Wai-Fah Chen.The Civil Engineering Handbook

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The Civil Engineering Handbook, Second Edition

Design Process

The design process of a proposed highway involves preliminary location study, environmental impact

evaluation, and ﬁnal design. This process normally relies on a team of professionals, including engineers,

planners, economists, sociologists, ecologists, and lawyers. Such a team may have responsibility for

addressing social, environmental, land-use, and community issues associated with highway development.

Preliminary Location Study

The preliminary location study involves collecting and analyzing data, locating feasible routes, determin-

ing preliminary horizontal and vertical alignments for each, and evaluating alternative routes to select

the best route. The types of data required are related to the engineering, social and demographic,

environmental, and economic characteristics of the area. Examples of such data are topography, land-

use pattern, wildlife types, and unit costs of construction. A preliminary study report is prepared and

typically includes a general description of the proposed highway, a description of alternative locations

and designs, projected trafﬁc volumes and estimated total costs, an economic and environmental eval-

uation, and a recommended highway location. Before the project is approved, it is common to hold

public hearings to discuss the preliminary study and environmental impacts.

Environmental Evaluation

Highway construction may impact the environment in a number of areas, including air quality, water

quality, noise, wildlife, and socioeconomics. For example, highways may cause loss or degradation of a

unique wildlife habitat and changes to migratory patterns. Socioeconomic impacts include displacement

of people and businesses, removal of historically signiﬁcant sites, and severance of the interpersonal ties

of displaced residents to their former community. It is therefore essential that environmental impacts of

alternative highway locations be fully evaluated.

Provisions of the National Environment Policy Act of 1969 require that an environmental impact

statement (EIS) be submitted for any project affecting the quality of the environment. The EIS must

describe the environmental impacts of the proposed action, both positive and negative; probable unavoid-

able adverse environmental impacts; secondary environmental impacts such as changes in the pattern of

social and economic activities; analysis of short- and long-term impacts; irreversible and irretrievable

commitments of resources; and public and minority involvement. Chapter 8, Section 10, provides more

details on the environmental process.

Final Design

The ﬁnal design involves establishing the design details of the selected route, including ﬁnal horizontal

and vertical alignments, drainage facilities, and all items of construction. The design process has been

revolutionized by advanced photogrammetric and computer techniques. For example, designers now can

have a driver’s eye view of a proposed highway alignment displayed on a monitor and readily examine

the effects of alignment reﬁnements. Further details on the design process are found in Garber and Hoel

[2001].

63.2 Fundamentals of Geometric Design

Geometric design involves a number of fundamentals and concepts that guide and control the manner

in which a highway is designed. These include highway types, design controls, sight distance, and simple

highway curves.

Highway Types

Classiﬁcation of highways into functional classes is necessary for communication among engineers,

administrators, and the general public. The functional classiﬁcation system facilitates grouping roads

that require the same quality of design, maintenance, and operation. The system also facilitates the logical

Geometric Design

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assignment of responsibility among different jurisdictions, and its structure of the design guidelines is

readily understood.

The highway functional classes, adopted separately for urban and rural areas, are locals, collectors,

and arterials. The principal arterial system includes freeways and other principal arterials. The two major

considerations in the functional classiﬁcation system are travel mobility and land access. Locals emphasize

the land access function; collectors provide a balanced service for both functions; and arterials emphasize

the mobility function. Design guidelines for locals, collectors, arterials, and freeways, in both urban and

rural locations, are presented by AASHTO in Chapters 5 through 8, respectively. Details on functional

system characteristics are found in FHWA [1989].

Design Controls

The major controls that inﬂuence the geometric design of highways include topography, the design

vehicle, driver performance, trafﬁc characteristics, highway capacity, access control and management, the

pedestrian, bicycle facilities, and safety. Other controls such as esthetics, environment, economics, and

public concerns are important but are reﬂected in either the preceding major controls or the preliminary

location study.

