MODULE 3.pdf

Naga College Foundation College of Engineering Module for 50Flex/Online Learning Course Title: Highway & Railroad Engineering Module No. 3 Credit Unit...

Naga College Foundation College of Engineering Module for 50Flex/Online Learning Course Title: Highway & Railroad Engineering Module No. 3 Credit Unit 3 units (Lecture) Period: Preliminary Period Professor: Engr. Francisco G. Naval C.E./Assistant Professor I CHAPTER TWO Designing the Highway Introduction Section 1 to 6 Introduction Consistency is the most important single rule in highway design. Highways are made for various types of vehicles and they are operated by drivers. These drivers expect the highway agency to provide them with a.) Clear information and guidance through a variety of road signs and b.) Avoiding abrupt changes in the traffic as well as the road standards. Roadway facilities are considered satisfactory if guides and directions could be planned properly to convey one single message at a time, and if these directions will be followed smoothly and easily without undue haste or changes in speed. It is recommended that highway signs and directions should be integrated as early as the preliminary layout studies. 1.) Definition of Terms AASHTO – American Association of State Highway and Transportation Officials Given the power to govern its own operation Engineering activities were implemented by the standing committees with the task of preparing specifications manual and standards, representing the current highway engineering practice. Roads and Highways – strip of land that have been cleared and further improve for the movement of people and goods. Roads – generally used to describe a public thoroughfare. It can also refer to railways. Highway – public road built by digging ditches on both sides and heaping up the earth on the middle creating a way higher than the adjacent land. Divided arterial highway for through traffic with full or partial control or access and generally provided with grade separation at major intersection. Freeway – an expressway with full control of access Control of Access – condition where rights of owners or occupants of adjoining land or other person’s access to light, air or view in connection with a highway is fully or partially controlled by public authority Full Control of Access – the authority to control access is exercised to give preference to through traffic by providing access connections to selected public road only. Crossing at grade or direct private driveway connections is not permitted. Page 2 of 12 Partial Control Access – the authority to control access is exercised to give preference to through traffic. There may be some crossing at grade and some private driveway connections allowed. Through Street or Through Highway – Every highway or portion thereof on which vehicular traffic is given preferential right of way, and at the entrance to which vehicular traffic from intersecting highways is required by law to yield right of way to vehicles on such through highway in obedience to either stop sign or yield sign erected thereon. Parkway – is an arterial highway for non-commercial traffic, with full or partial control of access usually located within a park or ribbon park-like development. Arterial Street – is an arterial route that carries traffic to the nearest point or through traffic. Arterial Route – main route or major route Basic Considerations in Planning Arterial Roadways: 1. Selection of the route 2. Studies of the traffic volume 3. Origin and destination 4. Accident experienced 5. Width should not be less than 15 meters 6. Must carry at least one lane of traffic in each direction 7. Should be at least one kilometer in length 8. Should skirt neighborhood areas rather than penetrate them 9. On grid design system streets, arterials are spaced at about 600 to 900 meters apart 10. Where accident hazard is not a factor, the minimum volume to justify arterial road is 300 vehicles per average hour during the day, and 450 vehicles hourly during peak periods. Sometimes, peak hour traffic volume on designated arterial road exceeds the capacity of two-way streets, and to increase the arterial capacity, the following solutions are enforced: 1. During peak hours, parking is prohibited on one or the side of the street 2. Parking is prohibited several meters away from each side of the road intersections or corners 3. Right turn is allowed on red signals any time with care 4. Left turns are eliminated on congested intersections 5. The direction of traffic is reversed in the center lane to provide more lanes in the direction of heavier traffic flow. Collector Street – form smaller mesh grid pattern where passengers are pick from service streets and carried to the arterial road. Local Road – street or road primarily for access to residence, business, or other adjoining properties. It is a road maintained by the local government. Highway Capacity – maximum number of vehicles that are reasonably expected to pass a given point over a given period of time usually expressed as vehicles per hour. 1. Under ideal conditions, one freeway lane can accommodate about 2,000 passenger cars per hour 2. Two-lane road can carry up to 1,000 passenger cars per hour in each direction As the volume of traffic approaches capacity, the average speed is