Chapter III Geometric design of Highways Tewodros N. www.tnigatu.wordpress.com tedynihe@gmail.com
Introduction Appropriate Geometric Standards Design Controls and Criteria Design Class Sight Distance Design Vehicle Traffic Volume and Design Speed Geometric Design Elements Horizontal Alignment Straights (Tangents) Circular Curves Super elevation Transition Curves Widening of Curves Vertical Alignment Vertical Curves Length Of Vertical Curves Sight Distances At Underpass Structures: Grades and Grade Control Cross-Section Lecture Overview
CROSS-SECTION ELEMENTS
CROSS-SECTION ELEMENTS A cross-section will normally consist of the carriageway, shoulders or curbs, drainage features, and earthwork profiles : Carriageway- the part of the road constructed for use by moving traffic, including traffic lanes, auxiliary lanes such as acceleration and deceleration lanes, climbing lanes, and passing lanes, and bus bays and lay-byes. Roadway- consists of the carriageway and the shoulders, parking lanes and viewing areas Earthwork profiles- includes side slopes and back slopes For urban cross-sections, cross-section elements may also include facilities for pedestrians, cyclists, or other specialist user groups. These include curbs, footpaths, and islands. It may also provide for parking lanes. For dual carriageways, the crosssection will also include medians.
CROSS-SECTION ELEMENTS Cross-section design basically involves two main tasks. The first and most important task is to use the values of the quality of design to estimate the control values of roadway and roadside design parameters. The second task involves judgment on the basis of terrain conditions, cost etc. The prime determinants of cross-section design are: The function that the road is intended to serve; The nature and volume of traffic to be accommodated; and The speed of the traffic. All these needs to be considered to design cross section elements so that we meet the overall objectives of safety, economy, convenience and minimum side effects.
TRAVEL LANE The lane width of a roadway greatly influences the safety and comfort of driving. From 3 to 3.6 m Width are generally used, but occasionally 2.7 m lane width is used in urban areas where the traffic volume is low and there is extreme right-of-way constraints. Lanes 3 m wide are acceptable on low speed facilities and lane widths of 3.6 m are desirable on both rural and urban facilities. 3.6 m lane provides desirable clearances between large commercial vehicles traveling in opposite directions on two-lane, two-way rural highways when high traffic volume and particularly high percentages of commercial vehicles are expected.
TRAVEL LANE The SA design guide recommends the use of 3.7 m lane width for high speed roads carrying high traffic volume, but discourages the use of wider lanes. A 3.4 meter lane is reasonably safe, even at a speed of 120 km/h, but is not a comfortable solution when applied over an extended distance. A reduction in the posted speed limit may be desirable if the lesser lane width is applied over several kilometers. Generally, lane width affects the comfort of driving, and capacity and safety of a road. Accident rate for large trucks increases as the traveled way of a two lane road decreases from 6.5 m. The capacity decrease significantly as the lane width decrease from 3.0 m.
TRAVEL LANE Minimum Width of a Traffic Lane: For vehicle traffic, the minimum desirable width of traffic lane is dependent on The dimensions of the design vehicle, The space required for unavoidable lateral movements and The conditions in adjacent lanes.
TRAVEL LANE For traffic lanes on roads outside built-up areas, the design lane width can be estimated by using the following relationship: min d WVL = W + S + v lm S f where : WVL min d = minimum desirable width of vehicle traffic lane (m) W v S lm = width of design vehicle (m) = space required for lateral movements (m) S f = Space required due to fear of sidewalk or objects (trees, parked vehicles etc.). Similar to S lm, S f can be estimated as:
TRAVEL LANE Based on standards and practice S lm = 0.15 0.30 at design speed of 30kph = 0.25 0.40 at design speed of 50kph = 0.40 0.50 at design speed of 70kph and above At design speed = 30kph: S f = 0.25 0.4 for sidewalk; and 0.5 0.6 for objects At design speed =50kph: S f = 0.35 0.5 for sidewalk; and 0.7 0.8 for objects At design speed =70kph: S f = 0.5 0.6 for sidewalk; and 0.9 1.0 for objects
TRAVEL LANE For vehicle traffic lanes on roads within built-up areas, the width can be estimated as WVL d min = [design speed/80] +2 Furthermore, the width of a bicycle lane on any road can be determined as WBL d min = [2+3a]/4 where WBL d min = width of bicycle lane and a = design number of bicycles riding side by side [2 a 4] For other types of conveyors, the minimum desirable width can be estimated as: WCL d min = W cv +0.5 Where WCL d min = minimum desirable width of special conveyor lane W cv = width of special conveyor (m)
SHOULDERS Shoulders are the usable areas immediately adjacent to the travelled way and are a critical element of the roadway cross-section. They provide: A recovery area for errant vehicles; A refuge for emergency and maintenance vehicles as well as stopped or disabled vehicles; Lateral support of the roadway structure; and In addition, shoulders support use of the road by other modes of transport, for example cyclists and pedestrians. Where bicyclists and pedestrians are to be accommodated on shoulders, a minimum usable shoulder width of 1.2 m should be used.