Design Vehicle

A design vehicle is a vehicle with representative weight, physical dimensions, and operating characteristics,

used to establish highway design controls for accommodating vehicles of designated classes. Each design

vehicle has larger dimensions and a larger minimum turning radius than most vehicles in its class. Four

general classes of design vehicles have been established: passenger cars, buses, trucks, and recreational

vehicles. The dimensions of 20 design vehicles within these general classes are given by AASHTO. The

design vehicle selected for geometric design is the largest vehicle likely to use the highway with consid-

erable frequency or a vehicle with special characteristics appropriate to a particular intersection for

determining the radii at intersections and the radii of

turning roadways

. A typical minimum turning

path for a single-unit (SU) truck design vehicle is shown in Fig. 63.1. Other vehicle characteristics such

as acceleration and braking capabilities, the driver’s eye height, and vehicle headlights also affect many

geometric design features.

Driver Performance

Highways should be designed to be compatible with driver capabilities and limitations. Information

about the performance of the drivers (how they interact with the highway and its information) is useful

in highway design and operations. Since it is not generally possible to reduce errors caused by innate

driver deﬁciencies, a “forgiving” design that lessens the consequences of failure should be implemented.

In addition, a positive guidance approach should be applied to design. Here are some examples:

•The design should focus a driver’s attention on the safety-critical elements by providing clear sight

lines and good visual quality.

•The design should take into account the longer reaction time required for complex decisions by

providing adequate decision sight distance.

• On high-speed facilities, guidance activities should be simpliﬁed because speed reduces the visual

ﬁeld, restricts peripheral vision, and limits the time available to process information.

Another important means to aid driver performance is the development of designs in accordance with

driver expectancies. Detailed information on driver attributes, driving tasks, and information handling

can be found in the FHWA report

A User’s Guide to Positive Guidance

[Alexander and Lunenfeld, 1990].

Trafﬁc Characteristics

Tr afﬁc characteristics include trafﬁc volume, directional distribution, trafﬁc composition, and speed.

Design volume and composition determine the highway type, required roadway width, and other geometric

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The Civil Engineering Handbook, Second Edition

features. The basic measure of the trafﬁc demand for a highway is the

average daily trafﬁc

(ADT). This

measure is used for selecting geometric design guidelines for local and collector roads. For other highways,

the design hourly volume (DHV), a two-way volume, is used and is generally deﬁned as the

30th highest

hour volume

of a designated year. The ratio of the DHV and ADT,

, varies only slightly from year to

year. For design of a new highway,

can be determined using existing trafﬁc volumes of similar highways.

The typical range of

is 12 to 18% for rural highways and 8 to 12% for urban highways.

For two-lane highways, the DHV is the total trafﬁc in both directions. For multilane highways, the

directional distribution of trafﬁc during the design hour should be determined. The directional DHV

can then be calculated by multiplying ADT by

and then by the percentage of trafﬁc in the peak direction

during the design hour. For recreational routes, in practice the DHV is selected as 50% of the volume

expected to occur during the few highest hours of the design year. Other volume characteristics required

for the design year include

peak-hour factor

and the percentages of trucks, buses, and recreational vehicles

in the design-hour volume.

Design speed is a selected speed used to determine various geometric design features of the roadway.

Selection of design speed is inﬂuenced by topography, adjacent land use, highway functional classiﬁcation,

and anticipated

operating speed

. Nearly all geometric design elements are directly or indirectly inﬂuenced

by design speed.

FIGURE 63.1

Minimum turning path for single-unit (SU) truck design vehicle. (From AASHTO,

A Policy on

Geometric Design of Highways and Streets

, Washington, D.C., 2001. With permission.)

Path of left

front wheel

Path of front

overhang

8.64 m min.

[28.3 ft]

13.26 m max.

[43.5 ft]

Assumed steering angle is 31.7°

CTR = Centerline turning

radius at front axle

Path of right

rear wheel

Scale

5 ft

10 ft

2.5 m

1.83 m

9.15 m

6.10 m

1.22 m

5 ft 10 ft

1 m

2.5 m

Scale

[8 ft]

[30 ft]

[20 ft]

[6 ft]

[4 ft]

Min. turning

radius = 12.80 m [42 ft]

2.44 m

CTR = 11.58 m [38 ft]

Geometric Design

-5

Highway Capacity

The required number of lanes of a highway depends on the DHV and the

level of service

intended for

the design year. The

Highway Capacity Manual

[TRB, 2000] deﬁnes six levels of service ranging from

level-of-service A (least congested) to level-of-service F (most congested). Table 63.1 shows the AASHTO-

recommended design levels of service for different highway classes and locations. These levels of service

are based on criteria for acceptable degrees of congestion.

