Classification and Cross Section of Roads

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Classification of Roads

  • Street – Road in built up area
  • By pass road – a road built to by pass congested area
  • Ring road – Circumferential road built around an urban area to enable free flow of traffic.
  • Boulevard – City road with landscape.
  • Ribbon development – Unauthorized development around roads / highways

Frontage Roads

  • Serve numerous functions, depending on the type of arterial they serve and the character of the surrounding area.
  • They may be used to control access to the arterial, function as a street facility serving adjoining properties, and maintain circulation of traffic on each side of the arterial.
  • Frontage roads segregate local traffic from the high speed through traffic and intercept driveways of residences and commercial establishments along the highway. Thus the through character of the highway is preserved.
  • Pedestrian – A pedestrian is any person on foot. Involvement of pedestrian is a major consideration in highway planning and design.

Pedestrian & Pedestrian Crossing

Pedestrian Crossing

– Level with road – Zebra Crossing

– Above level of road – foot over bridge (not preferred, 15 to 18 ft. height)

– Below level of road – subway (convenient 7 to 8 ft. height)

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Auxiliary Lanes and Mass Transit

  • Speed change lanes / Auxiliary lanes

– Lanes other than main driving lanes of roadway such as turning lanes, parking lanes, passing lanes or truck lanes etc are auxiliary lanes

  • Mass Transit – Movement of large number of people between small no of locations (physically means of transportation can be highways and railways)

Terrain

• Terrain / Topography – Causes speed of heavy vehicles to reduce below cars.

– Level – heavy vehicles can maintain same speed as a car.

– Rolling – speed of heavy vehicles reduces considerably than cars.

– Mountainous – heavy vehicles operate at crawling speed

Administrative Authorities

  • NHA (National Highway Authority)
  • PWD (Public Works Department)
  • C&W (Communication and Works Department)
  • Local Government

Commercial Vehicles

  • Rigid Chases – SU Trucks (2 to 4 axle cargo and power unit mounted on same frame)
  • Articulated – Two or more sections of a vehicle joined together in such a way that it makes it easy to turn corners e.g tractor and trailer combination.
  • Combination trucks (18 wheeler is most common). It has three axles on tractor and two on semi trailer. It consists of power unit or tractor and one or more trailers.
  • The trailers are either semi trailers ( i.e having one or more axle near its rear but no front axle.) or full trailers i.e having one or more axles at front and rear.

 

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Articulated Truck

 

 

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BUS

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Cross Section Elements

  • Number of travel lanes to be provided
  • Width and location of shoulders
  • Medians
  • Slopes
  • Embankments and Ditches

Components of a flexible pavement

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Cross section of a road

Platform

– horizontal surface raised above the surrounding ground.

Shoulder

– space between the roadway and ditch.

Median strip

– separation between two roadway.

Carriageway

– central part reserved for vehicle traffic. • Ditch

– channel that carries away water.

Sub-grade

– upper surface of prepared ground.
Base course-middle surface of a road.

Sub base-lower surface of a road.

Cross section of a road

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Cross Section of a Road

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Road Structure

Partial in Filling and Partial in Cutting

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In Cutting

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Highway Travel Lanes

  • The width of the surfaced road and number of lanes should be adequate to accommodate the type and volume of anticipated traffic and assumed design speed of vehicles.
  • Roads presently in use have traditionally been separated into generalized categories that include two-lanes, three-lanes, multilane undivided, multilane divided, and limited access highways.

Two Lane Highways

  • Two lane roads vary from low type roads, which follow the natural ground surface, to high speed primary highways with paved surface and stabilized shoulders.
  • As traffic density, vehicle speeds, and truck widths have increased, two lane highways have also increased in width from 4.8 m to the current recommended value of 7.2 m with 3 m stabilized or paved shoulders on either side along primary routes.

