Module 4: Geometric Criteria for Highways and Railways PDF

# Module 4: Geometric Criteria for Highways and Railways ## Principles of Geometric Design for Highways - Average Daily Traffic (ADT) - Under 200 - 200-400 - 400-1000 - 1000-2000 - Design Speed (kph) - Flat Topography: 60, 70, 70, 90, 80, 95, 90, 100 - Rolling Topography: 4...

# Module 4: Geometric Criteria for Highways and Railways ## Principles of Geometric Design for Highways - Average Daily Traffic (ADT) - Under 200 - 200-400 - 400-1000 - 1000-2000 - Design Speed (kph) - Flat Topography: 60, 70, 70, 90, 80, 95, 90, 100 - Rolling Topography: 40, 50, 60, 80, 60, 80, 70, 90 - Mountainous Topography: 30, 40, 40, 50, 50, 60, 60, 70 - Radius (meters) - Flat Topography: 120, 160, 160, 280, 220, 320, 260, 350 - Rolling Topography: 55, 85, 120, 220, 120, 220, 160, 280 - Mountainous Topography: 30, 50, 50, 80, 80, 120, 100, 160 - Grade (percent) - Flat Topography: 6.0, 6.0, 5.0, 3.0, 4.0, 3.0, 4.0, 3.0 - Rolling Topography: 8.0, 7.0, 6.0, 5.0, 5.0, 5.0, 5.0, 4.0 - Mountainous Topography: 10.0, 9.0, 8.0, 6.0, 7.0, 6.0, 7.0, 5.0 - Pavement Width (meters) - 4.0, 5.5-6.0, 6.10, 6.70, 6.70, 7.30 - Shoulder Width (meters) - 0.5, 1.0, 1.50, 2.00, 2.50, 3.00, 3.00 - Right of Way Width (meters) - 20, 30, 30, 30, 30, 60 - Superelevation (meters/meter) - 0.08 (max.), 0.08 (max.), 0.08 (max), 0.08 (max.) - Non Passing Sight Distance (meters) - Flat Topography: 70, 90, 90, 135, 115, 150, 135, 160 - Rolling Topography: 40, 60, 70, 115, 70, 115, 90, 135 - Mountainous Topography: 40, 40, 40, 60, 60, 70, 70, 90 - Passing Sight Distance (meters) - Flat Topography: 420, 490, 490, 615, 560, 645, 615, 675 - Rolling Topography: 270, 350, 420, 560, 420, 560, 490, 615 - Mountainous Topography: 190, 270, 270, 350, 360, 420, 420, 490 - Type of Surfacing - Gravel, crushed gravel or crushed stone, bitumen preservative treatment, single or double plant mix surface course, bituminous concrete surface course - Bituminous dense or open graded. - Bituminous concrete surface course. - Portland cement concrete pavement ## General Principles for Geometric Design - Cross Section - Profile/Vertical Alignment - Alignment/Horizontal Alignment ## Cross Section of Road and Rail Infrastructure  ## General Principles for Geometric Design - STRAIGHT: Shortest distance between 2 points, line of constant bearing. - Easy to design, set out and maintain. - Constant force on track from train wheels. - Shortest-quickest - distance to travel. - CIRCULAR CURVE: Line that is a fixed distance (radius) from a point (circle center). - Constant force from train wheels which can be offset by cant/superelevation. - Easy to design, set out and maintain. - TRANSITION: Curve of constantly changing radius - a spiral. - They avoid an instant change of radius, which would be very noticeable at high speed inside the train - see rules in following section. - Provide area for cant/super elevation - to be built up. ## Horizontal Geometric Design Consideration for Highways - Horizontal Alignment: Series of straights (tangents) connected by circular curves. It is common to introduce transition or spiral curves between the tangents and circular curves. - Major considerations in horizontal alignment design are: - Safety - Grade Profile - Type of Facility - Design Speed - Topography - Construction Cost ## Key components for horizontal alignment - Radius of Curve - Length of Curve - Superelevation - Widening - Spiral Curve/Transition ## The key steps in the design of horizontal curves are listed below. - Determine a reasonable maximum superelevation rate. - Decide upon a maximum side-friction factor (friction between the wheel and pavement). - Calculate the minimum radius for your horizontal curve. - Iterate and test several different radii until you are satisfied with your design. - Make sure that the stopping sight distance is provided throughout the length of your curve. Adjust your design if necessary. - Design the transition segments. ## Cant/Superelevation - Is the difference in height between the outer and the inner rail/road on a curve. It is provided by gradually raising the outer rail/road above the level of the inner rail/road. - Controlled by 4 factors - Climate conditions (amount of ice and snow) - Terrain (flat, rolling, mountainous) Type of area (rural or urban) Frequency of slow moving vehicles who might be influenced by high superelevation rates - Typical rates and values - Highest in common use = 10%, 12% with no ice and snow on low volume gravel-surfaced roads - 8% is logical maximum to minimize slipping by stopped vehicles  - The superelevation of the highway cross-section and the side-friction factor are two of the most crucial components in the design of horizontal curves. - The superelevation is normally discussed in terms of the superelevation rate, which is the rise in the roadway surface elevation as you move from the inside to the outside edge of the road. - For example, a superelevation rate of 10% implies that the roadway surface elevation increases by 1 ft for every 10 ft of roadway width. ## Side-Friction - The side-friction factor is simply the coefficient of friction between the design vehicle's tires and the roadway. - The coefficient of friction is a function of several variables, including the pavement type and the vehicle speed. - The side-friction factors that are employed in the design of horizontal curves should accommodate the safety and comfort of the intended users. - In general, studies show that the maximum side friction factors developed between new tires and wet concrete pavements range from about 0.5 at 30 km/h to approximately 0.35 at 100 km/h. For normal wet concrete pavement and smooth tires the value is about 0.35 at 