CHAPTER 3Introduction to Physical Design of Transportation
Facilities
3.1 THE DESIGN PROCESS
There are many ways to describe the design process for
transportation facilities or transportation systems. The overall
process of developing a transportation project is a mixture of
technical, legal, and political elements. In this process, there is
no clear distinction between what is usually referred to as
planning and the process known as design. Planning refers to the
more general and abstract parts of the process and design to the
more detailed and concrete, but both involve use of rational
processes to decide how to use available resources to achieve
goals. The overall process is a coordinated process of information
gathering, analysis, and decision making. In most all cases, it is
open-ended (that is, there is no one right answer, although some
answers may be better that others in terms of particular goals) and
iterative, so that various alternatives are proposed and evaluated
before the final decision is made.
Figure 3.1 is one way of representing the overall transportation
facility design process.
Specific steps:1. Deciding generally what sort of system or
facility is needed a highway, a mass transit route (or station), an
airport, even a whole system. This step is normally considered to
be a part of the planning process, and is the responsibility of
transportation planning officials and the political system as a
whole; nevertheless, design engineers are the key participants.2.
Demand analysis for the system or facility to be designed. In this
context, transportation demand analysis is an attempt to predict,
as accurately as possible, the number or type of trips which will
take place on a particular facility.3. Traffic performance
analysis. In this step, the designer establishes the relationship
between anticipated demand and the design features of the facility
or system. This step is often referred to as capacity analysis.4.
Size the facility or system, based on performance standards and the
traffic analysis. For a highway, for instance, this consists of
deciding the number of lanes to be provided at various locations.5.
Determine the location of the facility or system. This step
ordinarily requires consideration of several alternative locations.
Deciding between them may further require preliminary designs, cost
estimates, and environmental impact analyses, and will usually
involve public hearings and other public decision processes.6.
Determine the configuration and/or orientation of the facility or
system. Orientation refers to such matters as the direction of an
airport runway; configuration refers to things like transit system
route structures or selection of highway interchange types.7.
Identify physical design standards. These are often a matter of
policy within a given design organization, but the individual
designer must judge the applicability of given design standards to
particular situations.8. Geometric design. Geometric design refers
to establishment of horizontal and vertical alignments and cross
sections, based on considerations such as operating characteristics
of vehicles, design standards, and drainage. 9. Design auxiliary
systems, such as drainage, lighting, traffic control, and power
supply (for electrified rail lines)10. Design surface or guideway.
This refers to the design of pavement or track for land
transportation facilities.11. Estimate construction costs and
project impacts. Major cost item in the design of a transportation
facility include land (right-of-way, earthwork, structures, and
control devices). Final cost estimates are necessary before jobs
can go out to bid. It is also necessary to identify environmental
impacts and the cost of environmental mitigation.12. Evaluate
design. Designs should be evaluated continually throughout the
design process. Evaluation are based on criteria such as physical
feasibility; economy; and social, economic, and environmental
impacts.
3.2 DESIGN STANDARDS
Responsibility for the establishment of design standards varies,
depending on the type of facility. Design standards for the state
highways are established by the state departments of
transportation. These standards are usually based on the
recommended standards of the American Association of State Highway
and Transportation Officials (AASHTO).Establishment of design
standards for rail facilities is the responsibility of each
individual railroad company or transit authority. Recommended
standards are published by the American Railway Engineering
Association.The Federal Aviation Administration (FAA) has
established design standards for airport landing areas (runways,
taxiways, etc.)The physical performance of a transportation
facility, including its comfort and safety, is a result of the
interaction of vehicular characteristics, human characteristics,
and the characteristics of the transportation facility. Physical
design standards link physical performance to design elements such
as horizontal alignment, vertical alignment, cross section and
various design details.
Transportation system characteristics (or design elements) to
which design standards apply include the following:
Minimum radius of horizontal curve. This standard applies to
highways and railways. For a given design speed, minimum curve
radius is limited by maximum allowable side friction, which is
usually based on a comfort standard; maximum superelevation rate
(or banking) for the curve, and the necessity to maintain stopping
sight distance. Maximum rate of superelevation. This standard
applies to highways and railways. For highways, maximum
superelevation rate is limited by side friction and by presence of
roadside features such as driveways. The major concern here is to
prevent slow-moving vehicles from sliding to the inside curve under
slippery conditions. For railways, it is limited by the need ti
limit imbalances in the roads on the rails. Maximum grade. This
standard applies to highways, railways, and airport runways.