Three Lane Highways

  • Three lane roads were built in previous years. Their great advantage come from an operational improvement over the two lane road, with only a moderate increase in construction and right of way costs.
  • The center lane is either used as two way center left turn lane or alternate in the uphill direction as directional passing lane. The three lane road does appear to have a high accident rate, and construction of these types of roads is no longer recommended except in special cases

Four Lane Highways

  • On four lane highways traffic flows in opposite directions on each pair of lanes, and passing is accomplished within the lanes of forward movement and not in the lanes of opposing traffic.
  • Four lane highways provide at least four times the capacity of a two lane highway for the same assumed design speed. Sometimes up to eight lane highways are also used. The undivided multilane highway does, however, appear to have an accident rate higher than that of the two lane highway.

Four Lane Highways

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It is preferable that when traffic volumes are sufficient to require multilane construction, traffic separation is desirable.

Figure shows four lane undivided Highway

Divided Highways

  • Highways are frequently divided by a median strip to avoid conflicts of opposing traffic.
  • Minimum lane widths is 3.6 m, while 4 m lanes provided where many large trucks combinations are anticipated in divided highways.
  • The width of these median strips vary from 1.2 m to 18 m or more.
  • A median strip less than 1.2 m in width is considered as center line strip and its use is not recommended except for special conditions.
  • The median should be of sufficient width to maintain vegetation and support low growing plants that reduce the headlight glare of opposing traffic.
  • Median strips at intersections should be designed to permit necessary turning movements, which may require single or even dual turn lanes. It is recommended that width of medians should be such that additional travel lanes can be added in the future, by reducing the width of median.
  • Divided highways need not be of a constant cross section. The median strip may vary in width; the road may be at different elevations; and super elevation may be applied separately on each set of lanes.
  • In rolling terrain, substantial saving may be effected in construction and maintenance costs by this variation in design. This type of design also tends to eliminate the monotony of a constant width and equal grade alignment.

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Limited Access Highways

  • A very important feature of the design of a multilane highway is the control of access from adjacent property.
  • A limited access highway may be defined as a highway especially designed for through traffic, to which motorists and owners of abutting properties have only restricted right of access.
  • Limited or controlled access highways may consist of freeways that are open to all types of traffic or parkways from which all commercial traffic is excluded.
  • In urban areas, the design of a limited access facility is usually accompanied by providing frontage roads, parallel to the facility, which serve local traffic and provide access to adjacent land.
    • Such roads may be designed for either one way or two way operation.
    • Reasonably convenient connection should be provided between through traffic lanes and frontages. In general, spacing of access points along limited access facilities is 1.5 km (1 mile) or greater in urban areas and 4 to 7 km (3 to 5 miles) in rural areas.

Pavement Crown

  • Pavement crown is the raising of the centerline of the roadway above the elevation of the pavement edges.
  •  Pavement crowns have varied greatly throughout the years.
  •  On the early low-type roads, high crowns were necessary for good drainage and were commonly constructed at a 4 % slope rate or more (1/2 in. or more per foot).
  •  With the improvement of construction materials, road-building techniques and equipment innovations that permit closer control, pavement crowns have been decreased.
  •  Present-day high-type pavements with good control of drainage now have crowns as low as 1 percent slope rate (1/8 in. per foot).
  •  Low crowns are satisfactory when little or no settlement of the pavement is expected and when the drainage system is of sufficient capacity to quickly remove the water from a traffic lane to prevent a motor vehicle from hydroplaning.
  •  When four or more traffic lanes are used, it is desirable to provide a higher rate of crown on the outer lanes in order to expedite the flow of water from the pavement into the gutter or onto adjacent unpaved shoulder

Shoulders

  • Closely related to the lane width is the width of the shoulders.
  • It is necessary to provide shoulders for safe operation and to allow the development of full traffic capacity.
  • Well-maintained, smooth, firm shoulders increase the effective width of the traffic lane as much as 0.6 m (2 ft), as most vehicle operators drive closer to the edge of the pavement in the presence of adequate shoulders.
  • Shoulders should be wide enough to permit and encourage vehicles to leave the pavement when stopping.
  • The greater the traffic volume, the greater is the likelihood of the shoulders being put to emergency use.
  • A usable shoulder width of at least 3 m (10 ft) and preferably 3.6 m (12 ft) clear of all obstructions is desirable for all heavily traveled and high-speed highways.
  • In mountainous areas or low- type highways, a minimum width of 1.2 m (4 ft) may be provided; a width of 1.8 to 2.4 m (6 to 8 ft) is preferable to save extra cost.
  • Emergency parking pull-outs should be provided at proper intervals in mountainous area when shoulders are not provided.
  • For areas of terrain where guardrails or other vertical elements (such as retaining walls) are required, an additional 0.6 m (2 ft) of shoulder widening should be provided.
  • The slope of the shoulder should be greater than that of the pavement.
  • A shoulder with a high-type surfacing should have a slope of at least 3 percent (3/8 in. per foot)