SHOULDERS Minimum width of safety enhancement places [shoulders and parking lanes] The minimum desirable width of a safety enhancement place depends on whether the place is designated to be used as a shoulder or a parking lane. In the case of a shoulder the usable width is usually between 0.6m and 3.6m depending on the class of road and traffic volume. In general, the usable width of a shoulder can be determined as: min d WSL = W + C v Where: WSL min d = minimum desirable useable width of shoulder (m) W v = width of design vehicle (m) C w = clearance [usually between 0 and 1.5 meters] In the case of a parking lane, the width depends on the type of parking (parallel or angled) which will be allowed. In general, for parallel parking, the minimum and desirable widths are 3.0m and 3.6m respectively. w
SHOULDERS Where roadside barriers, walls or other vertical elements are present, it is desirable to provide a shoulder width enough that the vertical elements will be offset a minimum of 0.6 m from the outer edge of the usable shoulder. Shoulders widths as recommended by the ERA design Guide Design Standard Rural Terrain/Shoulder Width (m) Town Section Widths (m) Flat Rolling Mountainous Escarpment Shoulder Parking Foot Lane*** way Median! DS1 3.0 3.0 0.5 2.5 0.5 2.5 n/a 3.5 2.5 5.0 (min) (min) DS2 3.0 3.0 0.5 2.5 0.5 2.5 n/a 3.5 2.5 Barrier! DS3 1.5-3.0++ 1.5-3.0++ 0.5 1.5 0.5 1.5 n/a 3.5 2.5 n/a DS4 1.5 1.5 0.5 0.5 n/a 3.5 2.5 n/a DS5 * 0.0 0.0 0.0 0.0 n/a 3.5 +++ 2.5 n/a DS6** 0.0 0.0 0.0 0.0 n/a 3.5 +++ 2.5 n/a DS7 1.0 (earth) 1.0 (earth) 1.0 (earth) 1.0 (earth) n/a n/a + n/a + n/a DS8** 0.0 0.0 0.0 0.0 n/a n/a + n/a + n/a DS9** 0.0 0.0 0.0 0.0 n/a n/a + n/a + n/a DS10** 0.0 0.0 0.0 0.0 n/a n/a + n/a + n/a
SHOULDERS Shoulders widths as recommended by the SA design Guide
SHOULDERS Shoulders should be: continuous and have the same width on structures as approach roadways. Flush with the roadway surface and abut the edge of the traveled way. Sloped to drain way from the traveled way Bituminous and concrete surfaced roads 2 to 4 % Gravel surfaced shoulders 4 to 6 % Turf shoulders 6 to 8 % Where curbed, appropriate slope should be used with the drainage system to prevent ponding on the traveled way. If shoulders are to function effectively, they should be sufficiently stable to support occasional vehicle loads in all kinds of weather without severe distress.
SHOULDERS Where the traffic situation demands a dual-carriageway crosssection, the greatest width of shoulder, i.e. three meters, is called for. This width would apply to the outer shoulder. The inner shoulder need only be one meter wide: to protect the integrity of the pavement layers; to avoid drop-offs at the lane edge; and provide space for road markings provided the median island is not curbed, thus allowing a disabled vehicle to be moved clear of the adjacent lane.
TRAFFIC LANE Minimum desirable Number of Traffic Lanes for each category of conveyors: The desirable number of traffic lanes for each designated category of conveyors is dependent on the design entry flow rate and design service flow rate and can be estimated as: min DDFR( m) N d ( m) = DSFR Where: N min d (m) = desirable minimum number of lanes for a category m DDFR(m) = design demand flow rate for m in pcph per direction of flow DSFR = design service flow rate (pcph per traffic lane)
TRAFFIC LANE Cross Slope or Camber of a Traffic Lane Undivided travelled ways on tangents or on flat curves, have a crown or high point at the middle and a cross slope downward both edges. The cross slope may be a plane or rounded section or a combination. Rounded cross sections are parabolic and advantageous because the slope steepness towards the edge, but more difficult to construct. On divided highways each one-way travelled way may be crowned as on two-lane highways or it may have a unidirectional across slope across the entire width of the travelled way. The cross slope is largely dependent on the type of surfacing: high type 1.5-2 %; and low type surfacing 2-6%.