markedly reduced. AADT or ADT – Average Annual Daily Traffic or Average Daily Traffic, refers to traffic volume or flow on a highway as measured by the number of vehicles passing a partial station during a given interval of time, if the period is less than one year. Observed Traffic Volume – estimated 30th hour volume commonly used for design purposes Page 3 of 12 2.) The Design Speed - The design speed is the speed determined for design and correlation of the physical feature of a highway that influence vehicles operation. It is the maximum speed that can be maintained over specified section of the highway when weather and traffic conditions are so favorable that the design features of the highway govern. - Basic Design Features refers to the tightness and super elevation of curves, the sight distance, and grade. - AASHTO recommends that, the design speed be set to the greatest degree possible, to satisfy the needs of nearly all drivers both today and throughout the roads anticipated life. - For economic reasons, geometric features of certain road sections are designed for speed from 30 to 50 kilometers per hour. For control of over speeding vehicles, highway design adopted control by reducing the super elevation combined with easement curves, delineators, stripping signs and rumble strips for approaching sharp curves or blind curves. - The AASHTO practice is to classify first the highway as Rural or Urban, then as Freeways, Arterial, Collectors and Local. Rural collectors and local facilities are classified as flat, rolling or mountainous. - Although the design speed presently used is 80 km per hour, highway designers projected design speed up to 120 kilometers per hour to guarantee against future obsolescence as well as the increased margin of operating safety. - The high speed road design must anticipate future generation high speed vehicles and advance transportation system. - Most highways are governed by construction cost, but the concept of the relatively high speed design for freeways is regardless of cost. They are planned for a nationwide high volume traffic and high speed network. MINIMUM RECOMMENDED DESIGN SPEED DESIGN SPEED FACILITY URBAN Km/hr. RURAL Km/hr. Freeway 80 - 95 preferred 110 – 95 mountainous Arterial 64-95 but 48 in built up areas 80 – 110 Collectors 48 km/hr. See table below Local 32-48 km/hr. See table below AASHTO MINIMUM DESIGN SPEED KM/HR. FOR RURAL COLLECTORS AND LOCAL ROADS BASED ON CURRENT ADT Class Terrain Average Daily Traffic Collector 0-400 400-750 750-2000 2000-4000 Over 4000 Level 60 75 75 75 90 Rolling 45 60 60 75 75 Mountainous 30 45 45 60 60 Local 0-50 50-250 250-400 Over 400 Level 45 45 60 75 Rolling 30 45 45 60 Mountainous 30 30 30 45 Page 4 of 12 3.) Cross Section of Typical Highway Variables - The cross section of a typical highway has latitude of variables to consider such as: 1. The volume of traffic 2. Character of the traffic 3. Speed of the traffic 4. Characteristics of motor vehicles and of the driver Uniform Thickness - High volume traffic road facilities adopt cross section that is uniform in thickness from end to end of the improvement. - Low volume traffic facilities, modification of the features like the shoulders width in rough areas are usually employed to reduce cost. Accident Statistics - Cross section design generally offers the expected level of service for safety and a recent study showed that: 1. A 7.2 meters wide pavement has 18% less accident compared with pavement narrower than 5.50 m. wide 2. A 7.2 meters wide pavement has 4% fewer accidents than the 6.00 meters wide roadway. 3. Accidents records showed no difference between the 6.60 meters and the 7.20 meters wide pavement. 4. For the 6.00 m. and 7.20 m. wide pavement with 2.70 to 3.00 m. wide shoulder, recorded accident decreases by 30% compared to 0 to 0.60 m. wide shoulder. 5. And 20% compared with a 0.90 m to 1.20 meters wide shoulder. Standard Width - Original width = 4.5 m., then adjusted to 4.8 m. to 5.4 meters. - Standard width of the road = 3.00 first class paved one lane highway - Standard width for freeways = 3.60 meters - Two lane rural highways = 7.20 meters wide surface is required for safety - Collector roadway = 6.00 meters wide for low volume traffic including few trucks traveling - Local rural roadway = 4.80 meters for a 30km/hr design speed - Urban Roadway = 3.80 meters or 3.00 meters if space does not allow Page 5 of 12 Page 6 of 12 Page 7 of 12 4.) Road Shoulder Road shoulder or verge is that portion of the roadway between the edge of the traffic lane and the edge of the ditch, gutter, curb or side slope. Importance of Road Shoulders: 1. Serves as a place for vehicles to stop when disabled or for some other purposes. It reduces road accident. 2. The road capacity is decreased and accident opportunity increases if the shoulder is too narrow or omitted in the design. 3. Shoulder should be continuous along the full length of the roadway. It also adds structural strength to the road pavement. 4. It increases the horizontal sight distance on curves. It reduces accident potential when vehicle stop during emergencies. A continuous narrow line strip at the edge of the roadway that separates the shoulder, serves as guide to drivers during bad weather and poor visibility conditions. Policy on Geometric Design Recommends that: 1. Outside shoulder should be at least 3.00 or 3.60 meters wide if truck volume is more than 2.50 in the design hour (ADT) 2. The recommended width of left (median) shoulder is 1.20 to 2.40 meters with at least 1.20 meters paved. 