Access Control and Management

Access control refers to the interference regulations of public access rights to and from properties on the

roadside. These regulations include full control of access, partial control of access, and access manage-

ment. Full-controlled access facilities (such as freeways) have no at-grade crossings and have access

connections only with selected roads. With partial control of access, preference is given to through trafﬁc

to an extent, but there may be some at-grade crossings and driveway connections. Partial access control

can be achieved by driveway permits, zoning restrictions, and frontage roads. The extent of access control

is a signiﬁcant factor in deﬁning the functional type of a highway.

Access management, a new element of road design that applies to all types of roads, involves providing

(managing) access to land development while simultaneously preserving the ﬂow of trafﬁc (safety,

capacity, and speed) on the surrounding road system. It views the roadway and its surrounding activities

as part of a system with the goal of coordinating the planning and design of each activity. For more

details on access management, see Koepke and Levinson [1992].

Pedestrian

Interaction of pedestrians with trafﬁc is a major consideration in highway planning and design. Pedestrian

facilities include sidewalks, crosswalks, curb ramps for the handicapped, and grade separations. Sidewalks

are usually provided in urban areas and in rural areas with high pedestrian concentrations, such as schools,

local businesses, and industrial plants. Pedestrian crosswalks are provided at intersections and at midblocks.

For guidance on pedestrian crosswalk marking, refer to the

Manual on Uniform Trafﬁc Control Devices

(MUTCD). Curb ramps for the handicapped should be provided at all intersections that have curbs and

sidewalks and at midblock pedestrian crossings. Because these crossings are generally unexpected by

drivers, warning signs and adequate visibility should be provided. Pedestrian grade separations are nec-

essary when pedestrian and trafﬁc volumes are high or where there is abnormal inconvenience to pedes-

trians, such as at freeways. Design issues on safe accommodation of pedestrians are addressed in the

AASHTO

Guide for the Planning, Design, and Operation of Pedestrian Facilities

[AASHTO, 2002].

Bicycle Facilities

Design of bicycle facilities is an important consideration in highway design. Design measures to enhance

safety for bicycle trafﬁc on existing highways include paved shoulders, wider outside trafﬁc lanes, adjust-

ment of manhole covers to pavement surface, and provision of a smooth riding surface. The highway

TABLE 63.1

Guidelines for Selection of Design Levels of Service

Appropriate Level of Service for Speciﬁed

Combinations of Area and Terrain Type

Functional

Class

Rural

Level

Rural

Rolling

Rural

Mountainous

Urban and

Suburban

Freeway

Arterial

Collector

Local

Source: American Association of State Highway and Transportation

Ofﬁcials,

A Policy on Geometric Design of Highways and Streets

, Wash-

ington, D.C., 2001. With permission.

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The Civil Engineering Handbook, Second Edition

system can also be supplemented by providing speciﬁcally designated bikeways. Important elements of

bikeway design include design speed, bikeway width, superelevation, turning radii, grade, stopping sight

distance, and vertical curves. The

Guide for the Development of New Bicycle Facilities

[AASHTO, 1999],

in conjunction with the MUTCD [FHWA, 1988], provides guidance for bikeway planning and design.

Safety

Safety is a major consideration in the design of nearly all elements of highway geometric design, including

horizontal and vertical alignments, cross sections, roadsides, trafﬁc control devices, and intersections.

Safety must be reﬂected not only in new highway and major reconstruction projects but also in the

resurfacing, restoration, and rehabilitation (RRR) projects. AASHTO stresses the importance of estab-

lishing a safety evaluation program to identify safety hazards, evaluate the effectiveness of alternative

improvements, and allocate available funds to the most effective uses. The TAC guide includes explicit

evaluation of safety (collision frequency–design parameter relationships) for some highway elements,

such as horizontal alignment, vertical alignment, and truck climbing lanes [TAC, 1999].