TRAFFIC LANE Cross Slope or Camber of a Traffic Lane The four main causes of poor skid resistance on wet pavements are Rutting accumulates water in the wheel paths Polishing reduces the pavements micro-texture Bleeding covers the micro-texture Dirty covers micro texture
TRAFFIC LANE AUXILIARY LANES Auxiliary lanes at intersection, interchanges and on networks often help to facilitate traffic movements and are located immediately adjacent to the basic lanes. Auxiliary lanes at intersections and interchanges are mainly used as turning lanes (either to the left or to the right; the turning lanes are principally intended to remove slower vehicles, or stopped vehicles waiting for a gap in opposing traffic, from the through traffic stream hence increases the capacity of the through lanes) On network links, auxiliary lanes are climbing and passing lanes. Climbing lanes are often referred to as truck lanes, crawler lanes, overtaking lanes or passing lanes. The function of climbing lanes is, however, very different from that of passing lanes. Such added lanes should be as wide as the through-traffic lanes but not less than 3 m.
TRAFFIC LANE MEDIANS Median is the portion of a highway separating opposing directions of the traveled way and the principal functions are to: Separate opposing traffic reduce the probability of head-on accidents (achieved through selection of a suitable width of median or the use of median barrier) Provide recovery area during emergency (research has found that the minimum median width required for recovery of out of control vehicles and most of head-on crashes is 9 m), Provide stopping area for left and U-turning vehicles (the width of median should be sufficient enough to accommodate left turning vehicles) Provide refuge for pedestrians (pedestrians do not feel safe on median islands narrower than about 2m, suggesting that the median should have a width of the order of 5.5 to 6.0 meters. Reduce headlight glare
TRAFFIC LANE MEDIANS Median can be either raised, flush or depressed. Depressed medians are normally used in rural areas and raised medians in urban areas. This differentiation between rural and urban areas arises for two reasons: drainage and safety. According to AASHTO, median width vary between 1.2 up to 24 m or more depending on the availability of right-of-way Medians with a width of 9m or more allow for individual grading of the two carriageways, but crashes at intersections increases with increasing width of median by virtue of the long travel distances that they impose on turning vehicles.
CURBS Curbs are raised or near-vertical elements that are located adjacent to the traveled way and are usually used: To control drainage Improve aesthetic by delineating the pavement edge Reduce right-of-way Delineation of pedestrian walkways Assistance in orderly roadside development Are classified as Barrier curbs vertical, designed for preventing vehicles from leaving the road, but should not be used along high speed roads as it is not forgiving. Mountable curbs are designed so that vehicles can cross them readily when the need arises.
TRAFFIC BARRIERS Traffic barriers are used to prevent vehicles that leave the traveled way from hitting and object that has greater crash severity potential than the barrier itself. Traffic barriers include Longitudinal barriers (along roadsides and in medians): primarily to redirect errant vehicles. They can be flexible, semi-rigid, or rigid depending on the amount of deflection that can be accommodated when the barrier is struck. Median barrier a longitudinal structure used to prevent an errant vehicle from crossing the portion of a divided highway separating the traveled way for traffic in the opposite directions Roadside barrier protect vehicles from causing hazards onto roadside and shield pedestrians Crash cushions: primarily to decelerate errant vehicles to a safe stop for shielding from hitting rigid objects along the roadsides such as piers, overhead sign supports, abutments, and retaining walls. Bridge railings prevent vehicles, pedestrians or cyclists from falling off the structure.
DRAINAGE CHANNELS AND SIDESLOPES Modern highway drainage design should incorporate safety, good appearance, control of pollution, and economical maintenance. The interrelationship b/n drainage channel and side slopes is important for safety (good roadside design reduces the potential severity of accidents). Drainage channels should have adequate capacity for the design runoff, provide for unusual storm drainage, and be located and shaped to provide a safe transition from the roadway to the back-slope. The depth of channel should be sufficient to remove surface water without saturation of the subgrade. Side slopes should be designed to ensure roadway stability and to provide a reasonable opportunity for recovery area for an errant vehicle.
SIDEWALKS Pedestrian traffic is not encouraged in the road reserves of freeways, expressways or other high-speed arterials and accommodation of pedestrian traffic is usually handled elsewhere. Side walks provided on urban or rural roads When pedestrian traffic is high along main or high speed roads When shoulders are not provided on arterials even when pedestrian traffic is low In urban areas, sidewalks are provided along both sides of streets to serve pedestrians access to schools, parks, shopping centers, and transit stops. The width of a sidewalk should not be less than 1.5m and a minimum width of 2m should be provided near hospitals and where wheelchair traffic could be expected. If the sidewalk is immediately adjacent to the curbing, the minimum width should be increased by about 0.6 to 1m to accommodate utilities. The normal cross-slope on a sidewalk is 2%. A grade separated pedestrian facility allows pedestrians and motor vehicles to cross at different levels, either over or under a roadway.
