Design Criteria for Highways / railways and highway alignment

In order to provide roads that are safe to all road users, there should be a design criteria or standards for highway and railway that are suitable according to the limitations of human capacity. Therefore, it leads to the so-called "safe system approach" which encourages the following:

Simpler, self-explaining roads
Designing roads that enforce safe speeds
Forgiving roadsides (Clear Zone). Clear zones refer to areas provided adjacent to highways that are free from hazardous obstacles or steep slopes.
Functionality, Homogeneity, and Predictability of roads

CONTROLs in optimizing and improving highway design

HUMAN FACTORS AND DRIVER PERFORMANCE

Driver tasks (vehicle control and guidance)
Older pedestrians and drivers using roads and highways
Errors due to driver deficiencies and situational demands
Speed

VEHICLES

Analysis on such critical features such as radii at intersection and radii of turning roadways is therefore important classifying and considering the largest design vehicle. The following are the considerations used for the design of highway facilities:

Passenger cars may be selected when traffic generator is parking lot
Two-axle single unit truck for intersection design of residential streets and park roads
Three-axle single unit truck for design of collector streets and other facilities where large single unit trucks are used
Bus may be used for highway intersections with designated bus routes

TRAFFIC CHARACTERISTICS

Highway design and features must include traffic volumes and characteristics. Traffic volumes are considered crucial to road design and specifications as it directly influences geometric design features and other relevant studies.

PHYSICAL ELEMENT

Highway Capacity
Access control and management
Pedestrians (Sidewalk, crosswalks, traffic control features, etc.)
Bicycle facilities
Safety and environment

ECONOMIC FACTORS

It covers the total cost for highway improvement with its corresponding benefits (e.g., the ability of highways to provide just-in-time delivery services and other supply chain-related applications).

DESIGN CONTROLS OF HIGHWAY CONSTRUCTION

ANTICIPATED TRAFFIC VOLUME

Roads shall accommodate the number of vehicles which is estimated to pass it towards the end of its life. The number is called design volume. The minimum life of roads is assumed to be within 10-15 years for flexible pavement and 20 years for rigid.
Annual average daily traffic (AADT) is used in volume design. Other critical points such as intersections and peak traffic figures are also considered
Also known as the 30th highest hourly volume, the design volume (DHV) should be predicted on current traffic plus all traffic increases.
The usual traffic increase on highway improvement is between 50-150%, while expressways are in the range of between 80-200%

CHARACTER OF TRAFFIC

The weight dimension of the largest vehicle that will possibly use the highway for transport, as well as mobility and other characteristics are always taken into account for designing the road. Thus, the required width of lanes, which is the sum of widths of traffic lanes, dividing islands, curbs, gutters, ditches and other features relevant to width scale are also crucially important for the design

DESIGN SPEED

Design speed refers to the maximum safe speed that can be maintained over a specified section of the highway that the design features of highway govern. It is mainly attributed to the character of the terrain, man-made features and economic considerations. Introduction of higher and lower design speed should not be abruptly as there should be a permissible distance for drivers to change their speed in a gradual manner.

DESIGN TRAFFIC (VEHICLES)

These includes vehicle types and dimensions, turning radii and off tracking, resistance to motion, power requirements, acceleration and deceleration performance. Other factors that are considered include forces, as vehicles move with rolling, air, curve, and grade resistance.

HIGHWAY CAPACITY

Roadway factors such as geometric influence the overall road capacity and other performance measures. Other roadway factors include number of lanes, type of facility and its environment, lane widths, shoulder widths and lateral clearances, design speed, horizontal and vertical alignments, axle loads, and the availability of exclusive turn lanes at intersections

CLASSIFICATION OF HIGHWAY

Highway characteristics can be classified into multilane suburban, rural highways, and expressways. Unlike expressways, design standards for multilane tends to be lower as it has greater impact on drivers in terms of developed frontage and visual settings. Design standards are higher in comparison to urban streets. Thus, drivers on multilane highway are able to pass a slower moving vehicle without using lanes designated for oncoming traffic. In general, multilane highways have better design features than two-lane highways.