Maximum upgrades are limited by vehicle power/ weight ratios and
vehicle traction. Maximum downgrades are also limited by stopping
distances and sight distances. Maximum grade standards for
particular classes of roadway or railway are also influenced by
traffic levels and the need to maintain reasonable speeds on
upgrades. Minimum grades for some types of highways are limited by
the need to provide drainage. Minimum cross-slopes for highways,
runways, and taxiways are also limited by the need to provide
drainage. Minimum length of vertical curve. This standard applies
to highways, railways, and airport runways and taxiways. For
highways minimum length of vertical curve is limited by stopping or
passing sight distance requirements, vertical acceleration, and
appearance standards. For railways, minimum length of vertical
curve is also limited by the need to prevent jerk on couplings in
sag vertical curves. For runways and taxiways, minimum length of
vertical curve is limited by sight distance requirements. Edge
radii in roadway and taxiway intersections are limited by vehicle
turning radii. These, in turn, are related to vehicle wheelbase
dimensions. Minimum intersection setbacks (minimum distances to
obstruction to vision) are limited by stopping sight distance and
driver gap-acceptance behavior. Freeway ramp junction details are
limited by gap-acceptance behavior, steering behavior in entering
and exiting lanes, and vehicle acceleration and deceleration
capabilities. Horizontal and vertical clearances apply to all modes
of transportation. These are limited by vehicle dimensions and, in
the case of horizontal clearances for highways, by the need to
provide clear recovery zones for vehicles that run off the
road.
3.3 DESIGN SPEED AND SIGHT DISTANCE
Design speed (for highways) is defined as the maximum safe speed
that can be maintained over a specified section of highway when
conditions are so favorable that the design features of highway
govern.Design speeds vary depending on terrain and the anticipated
level and character of use of the facility. For highways, AASHTO
recommends the speeds given in table 3.2.
Sight Distances1. Stopping sight distance is the distance
required to see an object 150 mm high on the roadway.
The stopping sight distance s depends on the reaction of the
driver (including both perception time and the time required to
react physically) and the braking distance of the vehicle. That
is,
s = dr +db
Wheredr = the distance traveled during the drivers reaction
timedb = the braking distanceThe distance traveled during the
reaction time of the driver is just the speed of the vehicle times
the reaction time, or
dr = vtrwherev = design speedtr = drivers reaction time
(including perception time)
Braking distance can be computed by the formula,
db = wheredb = braking distanceg = acceleration of gravityf =
coefficient of friction between tires and pavementG = average
grade, dimensionless ratio (m/m)
For cases for which G varies (for instance, in a vertical curve)
an average velue for the entire brake reaction distance is used.
AASHTO also gives the mixed unit formula,
db = Where V is in kilometers per hour, db is in meters, and the
effect of grade is ignored.
Values of f, like assumed reaction times, are chosen to be
conservative, and ay vary with design speed. Table 3.3 gives values
of f recommended for AASHTO.
Example 3.1 Determine the minimum stopping sight distance on a
-3.5% grade for a design speed of 110 km/h.Total required stopping
sight distance:s = dr +dbReaction distance:dt = vtr = (110
km/h)(2.5 s) = 76.4 mBraking distance:f = 0.28 (Table 3.3)G = 0.035
(given)db = = = 194.4 m
Total sight distance:s = dr +db = 76.4 + 294.4 = 270.8 m
2. Passing sight distance is the distance required to see an
oncoming vehicle of a certain minimum size. The normally concerns
of these are only on two-way highway.
Calculation of passing sight distance is somewhat more
complicated, in that it depends on the relative speeds leading,
overtaking, ad oncoming vehicles, and on the minimum gap between
the oncoming vehicle and the vehicle being passed that the driver
of the passing vehicle will accept.
3.4 DESIGN DOCUMENTSRequired design documents for transportation
projects will vary somewhat depending on the type of facility. In
many cases, these detail will be reproduced from sets of standard
plans, which are maintained by the most design agencies. The four
basic elements are:
1. The plan view (or simply plan). This is the drawing of the
facility as it would look to an observer directly above it.2. The
profile. This drawing has elevation as its vertical axis, and
horizontal distance, as measured along the centerline of the
facility (or other recognized reference line) as its horizontal
axis.3. The geometric cross-section. This view has elevation as its
vertical axis and horizontal distance, measured perpendicular to
the centerline, as its horizontal axis.4. The superelevation
diagram. This applies to curved facilities, such as highways or
railways, only. It consists of a graph with roadway or railway
cross-slope (vertical axis) versus horizontal distance (horizontal
axis). The cross-slope is measured relative to the centerline or
some axis rotation for the facility.