CYCLE PATHS If adequately planned, designed and maintained, cycle paths can play an important role in the transportation system. It is important to realize that cyclists need sufficient space to operate with safety and convenience rather than simply being assigned whatever space is left over after the needs of vehicular traffic has been accommodated. The basic requirements of cyclists are: Space to ride (1 m); A smooth surface; Speed maintenance, and Connectivity. Facilities for bicycles can take the form of a: Shared roadway/cycle lane where motor vehicles and bicycles travel in a common lane; Cycle lane, which is part of the travelled way but demarcated as a separate lane; Shoulder lane, which is a smooth, paved portion of the shoulder, properly demarcated by pavement markings or traffic signs. Cycle paths may be located within the road reserve or in an independent reserve.
CYCLE PATHS CYCLE PATHS
CYCLE PATHS CYCLE PATHS
SIDE AND BACK SLOPES Side slopes should be designed to insure the stability of the roadway and to provide a reasonable opportunity for recovery of an out-of-control vehicle. Three regions of the roadside are important when evaluating the safety aspects: the top of the slope (hinge point), the side slope, and the toe of the slope (intersection of the fore slope with level ground or with a back slope, forming a ditch).
SIDE AND BACK SLOPES When the safety aspect in relation to side slope is considered it will be important to see the three zones categorized in the new ERA Geometric Design Manual: Recoverable, Non-recoverable and Critical. According to ASSHTO Design Manual (Roadside Design Guide) the definition of the three zones are as follows: Recoverable slope: all embankment slopes 4:1 (H:V) or flatter. Motorists who encroach on recoverable slopes can generally stop their vehicles or slow them enough to return to the roadway safely. A non-recoverable slope is defined as one which is traversable, but from which most motorists will be unable to stop or return to the roadway easily. Vehicles on such slopes typically can be expected to reach the bottom. All slopes in the range of 3:1 to 4:1 (H:V) are categorized as a non-recoverable slope. A critical slope is one on which a vehicle is likely to overturn. Slopes steeper than 3:1 (H:V) generally fall into this category. For the critical slopes (steeper than 3:1(H:V)) it is recommended to put warning signs. This is the recommendation according to AASHTO Design Manual. The important aspect, which needs to analyze, is the standards sated by AASHTO is for roads having high level of serviceability (minimum design speed 65km/hr).
SIDE AND BACK SLOPES Table : Comparisons of different cut slopes and recommendation for low speed road <60kph Materia l type Earth or Soil Height of Slope (m) New ERA Design Manual Side Slope (V:H) Cut Fill Back Slope ERA/ TCDE Geometric Design Manual Side Slope (V:H) Cut Fill Back Slope f Kenyan Road Design Manual Side Slope (V:H) Cut Fill Back Slope Tanzanian Road Design Manual Side Slope (V:H) Cut Fill Back Slope Highway Design Reference Guide* Side Slope (V:H) Cut Fill B k Sl Recommendatio n Side Slope (V:H) 0.0m-1.0m 1:4 1:4 1:3 1:3 1:3 1:4 1:3-1:4 1:3 for poor clay 1:3 1:3 1.0m-2.0m 1 1:3 1:3 1:2 1:2 1:2 1:2 1:2-1:2 2:1 for good clay 1:2 1:2 Over 2.0 m 1 1:2 1:2 2:3 2:3 2:3 2:3 2:3-2:3 1:2 2:1 for granular 2:3 2:3 to martial 2:3 Cut Fill Back Slope for Cut (V:H) Rock 0.0m-2.0m 1 1:3 1:3 4:5 1: 4:5 2:1 1: - - - 1) 4:1 to 2:1 for 1: 4:5 2:1 Over 2.0m 1 1:2 1:2 1:1 2 1:1 4:1 2 - - - Igneous rock 3:1 1:1 2 1:1 4:1 Weather 0.0m-2.0m 1 - - - - - - - - 2) 2:1 to 4:3 for 2:3 2:1 to to ed Rock Over 2.0m 1 - - - - - - - - sedim. rock 1:1 3:1 10:1 4:1 3) 4:1 to 4:3 for Meta. rock Decom 0.0m-1.0m - - - 1:3 1:3 - - - - 1:2 1:3 1:3 posed 1.0m-2.0m 1 - - - 1:2 1:2 - - - - to 1:2 1:2 Over 2.0 m 1 - - - 2:3 2:3 - - - - 1:1 2:3 1:1
ROAD SIDE DITCHES Are provided to remove the longitudinal and lateral incoming flood safely from the main road formation. Minimum depth of ditches should be 0.6m in mountainous and escarpment terrain, and 1.0m elsewhere, using a v-ditch configuration. Side drains should be avoided in areas with expansive clay soils such as black cotton soils. Where this is not possible, they shall be kept at a minimum distance of 4-6m from the toe of the embankment, dependent on functional classification (6m for trunk roads), as shown in Figure below. The ditch in this instance should have a trapezoidal, flat-bottom configuration.
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