

Guardrail or guardwalls should be installed at points of unusual

danger such as sharp curves and steep embankments, particularly at

those points that are unusual compared with the overall

characteristics of the road. The criteria used for warranting

guardrail installation on high-speed, high-volume highways do not

apply to the low-speed, low-volume traffic conditions on most park

roads.
33

In addition, placement and design of both guardrail and guiderail

should be as consistent as possible for both safety and appearance

throughout the length of a particular roadway. Choice of materials

and design should be sensitive to the setting or environment. In snow

country, design and location should recognize snow-removal methods.

Curbs

Except where necessary to control traffic or drainage or to provide

for road widening, the use of barrier curbs on roadways should be

avoided for safety, economy and simplicity. Where barrier curbs are

continuous along a low speed highway, they should be offset at least 1

foot and preferably at least 2 feet from the edge of traffic lane and

so carried across all structures. Barrier curbs introduced on bridges

or intermittently elsewhere should be offset at least 2 feet and

preferably 3 feet.
34

RESURFACING, RESTORATION & REHABILITATION PROJECTS

The primary purpose of the 3-R work the National Park Service will

undertake is to preserve and extend the service life of park roads and

to enhance their safety (23 U.S.C. 109(o)). Park roads on which

geometries were established several decades ago are capable in most

instances of providing safe, useful service. In such cases, minor

improvements will make those roads serviceable for many more years,

and the complete reconstruction which would be required on such roads

to meet current standards would be prohibitively costly and

environmentally objectionable.

A primary consideration in the development of 3-R projects is

application of criteria which will allow the necessary flexibility to

adjust to actual field conditions. Consequently, the geometric

information that follows is generally the minimum considered

acceptable. On 3-R work, the intent should be to improve above these

minimums, where feasible, and to ensure the highest level of safety

possible within existing conditions and constraints.

Guidance for 3-R projects can be found in FHWA's Direct Federal

Manual, 3(C)(1), Appendix I, "Direct Federal Geometric Design Criteria

for Non-Freeway 3-R Projects."

Traffic Volume - 3-R projects are undertaken primarily to meet

specific current needs. Where significant levels of rehabilitation

are involved, a desirable design volume should be established through

management decisions based on a 5- or 10-year traffic forecast.

Design Speed - Roads scheduled for 3-R work should be evaluated based

on the desirable design speed which would accommodate the current

running speed, but a minimum design speed should not be established. 

It is essential, when considering a project for a section of park

road, that the geometric conditions beyond the portion to be improved

also be evaluated and considered to obtain uniformity and to achieve

consistency in design over the entire route. Every attempt should be

made to maintain a uniformly safe running speed for a significant

segment of the roadway. Consideration should be given to providing a

transition between portions of a roadway having different design

speeds.

Pavement and Shoulder Widths - The criteria for roadway cross-sections

established for new construction and reconstruction apply to 3-R

projects, where feasible.
35

Grades, Curvature and Sight Distance - These geometric features would

usually require an older road to be reconstructed, if current

standards were to be achieved, and could not normally be achieved with

3-R work. The level of reconstruction necessary to satisfy these

criteria might also cause impacts on surrounding areas which the

Service would deem inappropriate.

During the project planning phase, however, all hazardous locations

along a roadway should be identified and accident records should be

analyzed to determine if roadway features are contributing to

accidents. If accidents relate to grade, cross-section, curvature or

sight distance, consideration should be given to reconstruction

improvements at critical locations (and/or to control of vehicular

types and speeds, improved signing, etc.)

Generally, however, the previously existing geometric features (such

as grade, curvature and sight distance) will be retained on 3-R

projects. In such cases, each vertical and horizontal curve should be

checked for stopping sight distance to determine if it is less than

that required for safety at the posted speed. If so, advisory limited

sight distance signs giving the appropriate speed should be installed. 

In some instances, where encroachment of vegetation has impaired

original sight distance, appropriate pruning or removal shall be

accomplished.

Bridges - Approaches for narrow bridges (including one-way bridges)

should be signed and delineated in accordance with MUTCD.

Historic Structures - A number of park roads and parkways, or

structures on them (e.g. bridges, walls and overlooks) are historic in

themselves, and are in some instances listed on the National Register. 

Preservation or restoration may be the only option for such historic

roadways or structures.
36

STRUCTURES, SIGNING 6 MAINTENANCE

Structures

The engineering design of bridges, culverts, walls, tunnels, and

ancillary structures should be in accordance with AASHTO Standard

Specifications for Highway Bridges. The design process should be

multi-disciplinary to address aesthetic, historical, and environmental

considerations.

