Lecture 4 Geometric - Highway Design - PDF

Highway Geometric  A geometric design is defined as the design of visible components of a highway. Therefore, the geometric designer can be considered as the architect of the roadway  The basic features of a highway are the carriageway itself, expressed in terms of the number of lanes used,...

Highway Geometric  A geometric design is defined as the design of visible components of a highway. Therefore, the geometric designer can be considered as the architect of the roadway  The basic features of a highway are the carriageway itself, expressed in terms of the number of lanes used, the central reservation or median strip and the shoulders (including verges), horizontal and vertical alignments, intersections and the length of acceleration and deceleration lanes and so on Highway geometrics are the elements of a road, which are visible to road users. H.G provide optimum efficiency in traffic operation, with max. Safety Highway geometric can be classified under headings: 1- Cross-section elements like camber, superelevation... etc. 2- Horizontal and vertical alignments. 3- Sight distance characteristics (Stopping sight distance, overtaking sight distance….... etc. Highway Design Control Geometric design of highway is the determination of layout and features visible on highway. The emphasis is more on satisfying the need of the driver as well as to ensure the safety of the vehicle, the comfort while driving and efficiency. Other related factors are also considered based on the project. Highway Geometric are affected by these factors 1-Topography: The area under construction may be plain, rolling, mountainous and steep. 2-Design speed: Highway design elements of a road is dependent on the design speed. 3-Traffic factors: Vehicular characteristic and human characteristic of road users also affect geometric standards of any road. 4-Volume and capacity factors: The roadway facility is not designed for the peak traffic flows but for slightly lower value. 5-Environmental and other factors: Landscaping, noise, Air pollution all affect road Geometrics. 6- Level of service. 7-Highway classification. 8- Cross-section of highway. 9-Fund. 10-Restrictions.  1- Cross-section elements  the principal elements of a highway cross section consist of the travel lanes, shoulders, and medians (for some multilane highways)  Marginal elements include and roadside barriers, kerbs, gutters, guard rails, sidewalks, and side slopes  The features of the cross-section of the pavement influence the life of the pavement as well as the riding comfort and safety Typical cross section for multilane highway (half section) Typical cross section for two lane highway Typical Cross Section Typical cross-section of road Rural Road Urban Road 1.2 : Pavement Surface Characteristics For safe and comfortable driving four aspects of the pavement surface are important the friction between the wheels and the pavement surface, smoothness of the road surface, the light refection characteristics of the top of pavement surface, and drainage to water 1.2.1: Friction Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Factors which affect friction coefficients 1. Type of road surface. Asphalt, concrete, earth, etc. 2. Pavement condition, rough, smooth, oil spills, muddy, wet, dry.... etc 3. Condition of tyres, old, new, make...etc. 4. Break efficiency. 5. Load on tyres (axle load). Skid and slip: 6. Tyer Pressure. Skidding is wheel skid without revolving. 7. Temperature of tyres and pavement. Slipping is opposite skidding, In slip distance traveled 8. Speed of the vehicle. by wheel less than normal. 9. Type of skid.  1.2.2: Unevenness  Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures.  Unevenness affects the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tires.  Pavement Unevenness is measured in terms of Unevenness index.  Unevenness index is a measure of unevenness which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road.  An unevenness index value – Less than 1500 mm/km - good – Less than 2500 mm/km – satisfactory – Greater than 3200 mm/km - uncomfortable even for speed of 55 kmph. 1.2.3: Light of reflection  White roads have good visibility at night, but caused glare during daytime.  Black roads has no glare during day, but has poor visibility at night  Concrete roads has better visibility and less glare 1.2.4 Drainage  The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers.  Further, both the geometry and texture of pavement surface should be such that it drain out all the water from the surface in less time.  Drains should be provided on one side or both side of the road pavement  Please Remember, the main enemy for pavement is water and water and water. 