This document discusses the factors that influence highway design, including functional classification, traffic volume, design speed, design vehicle, topography, and available funds. It describes the classification of highways as urban or rural and then further breaks them down into principal arterials, minor arterials, collectors, and local roads. The document also discusses various elements of highway geometric design, such as lane and shoulder width, medians, barriers, curbs, gutters, and sidewalks.

1. Chapt 2
 Geometric Design of Highway
Facilities

2.  geometric design deals with the dimensioning of
the elements of highways, such as vertical and
horizontal curves, cross sections, truck climbing
lanes, bicycle paths, and parking facilities.
 The fundamental objective of geometric design is
to produce a smooth-flowing and safe highway
facility, an objective that only can be achieved by
providing a reliable design standard that satisfies
the characteristics of the driver and the vehicles
that use the road.

3. FACTORS INFLUENCING HIGHWAY
DESIGN
 Highway design is based on specified design standards
and controls which depend on the following roadway
system factors:
 Functional classification
 Design hourly traffic volume and vehicle mix
 Design speed
 Design vehicle
 Cross section of the highway, such as lanes, shoulders,
and medians

4.  Presence of heavy vehicles on steep grades
 Topography of the area that the highway pass
through
 Level of service
 Available funds
 Safety
 Social and environmental factors

5.  These factors are often interrelated. For example, design
speed depends on functional classification which is
usually related to expected traffic volume. The design
speed may also depend on the topography, particularly
in cases where limited funds are available.
 particular highway will be designed are the level of
service to be provided, expected traffic volume, design
speed, and the design vehicle. These factors, attached
with the basic characteristics of the driver, vehicle, and
road, are used to determine standards for the geometric
characteristics of the highway, such as cross sections and
horizontal and vertical alignments

6. Highway Functional
Classification
 Highways are classified according to their
functions in terms of the service they provide.
The classification system facilitates a systematic
development of highways and the logical
assignment of highway responsibilities among
different authority. Highways and streets are
categorized as rural or urban roads, depending
on the area in which they are located.

7.  This initial classification is necessary because
urban and rural areas have significantly different
characteristics with respect to the type of land
use and population density, which in turn
influences travel patterns. Within the
classification of urban and rural, highways are
categorized into the following groups.

8. 一．Principal arterials
二．Minor arterials
三．Major collectors
四．Minor collectors
五．Local roads and streets

9.  Freeways are not listed as a separate functional class
since they are generally classified as part of the principal
arterial system. However, they have unique geometric
criteria that require special design consideration

10. Urban roads
 Urban roads consist of highway facilities within
urban areas as designated by responsible state and
local officials to include communities with a
population of at least 5000 people. Urban areas are
further subdivided into urbanized areas with
populations of 50,000 or more and small urban
areas with populations between 5000 and 50,000.
Urban roads are functionally classified into
principal arterials, minor arterials, collectors, and
local roads. A schematic of urban functional
classification is illustrated in Figure 15.1 for a
suburban environment.

12. Urban Principal Arterial System.
 This system of highways serves the major
activity centers of the urban area and consists
mainly of the highest-traffic-volume corridors. It
carries a high proportion of the total vehicle-
miles of travel within the urban area including
most trips with an origin or destination within
the urban area. The system also serves trips that
go around the central business districts (CBDs)
of urbanized areas.

13. Urban Minor Arterial System
 This system serves trips of moderate
length and places more emphasis on land
access than the primary arterial system.
All arterials not classified as primary are
included in this class. Although highways
within this system may serve as local bus
routes and may connect communities
within the urban areas, they do not
normally go through identifiable
neighborhoods.

14. Urban Collector Street System.
 The main purpose of streets within this
system is to collect traffic from local
streets in residential areas or in CBDs and
communicate it to the arterial system.
Thus, collector streets usually go through
residential areas and facilitate traffic
circulation within residential, commercial,
and industrial areas.

15. Urban Local Street System.
 This system consists of all other streets
within the urban area that are not included
in the three systems described earlier. The
primary purposes of these streets are to
provide access to abutting land and to the
collector streets. Through traffic is
discouraged on these streets.

16. Functional System of Rural
Roads
 Highway facilities outside urban areas
comprise the rural road system. These
highways are categorized as
 principal arterials,
 minor arterials,
 major collectors,
 minor collectors,
 and locals.

17. Rural Principal Arterial System.
 This system consists of a network of highways
that serves most of the expressway trips and a
substantial amount of intrastate trips. Virtually
all highway trips between urbanized areas and a
high percentage of trips between small urban
areas with populations of 25,000 or more are
made on this system..

18. Rural Collector System.
 Highways within this system carry traffic
primarily within individual counties, and
trip distances are usually shorter than
those on the arterial roads. This system of
roads is subdivided into major collector
roads and minor collector roads.

19. Rural Major Collector System.
 Routes under this system carry traffic
primarily to and from county seats and
large cities that are not directly served by
the arterial system. The system also
carries the main intracounty traffic

20. Rural Minor Collector System.
 This system consists of routes that collect
traffic from local roads and convey it to
other facilities. One important function of
minor collector roads is that they provide
linkage between rural hinterland and
locally important traffic generators such
as small communities.