70 km/h. In all cases the studies show a decrease in friction values for an increase in speed. | Speed (km/h) | Comfortable Side-Friction Factor | |---|---| | 40 | 0.21 | | 50 | 0.18 | | 55-80 | 0.15 | | >110 | <0.10 | ## Minimum Curve Radius - The minimum radius is a limiting value of curvature for a given design speed and is determined from the maximum rate of superelevation and the maximum side friction factor selected for design (limiting value of f). - The minimum radius for a horizontal curve is based on three factors: - the design speed - the superelevation, and - the side-friction factor. - The minimum radius serves not only as a constraint on the geometric design of the roadway, but also as a starting point from which a more refined curve design can be produced. - The smallest radius is also the curve that requires the most centripetal force. Use of sharper curvature for that design speed would call for superelevation beyond the limit considered practical or for operation with tire friction beyond what is considered comfortable by many drivers, or both. ## Horizontal Sight Distance - Sight Distance: distance along the center line of the road at which a driver has visibility of an object. - Can be the controlling aspect of horizontal curve design where obstructions are present near the inside of the curve. - To determine the actual sight distance that you have provided, you need to consider that the driver can only see the portion of the roadway ahead that is not hidden by the obstruction. - In addition, at the instant the driver is in a position to see a hazard in the roadway ahead, there should be a length of roadway between the vehicle and the hazard that is greater than or equal to the stopping sight distance. - M = (5730/D)*(1-cos(SD/200)) in feet - M = R*(1-cos(SD/200)) in feet Where: - R = radius - SD = sight distance - M = the distance between the centerline and obstruction. ## Horizontal Geometric Design Consideration for Highways - The horizontal alignment consists of straight sections of the road (known as tangents) connected by curves. The curves are usually segments of circles, which have radii that will provide for a smooth flow of traffic. - Simple curves with spirals - Broken Back: two curves same direction (avoid) - Compound curves: multiple curves connected directly together (use with caution) go from large radii to smaller radii and have R(large) < 1.5 R(small) - Reverse curves: two curves, opposite direction (require separation typically for superelevation attainment) ## General Principles for Geometric Design - TRANSITION CURVE (SPIRAL, CLOTHOID, ETC): - Rises on a steady curve until it has completed a loop, passing over itself as it gains height, allowing the railway to gain vertical elevation in a relatively short horizontal distance. - Is an alternative to a zig-zag. - There are five objectives for providing transition curve and are given below: - To introduce gradually the centrifugal force between the tangent point and the beginning of the circular curve, avoiding sudden jerk on the vehicle. This increases the comfort of passengers. - To enable the driver turn the steering gradually for his own comfort and security. - To provide gradual introduction of super elevation, and - To provide gradual introduction of extra widening. - To enhance the aesthetic appearance of the road. ## Obligatory Points - These are the control points governing the highway alignment. These points are classified into two categories. Points through which it should pass and points through which it should not pass. - Some of the examples are: - Bridge site - Mountain - Intermediate town - These were some of the obligatory points through which the alignment should pass. Coming to the second category, that is the points through which the alignment should not pass are: - Religious places - Very costly structures - Lakes/ponds etc ## Highway Intersections - INTERSECTION: is the area shared by the joining or crossing of two or more roads. The design and operational characteristics of each of the major interchange types are mentioned as follows and are discussed in the following sections. - Underpass: an underground passageway, completely enclosed except for openings for ingress and egress, commonly at each end. A tunnel may be for foot or vehicular road traffic, for rail traffic. - Overpass: a flyover, is a bridge, road, railway or similar structure that crosses over another road or railway. A pedestrian overpass allows pedestrians safe crossing over busy roads without impacting traffic. - Trumpet Interchange: used where one highway terminates at another highway. These involve at least one loop ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the continuous highway. - Diamond Interchange: simplest form of grade separated intersection between two roadways. The conflicts between through and crossing traffic are eliminated by a bridge structure. - Cloverleaf Interchange: eliminates all crossing movement conflicts by the use of weaving sections. It replaces a crossing conflict with a merging, followed some distance farther by a diverging conflict. - Partial Cloverleaf Interchange: parclo is a modification that combines some elements of a diamond interchange with one or more loops of a cloverleaf to eliminate only the more critical turning conflicts. - Directional Interchange: provides direct paths for left turns. These interchanges contain ramps for one or more direct or semi direct left turning movements. Interchanges of two freeways or interchanges with one or more very heavy turning movements usually warrant direct ramps. - Bridged Rotary: provided at circular junctions where traffic is permitted to flow in one direction around Central Island