The minimum design loading for new roadway bridges should be H-15. 

Higher design loadings may be appropriate, depending on use. The

vertical clearance at underpasses should be at least 14 feet above the

entire roadway width. The clear roadway widths for new and

reconstructed bridges should desirably be a minimum of the traveled

way plus shoulders plus four feet (2 feet on each side).

Bridge railings for Class II through V roads should be designed for

low volumes and speeds and to permit lateral scenic viewing, if

appropriate. Bridge railings should be located coincident with

barrier curbs except where sidewalks are provided.

Signing and Marking

Although safety and efficiency of operation depend to a major extent

on the geometric design of a road, they should be supplemented by

standard signing and marking to provide appropriate traffic control

and safety and to provide information to drivers. The Manual on

Uniform Traffic Control Devices (MUTCD), as supplemented by the

National Park Service Sign Manual, contains details regarding design,

location, and application of road signs and markings as they apply to

park roads and parkways. Application of standard signing and pavement

markings shall be incorporated as part of the design process. During

construction, project traffic control shall be provided by the

contractor, including erection and maintenance of all necessary

barricades; suitable and sufficient lights, as appropriate; and danger

signals, signs and other traffic control devices in conformity with a

project traffic control plan and with Part VI of the MUTCD.

Maintenance

Road design is but one component in the overall management of road

facilities, and must be considered in this light as part of a

continuing program of road and bridge facilities management and

operations. This process of road management commences with advance

planning and extends all the way through maintenance and operation.
37

To assure a continuously acceptable level of safe traffic service for

park visitors, it is essential to design park roads appropriately to

allow for maintenance and periodic resurfacing, restoration and

rehabilitation (3-R work) throughout the life of the road.

Road safety and efficiency of operation depend on adequate levels of

cyclic and preventative maintenance and repair, which are also

essential to protect the Service's extensive capital investment in the

physical facility constituted by park roads, parkways and bridges. 

Consequently, park roads shall be maintained to the standards to which

they have been constructed or reconstructed, and in a condition that

promotes safety and protects capital investment.

* * *
38
39

GLOSSARY OF SELECTED TERMS

AASHTO - Abbreviation for American Association of State Highway and

Transportation Officials.

ADT - cf. "Average Daily Traffic".

Aesthetics - A branch of philosophy dealing with beauty and the

beautiful and judgments of taste concerning them. In highway

engineering, aesthetic judgments have to do primarily with the

appearance of the highway or road as a whole, including the

roadside and its relationship to the natural and cultural

environment through which it passes.

Average Daily Traffic (ADT) - The average 24-hour volume, being the

total volume during a stated period divided by the number of days

in that period, normally a year or the number of days the road is

actually open to public travel.

Average Running Speed - For all traffic, or a component thereof, the

summation of distances traveled divided by the summation of

running times.

Backslope - In cuts, the slope from the bottom of the ditch to the top

of the cut.

Bridge - A structure exceeding 20 feet clear span measured along the

centerline of the roadway, which carries traffic over a

watercourse or opening.

Broken Back Curve - An arrangement of curves in which a short tangent

separates two curves in the same direction.

Capacity - The maximum number of vehicles which has a reasonable

expectation of passing over a given section of lane or roadway

during a given time period under prevailing roadway and traffic

conditions.

Centerline - (1) For a two-lane road, the centerline is the middle of

the traveled way; and for a divided road, the centerline may be

the center of the median. For a divided road with independent

roadways, each roadway has its own centerline. (2) The defined

and surveyed line shown on the plans from which road construction

is controlled.

Critical Length of Grade - That combination of gradient and length of

grade which will cause a designated vehicle to operate at some

predetermined minimum speed. A lower speed than this is

unacceptable and usually requires that an auxiliary climbing lane

be provided for slow-moving vehicles.
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Cross Section - The transverse profile of a road showing horizontal

and vertical dimensions.

Cross Walk - Any portion of a roadway at an intersection or elsewhere

distinctly delineated for pedestrian crossing by signs and by

lines or other markings on the surface.

Culvert - Any structure under the roadway with a clear opening of 20

feet or less measured along the center of the roadway.

Curve Widening - The widening of the traveled way on sharp curves to

compensate for the fact that the rear wheels of a vehicle do not

follow exactly in the track of the front wheels.

Curvilinear Alignment - A flowing alignment in which the majority of

its length is composed of circular and spiral curves.