Sight Distance

Sight distance is the length of the roadway ahead that is visible to the driver. It is a fundamental design

element in the safe and efﬁcient operation of a highway. Five basic types of sight distances must be

considered in design: (1) stopping sight distance (SSD), applicable on all highways; (2) passing sight

distance (PSD), applicable only on two-lane highways; (3) decision sight distance (DSD), needed at

complex locations; (4) preview sight distance (PVSD), applicable to horizontal curves, especially those

combined with vertical curves; and (5) intersection sight distance (ISD), needed for all types of inter-

sections. In addition, a special type of sight distance (called head-on sight distance) that may be needed

when parking occurs on both sides of a residential street has been addressed by Gattis [1991].

Stopping Sight Distance

Stopping sight distance is the distance that enables a vehicle traveling at or near the design speed to stop

before reaching a stationary object in its path. The SSD in feet is computed by

(63.1)

where

= the brake reaction time (sec)

= the design speed (mph)

= the deceleration rate (ft/sec

)

= the percent of grade divided by 100 (positive for upgrade and negative for downgrade)

The recommended design criterion for brake reaction time is 2.5 sec, which exceeds the 90th percentile

of reaction time for all drivers. The recommended design deceleration rate is 11.2 ft/sec

, which is the

comfortable deceleration rate for most drivers on wet surfaces. Design values of SSD for level grades

(

= 0) are shown in Table 63.2. These values are used for such application as vertical curve design,

intersection geometry, and placement of trafﬁc control devices. In Canada, SSD is based on the coefﬁcient

of friction between the tires and the roadway (rather than deceleration rate) and assumed operation

speed range for each design speed [TAC, 1999].

The SSD values of Table 63.2 are based on passenger car operation and do not explicitly consider truck

operation. Trucks need longer stopping distances than passenger cars. However, truck drivers can see

substantially farther beyond vertical obstruction than passenger cars, and this factor tends to balance the

additional braking lengths required for trucks. Therefore, separate SSD values for trucks and passenger

cars are not generally used in highway design. One exception is the case of horizontal sight restrictions,

where the greater eye height of truck driver is of little value. For this case, it is desirable to provide an

SSD that exceeds the values of Table 63.2.

SSD Vt

()

[]

147

30 32 2

Geometric Design

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Example 63.1

Compute the SSD for a highway with a 60-mph design speed and G = 0. From Eq. (63.1),

which is the same as the computed value in Table 63.2.

Passing Sight Distance

Passing sight distance is the distance required for a vehicle to overtake a slower moving vehicle safely on

a two-lane highway. The AASHTO model is based on certain assumptions for trafﬁc behavior and

considers PSD as the sum of four distances (Fig. 63.2): (1) distance during perception and reaction time,

and during the initial acceleration of the passing vehicle to the encroachment point on the left lane (

);

(2) distance traveled while the passing vehicle occupies the left lane (

); (3) distance between the passing

vehicle at the end of its maneuver and the opposing vehicle (

); and (4) distance traversed by an opposing

TABLE 63.2

Design Requirements for Stopping and Passing Sight Distances

Design

Speed

(mph)

Design

Speed

(mph)

Passing Sight Distance (ft)

Passed

Vehicle Speed

(mph)

Passing

Vehicle Speed

(mph) Calculated

Design

Stopping Sight Distance (ft)

Calculated

Design

111.9

196.7

300.6

423.8

566.0

727.6

908.3

115

200

305

425

570

730

910

707

1088

1470

1832

2133

2479

2677

710

1090

1470

1835

2135

2480

2680

Values are calculated using Eq. (63.1).

Values are based on Fig. 63.3.

Source: American Association of State Highway and Transportation Ofﬁcials,

A Policy on Geometric Design of

Highways and Streets

, Washington, D.C., 2001. With permission.

FIGURE 63.2

Elements of passing sight distance for two-lane highways. (From AASHTO,

A Policy on Geometric

Design of Highways and Streets

, Washington, D.C., 2001. With permission.)

SSD =¥¥+

()

=147 25 60

30 11 2 32 2

566 0

.ft

FIRST PHASE

SECOND PHASE

Passing vehicle

Opposing vehicle appears

when passing vehicle

reaches point A.