and one road axis is raised above the rotary intersection to allow grade separation. ## Railway Turnouts & Crossing - Turnouts: permit one track to cross another at grade. Such crossings can be designed as a rigid block or can include movable center points. - More switches means more flexibility of the network. - More switches also increases the change of one breaking down and disturbance of the railway network. - The length and the maximum allowable speed to pass the turnout in the divergent direction are given below as well: - 1:9 - 32 meter - 40km/h - 1:12 - 38 meter - 60 km/h - 1:15 - 47 meter - 80 km/h - 1:34 - 99 meter - 140 km/h  ## Horizontal Geometric Design Consideration for Railways - Level Crossings: Level crossings are provided on railway lines to allow road traffic to pass across the track. As the level of the passing road traffic is the same as that of the railway track, the crossing is referred to as a level crossing. - Major Level/Road Crossing: located at major road/street with a minimum of four (4) lanes. - Minor Level/Road Crossing: located at minor road/street with a maximum of two (2) lanes. - Key issues to be addressed on level crossings: - Vertical Clearance - Visibility - Signs and Markings - Safety and provisions for pedestrians and non-motorized road users - Geometric Implications of Chosen Design Vehicle ## General Principles for Vertical Curves - GRADE: is a measure of the inclination, or slope, of the roadway. It is defined as the rise over the run. - In other words, a 10% grade simply means that the elevation of the roadway increases by 10 feet for every 100 feet of horizontal distance. - The issues that surround the design of inclined roadway sections revolve around safety and level of service. - Research has also shown that most passenger cars are essentially unaffected by grades below 4-5%. Large commercial vehicles and recreational vehicles, on the other hand, are extremely sensitive to changes in grade. - For descending grades, the potential consequences of runaway vehicles are evident when you consider a highly populated area that is located at the base of a long, steep grade. - When two different or contrary gradients meet, they are connected by a curve in vertical plane is called a vertical curve. It is advisable to introduce a vertical curve in road and in railway work in order to round off the angle and to obtain a gradual change in grade so that abrupt change in grade is avoided at the apex. - It gives adequate visibility and safety to the traffic. - It gives gradual change in grade or slope. - It gives adequate comfort to the passengers. ## Vertical Geometric Design Consideration for Highways - Sight Distance - Ability to see ahead is of the utmost importance in the safe and efficient operation of the highway. - Distance at which a driver of a vehicle can see an object of specified height on the road ahead, assuming adequate sight and visual acuity and clear atmospheric conditions. - Stopping (Non-Passing) Sight Distance (SSD): - Minimum distance required for a vehicle, travelling at the design speed, to stop before reaching an object in its part. - Sum of the distance travelled during perception, brake reaction time, and the distance travelled whole breaking to a stop on wet pavements. - SSD consists of distances travelled during these time intervals: - Time for the driver to perceive the hazard - Time for reaction - Time to stop the vehicle after the brakes are applied. - Decision Sight Distance: Distance needed for a driver to detect an unexpected or otherwise difficult to perceive information source or condition in a roadway environment that maybe visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete complex maneuvers. - Passing Sight Distance: - Minimum distance required to safely make a normal passing maneuver on 2-lane highways at passing speeds common to nearly all drivers, commensurate with design speed. - The minimum passing sight distance for a 2-lane highway is determined as the sum of four distances: - Initial maneuver distance: distance traversed during perception and reaction time and during the initial acceleration to the point of encroachment on the left lane. - Distance travelled while the passing vehicle occupies on the left lane. - Distance between the passing vehicle at the end of its maneuver and the opposing vehicle. - Distance traversed by an opposing vehicle for two-thirds of the time the passing vehicle occupies the left lane, or 2/3 of the distance travelled whole the passing vehicle occupies the left lane. - Calculating the passing sight distance required for a given roadway is best accomplished using a simple model. The model that is normally used incorporates three vehicles, and is based on six assumptions: 1. The vehicle being passed travels at a constant speed throughout the passing maneuver. 2. The passing vehicle follows the slow vehicle into the passing section. 3. Upon entering the passing section, the passing vehicle requires some time to perceive that the opposing lane is clear and to begin accelerating. 4. While in the left lane, the passing vehicle travels at an average speed that is 10 mph faster than the vehicle being passed. 5. An opposing vehicle is coming toward the passing vehicle. 6. There is an adequate clearance distance between the passing vehicle and the opposing vehicle when the passing vehicle returns to the right lane. - Headlight Sight Distance: For night driving on highways without lighting, the length of visible roadway is that roadway that is directly illuminated by the headlights of the vehicle. For certain conditions the minimum stopping sight distance values used for design can exceed the length of visible roadway. ==End of OCR for page 1==