Cut Section - That part of the roadway which, when constructed, is

lower in elevation than the original ground.

Design Speed - A speed selected for purposes of design and correlation

of the physical features of a road that influence vehicle

operation. It is the maximum safe speed that can be maintained

over a specified section of the road when conditions are so

favorable that the design features of the road govern.

Design Vehicle - A selected motor vehicle, the weight, dimensions, and

operating characteristics of which are used as a control in road

design.

Design Vehicle Turning Radius - The turning radius of a Design

Vehicle, used primarily to determine the minimum radius used in

the design of turning and intersecting roadways.

Design Volume - A volume determined for use in design, representing

the traffic expected to use the road.

Embankment - A raised earth structure on which the roadway pavement

structure is placed.

Fill Section - cf. "Embankment".

Foreslope - The slope from the edge of the surfaced shoulder to the

top of the subgrade, or the bottom of the ditch in cuts.

Functional Classification - The grouping of individual roads in a road

system according to their purpose or function and the type of

traffic or use they serve.
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Geometric Design - The arrangement of the visible elements of a road,

such as alignment, grades, sight distances, widths, slopes, etc.

Grade - (1) The profile of the center of the roadway, or its rate of

ascent or descent. (2) To shape or reshape an earth road by means

of cutting or filling. (3) Elevation.

Grade Separation - A structure which provides for traffic to pass over

or under another road or railroad.

Horizontal Alignment - Horizontal geometries of the roadway.

Horizontal Curve - A curve or transitional by means of which a road

can change direction to the right or left.

Hourly Volume - The number of vehicles passing over a given section of

lane or roadway during one hour.

Intersection - The general area where two or more roads join or cross,

within which are included the roadway and roadside facilities for

traffic movements in that area.

Intersection Angle - The angle between two intersecting roads.

Lanes -

Auxiliary Lane - The portion of the roadway adjoining the

traveled way for parking, speed change, turning, storage for

turning, weaving, truck climbing, or for other purposes

supplementary to through traffic movement.

Median Lane - A speed-change lane within the median to

accommodate left-turning vehicles.

Parking Lane - An auxiliary lane primarily for the parking of

vehicles.

Speed-Change Lane - An auxiliary lane, including tapered areas,

primarily for the acceleration or deceleration of vehicles

entering or leaving the through traffic lanes.

Turn Lane - A traffic lane within the normal surfaced width of a

roadway, or an auxiliary lane adjacent to or within a median,

reserved for vehicles turning left or right at an intersection.

Traffic Lane - The portion of the traveled way for the movement

of a single line of vehicles in one direction.
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Level of Service - A qualitative rating of the effectiveness of a road

relative to the service it renders to its users, measured in

terms of a number of factors, such as operating speed, travel

time, traffic interruptions, freedom to maneuver and pass,

driving safety, comfort, and convenience.

Median - The portion of a divided roadway separating the traveled ways

for traffic in opposite directions.

Merging - The converging of separate streams of traffic into a single

stream.

Multi-Lane Road - A road having two or more lanes for traffic in each

direction, or four or more lanes for traffic in two directions. 

It may be one-way or two-way, divided or undivided.

Overlook (Scenic Overlook) - A roadside area provided for motorists to

stop their vehicles beyond the traveled-way, primarily for

viewing scenery in safety.

Passing Opportunity - A section of two-lane, two-directional road

where sufficient clear sight distance exists to allow a safe

passing maneuver to be performed.

Passing Sight Distance - The minimum sight distance that must be

available to enable the driver of a vehicle to pass another

safely and comfortably without interfering with the speed of an

oncoming vehicle traveling at the design speed should it come

into view after the overtaking maneuver is started.

Pavement Markings - Devices or paint placed on the roadway to mark

pavement for vehicular and pedestrian traffic control.

Pavement Structure - The combination of subbase, base course, and

surface course placed on a subgrade to support the traffic load

and distribute it to the roadbed.

Base Course - The layer or layers of specified or selected

material of designed thickness placed on a subbase or a subgrade

to support a surface course.

Subbase - The layer or layers of specified or selected material

or designed thickness placed on a subgrade to support a base

course.

Subgrade - The top surface of a roadbed upon which the pavement

structure and shoulders, including curbs, are constructed.
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Subgrade Treatment - Modification of roadbed material by

stabilization.

Surface Course - One or more layers of a pavement structure

designed to accommodate the traffic load, the top layer of which

resists skidding, traffic abrasion, and the disintegrating

effects of climate. The top layer is sometimes called "Wearing

Course".