1/3 d

2/3 d

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The Civil Engineering Handbook, Second Edition

vehicle for two-thirds of the time the passing vehicle occupies the left lane (

). The calculations of d

through

are described in AASHTO, and the sum of the four elements is shown in Fig. 63.3.

The design values of PSD are shown in Table 63.2. The speed of the passed vehicle is assumed to be

the

average running speed

, while the speed of the passing vehicle is 10 mph greater. The design values

apply to a single passing only. For passing maneuvers on upgrades, the passing sight distance should be

greater than the design values shown in Table 63.2. However, speciﬁc adjustments for design use are

currently unavailable.

Decision Sight Distance

Decision sight distance is required at complex locations to enable drivers to maneuver their vehicles safely

rather than stop. It is the distance required for a driver to detect an unexpected hazard, recognize the

hazard, decide on proper maneuvers, and execute the required action safely. Examples of complex

locations where provision of DSD is desirable include complex interchanges and intersections, toll plazas,

lane drops, and areas where sources of information (such as signs, signals, and trafﬁc control devices)

compete. Design values for DSD, based on empirical data, are shown in Table 63.3. Since decision sight

distance affords drivers sufﬁcient length to maneuver their vehicles, its value is much greater than the

stopping sight distance.

The decision sight distance is computed by

(63.2)

(63.3)

where

= the premaneuver time for maneuvers A and B or the total premaneuver and maneuver

time for maneuvers C to E (sec), based on the notes in Table 63.3

= the design speed (mph)

= the deceleration rate (ft/sec

)

Note that for maneuvers A and B in Table 63.3, the premaneuver time is increased above that of SSD

(2.5 sec) to allow the driver additional time to detect and recognize the roadway and trafﬁc environment,

identify alternative maneuvers, and initiate a response at critical locations. The braking distance from

the design speed is then added to the premaneuver component, as noted in Eq. (63.2). For maneuvers

C to E, the braking component is replaced with a maneuver distance based on maneuver times between

FIGURE 63.3 Total passing sight distance and its components — two-lane highways. (From AASHTO, A Policy on

Geometric Design of Highways and Streets, Washington, D.C., 2001. With permission.)

Total = d

+ d

10 20

40 6050 70 9080

Average speed - Passing vehicle (mph)

3500

2500

3000

2000

1500

1000

500

Elements - Passing sight distance (ft)

DSD for maneuvers A and B=+

()

147 1075

..Vt V a

DSD for maneuvers C, D, and E=

()

147. Vt

Geometric Design 63-9

3.5 and 4.5 sec that decrease with increasing speed. Where it is not feasible to provide DSD, designers

should move the location or use suitable trafﬁc control devices to provide advance warning of the

conditions to be encountered.

Preview Sight Distance

Preview sight distance is the distance required by a driver to perceive a horizontal curve and properly

react to it. AASHTO implicitly recognizes PVSD by recommending that sharp horizontal curvature should

not be introduced at or near the top of a pronounced crest vertical curve or near the low point of a

pronounced sag vertical curve. The preview sight distance has been suggested by Gattis and Duncan

[1995] for horizontal curves. For three-dimensional alignments, PVSD is the sum of two components

(Fig. 63.4): the tangent component, S

, and the curve component, S

[Hassan and Easa, 2000]. The tangent

component is the distance required for the driver to react and adjust the speed before reaching the curve.

The curve component is the distance on the horizontal curve required for the driver to detect its existence.

Preliminary design values of PVSD have been established.

Intersection Sight Distance

Intersection sight distance is the distance provided at an intersection to allow approaching vehicles (at

an uncontrolled or yield-controlled intersection) to see any potentially conﬂicting vehicles in sufﬁcient

time to slow or stop safely and to allow stopped vehicles (at a stop or signal-controlled intersection) to

enter or cross the intersection safely. Details on the calculation of intersection sight distance are presented

later in this chapter.