Guardrail

  • Guardrail should be provided

– where fills are over 2.4 m (8 ft) in height

– when shoulder slopes are greater than 1:4

– In location, where there is sudden change in alignment

– Where a greater reduction in speed is necessary.

Guardrail

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  • In locations with deep roadside ditches, steep banks, or other right-of-way limitations, it is often necessary to steepen the side slopes and to require the use of guardrail.
  • The width of the shoulders is increased approximately 0.6 m (2 ft) to allow space for placing the posts where guard rails are used.

Crash Cushion

• Crash Cushion” is another useful safety feature on our roads.

• It is an energy absorbing crash attenuator with yellow and black chevron sign placed in front of the hard gore areas along expressways and major arterial roads.

• Gore area refers to location where the road is

diverged into more directions, whereas hard

gore refers to area with concrete platform.

• As the term “Crash Cushion” suggests, it is designed to help “soften” the impact of crashes.

• By redirecting the vehicles in the event of a collision, crash cushion helps to absorb the impact and minimize the damage caused to the impacting vehicle and thereby reduce the severity of injuries.

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Crash Cushion

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Curbs, Curb and Gutter, and Drainage Ditches

  • The use of curbs is generally confined to urban and suburban roadways. The design of curbs varies from a low, flat, angle-type to a nearly vertical barrier-type curb.
  • In areas where sidewalks are not provided, curbs adjacent to traffic lanes should be low in height and constructed with a flatter vertical angle so as not to create an obstruction.

Mountable Curb

The face of the curb should be no steeper than 45° so that vehicles may drive over the curb without difficulty.

Curbs at parking areas and adjacent to sidewalks should be 150 to 200 mm (6 to 8 in.) in height.

Clearance should be sufficient to clear passenger car bumpers and to permit the opening of car doors without scraping.

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Curbs, Curb and Gutter, and Drainage Ditches

 

  • Storm water drainage will also affect the shape and height of the curb.
  • From a pedestrian’s viewpoint, curbs should be limited to one step in height. Curbs, Curb and Gutter, and Drainage Ditches
  • Drainage ditches should be located and shaped to avoid creating a hazard to traffic safety.
  • Under normal conditions, ditches should be low enough to drain the water from under the pavement.

Slopes

  • Side slopes and back slopes may vary considerably depending on soil characteristics and the geographic location of the highway.
  • Well-rounded flat slopes present a pleasing appearance and are most economical to build and maintain.
  • Side slopes of 1:4 are used a great deal in both cut and fill sections up to about 3 m (10 ft) in depth or height, but where the height of cut or fill does not exceed 1.8 m (6 ft), a maximum side slope of 1:6 is recommended.
  • Slopes may be as high as 1:1.5 Slopes as high as 1:1 are generally not satisfactory and exhibit long-term maintenance problem.
  • In certain fill sections, special slopes may be built with riprap, mechanically stabilized fabric applications, reinforced concrete cribbing, and various types of retaining walls.

Right of Way

  • The right-of-way width for a two-lane highway on secondary roads with an annual average daily traffic volume of 400 to 1000 vehicles, as recommended by the American Association of State Highway and Transportation Officials (AASHTO), is 20 m (66 ft) minimum and 25 m (80 ft) desirable.
  • Along the Interstate Highway, minimum widths vary, depending on local conditions, from 46 m (150 ft) without frontage roads and 76 m (250 ft) with frontage roads, to 60 to 90 m (200 to 300 ft) for an eight-lane divided highway without frontage roads.
  • Sufficient right-of-way should be acquired in order to avoid the expense of purchasing developed property or the removal of other physical encroachments from the highway right-of-way.

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