ACCIDENT INFORMATION

Study of accidents according to location, type, severity, contributing circumstances, environmental conditions, and time periods may suggest possible safety deficiencies.

GEOMETRIC DESIGN FOR HIGHWAYS AND RAILWAYS

The alignment of highway is a three-dimensional problem measured in x,y, and z coordinates. But due to complexity of three dimensional design computations, highway alignment problems are reduced into two dimensional alignment; Horizontal alignment consisting of x and z coordinates (constant elevation) and Vertical alignment which consists of y coordinates (elevation). Instead of using x and z coordinates, 2D alignment is then simplified into highway positioning and length, defined as distance along the highway from a specified point

Here is the notation for stationing distances in a highway:

Metric

1 Station = 1000.000 m

If we begin at 0.000m and measure 400.000 along the reference line, we are at station 0+400.00

US Survey Foot

1 Station = 100.00'

If we begin at 0.00' and measure 200.00' along a reference line, I am at station 2+00.00

HIGHWAY ALIGNMENT IN 3D VIEW

HIGHWAY ALIGNMENT IN 2D VIEW

VERTICAL ALIGNMENT

Vertical alignment specifies elevation of points along the roadway. The elevation of roadway points are determined in order to provide an acceptable level of driver's safety, driver comfort, and proper drainage (from rainfall runoff). However, establishing transition of roadway elevations between two grades is difficult to achieve in vertical alignment. This can be made using vertical curves

Vertical curves can be classified into two types:

Crest Vertical Curve

These are the curves that connect inclined sections of roadway, forming a crest.

2. Sag Vertical Curve

Sag vertical curves are curves connecting descending grades, forming a bowl or sag. It is characterized by a positive change in grade, which means vehicles travelling over sag vertical curves are accelerated upward.

Elements of vertical curves can be solved via:

a.) Solution using grade diagram

b.) Using analytic geometry

ELEMENTS OF A VERTICAL CURVE

PC = Point of curvature

PT = Point of tangency

PI = Point of intersection of the tangents

L = Length of the parabolic curve

S1 = Horizontal distance from PC to the highest (lowest) point of the summit (sag) curve

S2 = Horizontal distance from the PT to the highest (lowest) point of the summit (sag) curve

h1 = Vertical distance between PC and the highest (lowest) point of the summit (sag) curve

h2 = Vertical distance between PT and the highest (lowest) point of the summit (sag) curve

g1 = grade (in percent) of the back tangent (tangent through PC)

g2 = grade (in percent) of forward tangent (tangent through PT)

A = change in grade from PC to PT

a = vertical distance between PC and PI

b = vertical distance between PT and PI

H = vertical distance between PI and the curve

Here are the following formulas and examples for solving symmetrical parabolic curves using grade diagram

Part 2.pdf

UNSYMMETRICAL PARABOLIC CURVE

Unsymmetrical parabolic curve happens when two or more parabolic curves are placed adjacent to each other. Below is an example of solving missing elements of an Unsymmetrical Parabolic curve using grade diagram.

ASSESSMENT TASKS IN CLO2

Problem Set #1

Problem Set #1_BARTOLOME.pdf

Problem Set #2

Problem Set #2_BARTOLOME.pdf

REFLECTION/S

When designing the highway, there are factors and physical classifications that needs to be considered. Based on what I've learned throughout the topics under CLO2, the design criteria for highway and other road constructions heavily relies on geometric design elements and controlling factors. In order to achieve maximum safety of drivers, improve vehicle performance and visual settings, and have a traversable, self-explaining roads, transportation engineers should take into account the characteristics that are essential for improving functionality of roads and highways. Thus, design controls are vitally important as anticipation of larger number of vehicles has always been a common issue for roadway users especially in urban areas.

Furthermore, understanding the geometric design elements is also important in highway design. The key concepts and elements of highway alignment greatly helped in solving any highway alignment problem as I am able to correlate design elements and the performance characteristics of vehicles.

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