Pedestrian Crossing (Crosswalk) - An area clearly marked for the

passage of pedestrians at street junctions or other locations

where drivers must yield the right-of-way by stopping to enable

pedestrians to cross safely.

Profile - A longitudinal section of a roadway, drainage course, etc.

Recreation Vehicle (RV) - A vehicle specifically designed for

recreational use, usually considerably larger than a passenger

car and frequently containing kitchen, sleeping and toilet

facilities.

Resurfacing - The placing of one or more new courses on an existing

road surface.

Reverse Curve - A curve consisting of two arcs of the same or

different radii curving in opposite directions and having a

common tangent or transition curve at their point of junction.

Road (Highway) - A general term denoting a public way for purposes of

vehicular travel, including the entire area within the right-ofway.

Roadside - A general term denoting the area adjoining the outer edge

of the roadway. Extensive areas between the roadways of a

divided road may also be considered roadside.

Roadway - The portion of a highway, including shoulders, for vehicular

use. A divided highway has two or more roadways.

Running Speed - cf. "Average Running Speed".

Service Volume - The maximum number of vehicles that can pass over a

given section of a lane or roadway in one direction on a multilane road or in both directions on a two-lane road during a

specified time period, which is stated as an hourly volume and

which varies according to the level of service.
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Shoulder - The portion of the roadway contiguous with the traveled way

for accommodation of stopped vehicles, for emergency use, and for

lateral support of base and surface courses.

Sight Distance - The length of roadway ahead visible to a driver.

Skid Resistance - The frictional force between a locked tire and a

pavement, which force resists motion.

Slope - The face of an embankment or cut section; any ground the

surface of which makes an angle with the plane of the horizon.

Speed - The rate of movement of a vehicle, generally expressed in

miles per hour (mph).

Spiral Curve - cf. "Transition Curve".

Standard - Criteria having recognized and usually permanent values

which are established formally as a model or requirement.

Stopping Sight Distance - The distance required by a driver of a

vehicle, traveling at a given speed, to bring his vehicle to a

stop after an object on the roadway becomes visible, including

the distance traveled during the perception and reaction times,

as well as the vehicle braking distance.

Superelevation - The elevating of the outside edge of a curve to

partially offset the centrifugal force generated when a vehicle

rounds the curve.

Superelevation Runoff - The transition distance between normal crown

and fully superelevated roadway.

Sustained Grade - A continuous road grade of appreciable length and

consistent, or nearly consistent, gradient.

Traffic - All types of vehicles, together with their loads, either

singly or as a whole, including pedestrians, while using a

roadway for the purpose of transportation.

Traffic Control Device - A sign, signal, marking or other device

placed on or adjacent to a street or highway by authority of a

public body or official having jurisdiction to regulate, warn, or

guide traffic.

Traffic Control Signal - Any device--whether manually,

electrically, or mechanically operated--by which traffic is

alternately directed to stop and permitted to proceed.
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Traffic Markings - All lines, patterns, words, colors, or other

devices--except signs--set into the surface of, applied upon, or

attached to the pavement or curbing or to objects within or

adjacent to the roadway, officially placed for the purpose of

regulating, warning, or guiding traffic.

Traffic Sign - A device mounted on a fixed or portable support

whereby a specific message is conveyed by means of words or

symbols, officially erected for the purpose of regulating,

warning, or guiding traffic.

Traffic Signal - A power-operated traffic control device by which

traffic is regulated, warned, or alternately directed to take

specific actions.

Traffic Volume - The number of vehicles passing a given point during a

specified period of time.

Transition - A section of variable pavement width required when

changing from one width of traveled way to a greater or lesser

width.

Transition Curve (Spiral) - A curve of variable radius intended to

effect a smooth transition from tangent to curved alignment.

Traveled Way - The portion of the roadway for the movement of

vehicles, exclusive of shoulders and auxiliary lanes.

Turn Angle - cf. "Angle of Turn".

Turning Path - The path of a designated point on a vehicle making a

specified turn. (cf. also "Minimum Turning Path").

Turning Track Width - The radial distance between the turning paths of

the outside of the outer front tire and the outside of the rear

tire which is nearest the center of the turn.

Vertical Alignment (Profile Grade) - The trace of a vertical plane

intersecting the top surface of the proposed wearing surface,

usually along the longitudinal centerline of the roadbed, being

either elevation or gradient of such trace according to the

context.

Vertical Curve - A curve on the longitudinal profile of a road

providing a change of gradient.

Wearing Course (Surface Course) - The top layer of a pavement.