21. Rural Local Road System.
 This system consists of all roads within
the rural area not classified within the
other systems. These roads serve trips of
relatively short distances and connect
adjacent lands with the collector roads.

22. Highway Design Standards
 Selection of the appropriate set of geometric
design standards is the first step in the design
of any highway. This is essential because no
single set of geometric standards can be used
for all highways. For example, geometric
standards that may be suitable for a scenic
mountain road with low average daily traffic
(ADT) are inadequate for a freeway carrying
heavy traffic. The characteristics of the
highway should therefore be considered in
selecting the geometric design standards.

23. Design Hourly Volume
 The design hourly volume (DHV) is the
projected hourly volume that is used for
design. This volume is usually taken as a
percentage of the expected ADT on the
highway .

24. Design Speed
 Design speed is defined as a selected
speed to determine the various geometric
features of the roadway. Design speed
depends on the functional classification of
the highway, the topography of the area in
which the highway is located, and the land
use of the adjacent area. For highway
design, topography is generally classified
into three groups: level, rolling, and
mountainous terrain

25.  Level terrain is relatively flat. Horizontal and vertical
sight distances are generally long or can be achieved
without much construction difficulty or major expense.
 Rolling terrain has natural slopes that often rise above
and fall below the highway grade with occasional steep
slopes that restrict the normal vertical and horizontal
alignments.
 Mountainous terrain has sudden changes in ground
elevation in both the longitudinal and transverse
directions, thereby requiring frequent hillside
excavations to achieve acceptable horizontal and vertical
alignments.

27. Design Vehicle
 A design vehicle is selected to represent all
vehicles on the highway. Its weight,
dimensions, and operating characteristics are
used to establish the design standards of the
highway. The vehicle type selected as the
design vehicle is the largest that is likely to
use the highway with considerable frequency.
The selected design vehicle is used to
determine critical design features such as
radii at intersections and turning roadways as
well as highway grades.

28. Cross-Section Elements
 The principal elements of a highway cross
section consist of the
 travel lanes,
 shoulders,
 and medians (for some multilane highways).
 median and roadside barriers,
 curb,
 gutter,
 guard rails,
 sidewalks, and side slopes.

30. Width of Travel Lanes.
 Travel lane widths usually vary from 9 to
12 ft. Most arterials have 12-ft travel lanes
since the extra cost for constructing 12-ft
lanes over 10-ft lanes is usually offset by
the lower maintenance cost for shoulders
and pavement surface, resulting in a
reduction of wheel concentrations at the
pavement edges. On two-lane, two-way
rural roads, lane widths of 10 ft or 11 ft
may be used,

31. Shoulders.
 The shoulder of a pavement cross section
is always nearby with the traveled lane so
as to provide an area along the highway
for vehicles to stop when necessary. In
some cases, bicycles are permitted to use
a highway shoulder particularly on rural
and collector roads. Shoulder surfaces
range in width from 2 ft on minor roads to
12 ft on major arterials.

33. Medians.
 A median is the section of a divided
highway that separates the lanes in
opposing directions. The width of a
median is the distance between the edges
of the inside lanes, including the median
shoulders. The functions of a median
include.

34.  Providing a recovery area for out-of-control
vehicles
 Separating opposing traffic
 Providing stopping areas during emergencies
 Providing storage areas for left-turning and U-
turning vehicles
 Providing refuge for pedestrians
 Reducing the effect of headlight glare
 Providing temporary lanes and cross-overs
during maintenance operations

35. Roadside and Median Barriers
 A median barrier is defined as a longitudinal
system used to prevent an misbehaving vehicle
from crossing the portion of a divided highway
separating the traveled ways for traffic in
opposite directions. Roadside barriers, on the
other hand, protect vehicles from obstacles or
slopes on the roadside. They also may be used to
shield pedestrians and property from the traffic
stream.

37. Curbs and Gutters.
 Curbs are raised structures made of either
Portland cement concrete or bituminous
concrete (rolled asphalt curbs) that are
used mainly on urban highways to
describe pavement edges and pedestrian
walkways. Curbs are also used to control
drainage, improve aesthetics, and reduce
right of way. Curbs can be generally
classified as either vertical or sloping.

39. Gutters
 Gutters or drainage ditches are usually
located on the pavement side of a curb to
provide the principal drainage facility for
the highway. They are sloped to prevent
any hazard to traffic, and they usually
have cross slopes of 5 to 8 percent and are
1 to 6 ft wide. Gutters can be designed as
V-type sections or as broad, flat, rounded
sections

41. Guard Rails.
 Guard rails are longitudinal barriers placed
on the outside of sharp curves and at sections
with high fills. Their main function is to
prevent vehicles from leaving the roadway.
They are installed at embankments higher
than 8 ft and when shoulder slopes are
greater than 4:1. Shapes commonly used
include the W beam and the box beam. The
weak post system provides for the post to
collapse on impact, with the rail deflecting
and absorbing the energy due to impact.

43. Sidewalks.
 Sidewalks are usually provided on roads in
urban areas, but are uncommon in rural areas.
Nevertheless, the provision of sidewalks in
rural areas should be evaluated during the
planning process to determine sections of the
road where they are required. For example,
rural principal arterials may require
sidewalks in areas with high pedestrian
concentrations, such as adjacent to schools,
industrial plants, and local businesses