SPECIALTRANSPORTATION STRUCTURES (Notes for Guidance ) Highway Design Procedures/Route Geometric Design/Horizontal Alignment/Sight distance, widening and superelevation in curves Radu ANDREI, PhD, P.E., Professor of Civil Engineering Technical University “Gh. Asachi” IASI Lecture Ten
Horizontal Alignment/Sight distance, widening and superelevation in curves Sight distance in horizontal alignment Widening on curves Problems Additional Readings
Horizontal Alignment/Sight distance Checking of sight distance across the inside of curves is another important element of horizontal alignment design. Different sight obstructions such as trees in wooden areas, existing buildings, guard rails have to be removed or adjusted in order to provide adequate sight distance. If they can not be removed, we have to adjust the road route, its alignment and cross section.
Horizontal Alignment/Sight distance The checking of the geometry of the road is usually done by using the design speed V and the proper minimum sight distance. Design charts have been drafted in order to help the designer to check the geometry of the road, but when such chart do not apply, it will be necessary to undertake field measurements or to make measurements on plans. The criteria for the determination of the minimum sight distance on a horizontal curve are derived further in relation with the figure from the next slide.
Horizontal Alignment/Determination of the minimum sight distance on a horizontal curve
Horizontal Alignment/Determination of the minimum sight distance ion a horizontal curve In relation with the figure from the previous slide, the driver of the vehicle from point B, manifest a tendency to shorten the way, travelling the lane designated to the opposite traffic, this being the most unfavourable case which may happen in practice. This behaviour disturbs the driver of the vehicle in point A, who travels regular on his designated line. In order to avoid collision, both drivers have to see each other in proper time, in order to make the necessary movements for correction. So the driver in point A has to put the break and the driver in point B has to leave the undesignated lane.
Horizontal Alignment/Determination of the minimum sight distance ion a horizontal curve Assuming that visibility is assured, the sight distance E, measured along the travel AB will be given by the relation:E = EA + EB = VA* t + d +3,5 VB In this relation, the time of perception is : t= 1 second, d is the distance travelled by the vehicle A in 2,5 seconds, and 3,5 VB is the distance travelled by the vehicle B, whose driver needs 1 second for the perception of the coming of the vehicle A and 2,5 seconds to change the lane.
Horizontal Alignment/Determination of the minimum sight distance ion a horizontal curve using the equation from previous slide, the sight distance of any horizontal curve can be determined. In order to cover all the possible situations which may be encountered, the field of visibility may be determined, by tracing the all possible visual radii, connecting the two vehicles, as shown in the next slide.
Horizontal Alignment/Determination of the field of visibility in horizontal alignment by using the minimum sight distance
Horizontal Alignment/ The minimum recommended visibility and overpass sight distances ( STAS 863-85) Design speed ( Km/h)/Minimum visibility distance for curves ( m)/ The overpass distance (m) V vd od 100280400 80230325 60140250 50110200 4070150 3060125 2550 100
Horizontal Alignment/widening on curves In practice the rear wheels of a vehicle will truck around a curve of a shorter radius than the front wheels. Thus the vehicle occupies more pavement width than it does on the tangent. When the curve is of small radius and the length l of the vehicle is relatively large, the need for an extra-width may become significant. In order to make operating conditions for vehicles on curves, similar with those encountered on tangents, it is necessary to widen the carriage way.
Horizontal Alignment/widening on curves In current design practice there are various modes of applying widening of pavements on curves. An approximate formula for determining the widening necessary for the two lane carriageway is given by literature , as follows : W = l2/R where : l = the length of the vehicle; R = the radius of curve
Horizontal Alignment/widening on curves As usually, in practice, curves with the radius greater than 300m do not need widening. Below, are given the recommended values of widening, for various values of radii Radius (m): 20 25 30 35 40 60 80 100 125 125…300 --------------------------------------------------------------------------------------------------- widening (m): 2,00 1,60 1,35 1,18 1,00 0.65 0,50 0,40 0,35 030…0,25
Horizontal Alignment/widening on curves Widening should be attained gradual on the approaches to the curves in order to ensure a reasonable smooth alignment on the edge of pavement and to fit the paths of vehicles entering or leaving the curve. On simple curves, widening should be applied on the inside edge of pavement only. Curve widening should be attained gradually over a length sufficient to make the whole of the pavement fully used. The maximum widening should be used for the entire length of the circular curve, and should vary from zero at the origin of the spiral to the full amount of the at the point of circular curve, as shown in next slide.
Horizontal Alignment/widening on curves Horizontal Alignment/widening on curves Variation of the widening in case of a connection of tangents with two symmetrical clothoids
Problems WORKSHOP No.1 1. After connecting the tangents with simple circular arcs, determine the value of the minimum sight distance for each circular curve and based on this determine graphically, the field of visibility on the inside of each curve, by tracing the all possible radii of visibility,connecting two vehicles.
Problems WORKSHOP No.2 1. After introducing symmetrical transition curves, determine the values of the widening, apply and and represent them graphically along the symmetrical spirals and along the remaining circle arc.
Additional Readings Andrei R. Land Transportation Engineering, Technical Publishers, Chisinau, 2002Andrei R. Land Transportation Engineering, Technical Publishers, Chisinau, 2002 Garber j.N., Hoel A.,L, Traffic and Highway Engineering, revised second edition, PWS Publishing,1999Garber j.N., Hoel A.,L, Traffic and Highway Engineering, revised second edition, PWS Publishing,1999
Additional Readings Zarojanu Gh.H. Popovici D., Drumuri- Trasee, Editura VENUS, Iasi,1999 Belc F. Cai de comunicatie terestre. Elemente de proiectare, Editura Orizonturi Universitare, Timisoara, 1999 STAS 863-85 Road works. Geometrical elements of Lay out. Design specifications
Additional Readings Hikerson F.T. RouteLocation and Design, Mc GRAW- HILL, Fifth Edition, 1967 Civil Engineer's Reference Book, 3-rd Edition, Butterworths, London, 1975 Dorobantu si al. Drumuri. Calcul si Proiectare, Editura tehnica bucuresti, 1980