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.