3. For 6 or more lanes, the median shoulder should be 3.00 meters wide or 3.60 meters if truck volume in the design hour exceed 2.50 4. For arterials with ADT less than 400 the usual shoulder width is fixed at 1.20 meters minimum, although 2.40 meters wide is much preferred. 5. When the design hour volume ADT exceeds 400, the usual minimum shoulders width is 2.40 meters although 3.60 meters is recommended. 6. For Urban Arterial Road, similar shoulder without curb is suggested unless needed for proposed drainage. 7. The width of median shoulder on four lane divided arterials is fixed at 90 centimeters as minimum. 8. For six or more lanes, 2.40 to 3.00 meters shoulder width is recommended. 9. For Rural Collectors Roadway, 60 centimeters wide graded shoulder is required for ADT’s less than 400. 10. For ADT’s over 2,000, 2.40 meters wide shoulder is recommended WIDTH OF ROADWAY FOR UNDIVIDED HIGHWAY Total Travelway Shoulder Width Roadway Width Speed of Reference 14.00 m. 3.25 20.50 100 – 80 – 60 – 40 12.00 m. 3.25 18.50 60 – 40 7.00 m. 2.75 12.50 100 – 80 – 60 – 40 7.00 m. 2.00 11.00 80 – 60 – 40 6.70 m. 2.75 12.20 100 – 80 – 60 – 40 6.70 m. 2.00 10.70 80 – 60 – 40 6.00 m. 2.75 11.50 80 – 60 – 40 6.00 m. 1.50 9.00 80 – 60 – 40 Page 8 of 12 Page 9 of 12 Page 10 of 12 5.) Cross, Cut or Fill Slope Cross Slope The cross slope is provided in all tangent sections of the roadway. Slope usually falls in both directions from the centerline of the two lane highway except where super elevation of curves directs all water towards the inside. High type pavement crown or slope is often 1% to 2%. Cut or Fill Slope Earth fill of normal height is safe on a slope of 1:2 ratios. The first number is the horizontal distance and the second number is the vertical distance. Slope of cuts through an ordinary undisturbed earth fill remain in place with a ratio of 1:1 slope. Rock cuts could be as steep as 1:2 and sometimes 1:4 proven to be stable. Advantage of Flat Side or Back Slope 1. With back slope of 3:1 or even flatter, cars could be directed to back into the road and will come to stop or continue down the slope with no risk of overturning. 2. Flat fill slopes are visible from the vehicles at full extent giving the roadway safer appearance. 3. With visible slope for being low and flat, vehicles could be positioned or parked closer to the edge, and on two lane roadway, facilities parking would be farther from the opposing traffic. Recommended Policy on Geometric Design 1. The 6:1 slope ratio could be adopted on embankment less than 1.20 meters high, and 4:1 ratio on a higher fill. 2. The 2:1 slope is allowed to heights greater than 6.00 meter 3. Cut slope should not be steeper than 2:1 ratio except on solid rock or special kind of soil. Page 11 of 12 6.) Highway Median & Grade Line Highway Median Advantages of median in most highways 1. Effective means of reducing headlight glares 2. Offers refuge between opposing traffic stream of cross traffic 3. Pedestrian could traverse each stream at separate maneuvers 4. Provides available space for left turn 5. It makes turning of vehicles smooth and safe operation 6. For rural sections of freeway, the 18 to 27 meters wide median is being adopted 7. Medians with 6 to 18 meters wide allow drivers to cross each roadway separately. 8. A 4.20 to 6.60 meters median width provides protection for turning vehicles. 9. Curved median with 1.20 to 1.80 meter width serves as partition – separation of opposite traffic control devices 10. Cross slope of median should not be greater than 6:1 but preferably 10:1 11. Dense planting of rose hedges serves as safety crash barriers Narrow median, 4 means of reducing cross median accidents: 1. Provide deterring devices. Two sets of double strip painted on the existing pavement, raised diagonal bars, low curbing and shallow ditches. 2. Provide non-traversable energy absorbing barriers. The line chain link fence 1 meter high supported by steel post augmented by cables at the bottom and midpoint. 3. Provide non-traversable rigid barriers or metal guardrail. 4. Provide G.M. barriers. Concrete barrier transported and arranged by means of crane. Page 12 of 12 Grade Line Grade line is the longitudinal profile of the highway as a measure how the centerline of the highway rises and fall. It appears on the profile taken along the road centerline. It is a series of straight lines connected by parabolic vertical curves to which straight grades are tangent. In laying the grade line, the designer must consider the following: 1. Where earthwork is minimal and consistently meeting sight distances in relation to grade line, economy is one main consideration. 2. In mountainous areas, the grade line must be considered balanced excavation against embankments to get the minimum overall cost 3. In flat area, the grade line is set almost parallel to the ground surface but sufficiently above the ground for drainage purposes. 4. Undesirables native soil should be provided with sufficient covering 5. Grade line elevations along the river or stream is governed by the expected level of eater flood.