Design Heights for Sight Distances

The AASHTO design driver’s eye height and object height used for measuring various sight distances are

shown in Table 63.4. Sight distances are measured from a 3.5-ft driver’s eye height to a 2.0-ft object height

for SSD and DSD and a 3.5-ft object height for PSD and ISD. The object height of 3.5 ft is based on a

design vehicle height of 4.35 ft, less an allowance of 10 in. to allow another driver to recognize the vehicle

TABLE 63.3 Design Requirements for Decision

Sight Distance

Design

Speed

(mph)

Decision Sight Distance (ft)

Avoidance Maneuver

AB C D E

220

330

465

610

780

970

490

690

910

1150

1410

1685

450

600

750

990

1105

1260

535

715

890

1125

1275

1455

620

825

1030

1280

1445

1650

Avoidance maneuvers:

A: Stop on rural road (t = 3.0 sec)

B: Stop on urban road (t = 9.1 sec)

C: Speed, path, or direction change on rural road

(t varies from 10.2 to 11.2 sec)

D: Speed, path, or direction change on suburban

road (t varies from 12.1 to 12.9 sec)

E: Speed, path, or direction change on urban road

(t varies from 14.0 to 14.5 sec)

Source: American Association of State Highway and

Tr ansportation Ofﬁcials, A Policy on Geometric Design

of Highways and Streets, Washington, D.C., 2001. With

permission.

63-10 The Civil Engineering Handbook, Second Edition

as the object. In Canada, sight distances are measured from a 1.05-m driver’s eye height to an appropriate

object height for SSD, depending on the prevailing conditions used (ranging from 0 to 0.38 m), a 0.15-m

object height for DSD, and a 1.3-m object height for PSD [TAC, 1999]. For PVSD, a 1.05-m driver’s eye

height (daytime) or a 2-ft vehicle’s headlight height (nighttime) and a zero object height have been

recommended [Hassan and Easa, 2000].

Simple Highway Curves

Two basic curves are used for connecting straight (tangent) roadway sections in geometric design: a

simple circular curve for horizontal alignment and a simple parabolic curve for vertical alignment. Other

options include spirals, compound curves, and reverse circular curves for horizontal alignment; and

unsymmetrical curves and reverse parabolic curves for vertical alignment. Details on the geometry of

these curves can be found in Meyer and Gibson [1980].

Simple Horizontal Curves

A simple horizontal curve with radius R and deﬂection angle I is shown in Fig. 63.5. The basic elements

required for laying out a horizontal curve are tangent distance T, external distance E, middle ordinate

M, length of chord C, and curve length L. These elements can be easily computed in terms of R and I.

For example, the tangent distance T equals R tan (I/2) and the curve length L equals R I

/180. The

horizontal curve can also be described by the degree of curve (D = 5730/R, where D is in degrees and

R is in feet), instead of the radius. However, the degree of curve is no longer used in current geometric

design guides.

FIGURE 63.4 Illustration of preview sight distance. (From Hassan, Y. and Easa, S.M., J. Transp. Eng. ASCE, 126, 13,

2000.)

TA BLE 64.4

Design Heights for Daytime Sight Distances

Sight Distance

Type

Driver’s Eye Height, H

(ft)

Object Height, H

(ft)

SSD

DSD

PSD

PVSD

ISD

3.5

2.0

3.5

BVC

PC (or TS)

PT (or ST)

EVC

(a) Vertical Alignment

(b) Horizontal Alignment

PVSD origin

(driver eye or

vehicle headlight)

PVSD object

(pavement marking)

PVSD

Geometric Design 63-11

Simple Vertical Curves

Vertical curves are normally parabolic. A simple vertical curve may be a crest vertical curve or sag vertical

curve, as illustrated in Fig. 63.6. In this ﬁgure, G

and G

are the grades of the ﬁrst and second tangents

(in percent), A is the absolute value of the algebraic difference in grades (in percent), L is the curve length

measured in a horizontal plane, VPI is the vertical point of intersection, VPC is the vertical point of

curvature, and VPT is the vertical point of tangent. For a simple vertical curve, VPI lies in the middle of

the curve. The vertical curve parameter, K, is deﬁned as (with L in feet and A in percent)

FIGURE 63.5 Geometry of simple horizontal curves.

FIGURE 63.6 Types of vertical curves. (From AASHTO, A Policy on Geometric Design of Highways and Streets,

Washington, D.C., 2001. With permission.)

I/2I/2

G

(a) Crest Vertical Curves

(b) Sag Vertical Curves

TYPE I

TYPE III

TYPE II

TYPE IV

VPC

VPI

VPT