1.3 :Camber Camber is the cross slope provided to raise middle of the road surface in the transverse direction to drain off rain water from road surface. Too steep slope is undesirable for it will erode the surface. Camber is measured in 1 in n or n% (Eg. 1 in 50 or 2%) The objectives of providing camber are: 1) Surface protection especially for gravel and bituminous roads 2) Sub-grade protection by proper drainage. 3) Quick drying of pavement which in turn increases safety. Types of Camber IRC Values for camber Figure : Different types of camber (cross-slope) 1.4 :Width of Travel Lanes  Width of the carriageway or the width of the pavement depends on the width of the traffic lane and number of lanes  Width of a traffic lane (one lane width) depends on the width of the vehicle and the clearance. Side clearance improves operating speed and safety.  In general, travel lane widths usually vary from 2.75m to 3.75m.  On two lane, two-way rural roads, lane widths of 3m or 3.65m may be used, but two factors must be considered when selecting a lane width less than 3.65m wide  When pavement surfaces are less than 6.75m, the crash rates for large trucks tend to increase and, as the lane width is reduced from 3.65m, the capacity of a highway significantly decreases. Lane widths of 3m are therefore used only on low-speed facilities.  It should be noted that the maximum permissible width of a vehicle is 2.44m and the desirable side clearance for single lane traffic is 0.68 m.  This require minimum of lane width of 3.75 m for a single lane road  No. of travels lanes in both directions are usually equal.  However, the side clearance required is about 0.53 m, on either side or 1.06 m in the centre. Therefore, a two-lane road require minimum of 3.5m for each lane  In Iraq, a lane width of 3.75m is generally used for multilane highways while standard lane width is 3.60m. Table 1 represents the lane width as recommended by AASHTO Green book Table 1 Lane width for single and two lane roads 1.5: Shoulders  The shoulder of a pavement cross section is always contiguous with the travelled lane to provide an area along the highway for vehicles to stop when necessary  Shoulder surfaces range in width from 0.6m on minor roads to 3.65m on major arterials. Shoulders are also used to laterally support the pavement structure.  Minimum shoulder width of( 1.80-2.40)m may be considered for low- volume highways  Asphalt and concrete – surfaced shoulders should be sloped from 2% to 6%, aggregate and untreated granular shoulders from 4% to 6%. In other words, slope of shoulder depends on the type of constructed materials. Graded and usable shoulders Functions of shoulders a. accommodation of stopped vehicles (disabled vehicles, bus stops) a. emergency use b. lateral support for the pavement c. space for roadside facilities d. space for bicycles and pedestrians e. driving comfort (freedom from strain) f. improvement in sight distance g. improvement in capacity  1.6 :Medians  A median is the section of a divided highway that separates the lanes in opposing directions  The width of a median is the distance between the edges of the inside lanes, including the median shoulders  It is the physical or painted separation provided on divided highways between two adjacent roadways.  In general, median widths are in the range from 1.2m to 24m or even more at some cases The functions of a median include 1. Providing a recovery area for out-of-control vehicles 2. Separating opposing traffic 3. Providing stopping areas during emergencies 4. Providing storage areas for left-turning and U-turning vehicles 5. Providing refuge for pedestrians 6. Reducing the effect of headlight glare Medians can either be raised, flush, or depressed as follows: 1.6 : Curbstone  curbs are raised structures made of either Portland cement concrete or bituminous concrete (rolled asphalt curbs) that are used mainly on urban highways to delineate pavement edges and pedestrian walkways  curbs are also used to control drainage, improve aesthetics, and reduce right of way  They can be generally classified as either vertical or sloping Kerbs indicate the boundary between the carriage way and the shoulder or islands or footpaths. Functions of curbs a. Drainage control b. Roadway edge delineation c. Enhanced appearance d. Right-of-way reduction e. Delineation of pedestrian walkways f. Reduction of maintenance operation Different types of kerbs are shown in Figure  Low or mountable kerbs: This type of kerbs are provided such that they encourage the traffic to remain in the through traffic lanes and allow the driver to enter the shoulder area with little deficiency  Semi-barrier kerbs: when the pedestrian traffic is high, these kerbs are provided  Barrier kerbs: they are designed to discourage vehicles from leaving the pavement  Submerged kerbs: They are used in rural roads. The kerbs are provided at pavement edges between the pavement edge and shoulders. They provide lateral confinement and stability to the pavement 1.7 : Gutters or drainage ditches are usually located on the pavement side of a curb to provide the principal drainage facility for the highway. They are sloped to prevent any hazard to traffic, and they usually have cross slopes of 5 to 8 percent and are 0.3 to 1.8 m wide. Gutters can be designed as V-type sections or as broad, flat, rounded sections. 2. Guard Rails Guard rails are longitudinal barriers placed on the outside of sharp curves and at sections with high fills Their main function is to prevent vehicles from leaving the roadway They are installed at embankments higher than 2.4m and when shoulder slopes are greater than 4:1. Shapes commonly used include the W beam and the box beam Guard rail Other elements  Side Walk  Sidewalks are usually provided on roads in urban areas, but are uncommon in rural areas.  Generally, sidewalks should be provided when pedestrian traffic is high along main or high-speed roads in either rural or urban areas  Sidewalks should have a minimum clear width of 1.25 m in residential areas and a range of 1.25 to 2.5 m in commercial areas.  Cycle tracks are provided in urban areas when the volume of cycle tracks is high Minimum width of 2 meter is required, which may be increased by 1 meter for every additional track.  3.Side Slope  Side slopes are provided on embankments and fills to provide stability for earthworks  They also serve as a safety feature by providing a recovery area for out-of-control vehicles  Three regions of the roadside are important to reducing the potential for loss of control for vehicles that run off the road:(hinge point), fore slope, toe of the slope.  The slope of earthwork in filling or in cutting is called Side slope. It imparts stability to the earthwork.  Retaining walls should be considered where slopes would be steeper than 1:2 Recommended values for foreslopes and backslopes are provided in Table. Guide for earth slope design Roadside regions  4.Right of Way  The right of way is the total land area acquired for the construction of a highway.  The width should be sufficient to accommodate all the elements of the highway cross section, Rights of way are purchased prior to the construction of a new road, and usually enough extra land is purchased.  any planned widening of the highway, and public-utility facilities that will be installed along the highway.  Sometimes, rights of way are left vacant after the initial roadway facility is constructed to allow for future highway expansion. Requirements of area for right of way are as follows: For 2 lane road = 150 ft (45 m )width of area For 4 lane road = 250 ft (75 m)width of area For 8 lane road = 300 ft (90 m)width of area  For Iraqi Expressway No One, a right- of- way width of 260 m has been provided, which included service roads. The right of way width is governed by:  Width of formation: It depends on the category of the highway and width of roadway and road margins.  Height of embankment or depth of cutting: It is governed by the topography and the vertical alignment  Side slopes of embankment or cutting: It depends on the height of the slope, soil type etc.  Drainage system and their size which depends on rainfall, topography etc.  Sight distance considerations: On curves, there is restriction to the visibility on the inner side of the curve due to the presence of some obstructions like building structures etc  Reserve land for future widening: Some land has to be acquired in advance anticipating future developments like widening of the road. Design of On – Street Parking Facilities On-street parking facilities may be designed with parking bays parallel or inclined to the curb For different parking angles, number of parking bays can be determined as: Street Space Used for Various Parking Configurations Design of Off – Street Parking Facilities – Surface Car Park Herringbone Layout of Parking Stalls in an On-Surface Lot Parking Stall Layout Design of Off-Street Parking Facilities—Garages Parking garages consist of several platforms, supported by columns, which are placed in such a way as to facilitate an efficient arrangement of parking bays and aisles Access ramps connect each level with the one above. The gradient of these ramps is usually not greater than 1:10 on straight ramps and 1:12 on the centerline of curved ramps The radius of curved ramps measured to the end of the outer curve should not be less than 70 ft and the maximum superelevation should be 0.15 ft /ft. The lane width should not be less than 16 ft for curved ramps and 9 ft for straight ramps. Ramps can be one-way or two-way, with one-way ramps preferred When two-way ramps are used, the lanes must be clearly marked and where possible physically divided at curves and turning points to avoid head-on collisions, as drivers may cut corners or swing wide at bends.  Gutter to drain out water Medians Off – street parking - garage Shoulders