Design of Road Curves for Safety - Essay Example

Design of Road Curves for Safety Introduction: A road is considered safe if it has a very less histories of accidents or collisions. But all roads would have had at least a few cases of minor collisions . Among the different road components, the curves are the major points of concern along the road system (Glennon, 2003). Even, among the most prominent industrial countries like France and Denmark , the available data show that curves constitute the major locations that have high incidence of accidents - about 20 percent of the reported figures. In the developing countries too these figures touch peak proportions of 70 percent of the total accidents reported in those places (Kirk, 1997). The majority of the accidents in these places are normally classified into two types : smashing at an object after running out of road and rolling over resulting from the losing control of the vehicle. In most of the cases the reasons identified are the excessive speed of the drivers entering the curves , which would lead to misjudging the sharpness of the bend , the sudden sharp turn of the curve after series of gentle curves or the change in surface characteristics that would lead to unsafe road curves. It is widely observed that the road surface would undergo significant change like polishing of the road surface aggregate on the bend than that of the rest of road due to the larger forces on road due to higher side thrust that results in unsafe road conditions on curves. Also , the problems due to inappropriate assumptions in the geometric design of the road like possibility of large number of drivers exceeding the design speed at curves is another important reason for higher accident rates at curves. Loss of super elevation due to poor maintenance could also lead to horizontal curves becoming more problematic than designed. Impact of curves on safety One of the earlier studies in the effect of curves on the safety of the roads in along London to Birmingham was reported by Hauer (2001). The data obtained in the field studies analysed are presented in the Table 1 Particulars of road geometric Grade of road Injury, Accidents (Million Vehicles Miles) Straight road - 1.8 0.8 -1.2 0.4 10.4 0.6 0.4 0.25 1.2 0.35 1.8 0.3 Road curves to right -1.2 0.3 -0.4 1.05 1.8 0.85 1.1 0.4 0.5 0.5 Road curves to left -1.8 0.6 0.4 0.35 1.1 0.4 1.8 0.45 Table 1 Effect of curves on road safety The results also showed that the effect of vertical curves on the road safety was not very significant. The most striking feature was the combination of grades with the horizontal alignment. Higher the grades as well as higher degree of curves resulted in the increased rate of accidents. Also, the combination of grades with alignment too resulted in the cumulative effect on the accident occurrence. In an another initiative , the detailed road safety audit exercise conducted in UK brought out the different aspects of the road that often lead to accidents (Motoring Policy, n.d.). According to vehicle user, the major reasons for the accidents at the curves are due to higher actual speed of vehicles than the design speed, obstruction to sign boards due to overgrown vegetation and absence of safety fences. Prevailing Guidelines for design of curves In UK, the guidelines are as per the UK departmental Standard TD 9/93 "High Way Link Design". But often less attention is paid in strictly adhering to the norms and specifications. And the road curves are built to alternate framework. This ultimately lead to non-uniform road networks and driver confusion due to poor visibility etc (Department of Transport, 1987). The safe guidelines for the design of curves are given as follows. Design speed is the one of the most important parameter that links different factors that ensures safe driving conditions. The design speed is given as Vdesign = 127.R.(e + f ), where Vdesign is in Km /hr, R is the radius in meters, e is the super elevation in metres per metre and f is the side friction factor (Transport Research Laboratory, 1988). The guidelines that are presently followed in the design of vertical and horizontal curves for different design speeds are given in the Table 2. For those radii less than that mentioned in the Table, the super elevation shall be provided as follows Where V is the design speed in Kmph, R is the radius of curve in metre and e is the super-elevation % Table 2 Design data for road curves (a) Horizontal Curvature Design Speed (Kmph) 120 100 85 70 60 Minimum R with out elimination of adverse camber and transitions. 2880 2040 1440 1020 720 Minimum R with super elevation of 2.5 % 2040 1440 1020 720 510 Minimum R with super elevation of 3 % 1440 1020 720 510 360 Desirable minimum with super elevation of 5 % 1020 720 510 360 255 One step below desirable minimum R with super elevation of 7 % 720 510 360 255 180 Two step below the desirable minimum R with super elevation of 7 % 510 360 255 180 127 (b) Vertical Curvature Desirable minimum crest K value 182 100 55 30 17 One step below desirable minimum crest K value 100 55 30 17 10 Absolute minimum sag K value 37 26 20 20 13 Need for more prudent approach. The design criteria used in various countries for the design of curves varies from one nation to the other. Majority of them relies on the operating speed observed on the curves of different radius. Also considerable of flexibility need to be incorporated like not confirming to single values of maximum super elevation rate or providing freedom to the designers for applying a limiting values of curve radius. This might lead to the situation of having identical roads curves with different super elevation values. Research work in this direction have helped to evolve systems that could ensure horizontal alignment consistence by proposing the curve radius that would help the driver to maneuver the curve effectively by helping to select the speed besides assessing the centrifugal acceleration. A typical comparison of these factors across United States ,Australia and United Kingdom based on the published information is given in Table 3 (Kanellaidis, 1999). Table 2 Comparison of Design Criteria (Kanellaidis, 1999) Particulars United States Australia U K Design Speed (Vd) The design speed, Vd , is chosen based on the category to which road is classified, land use and terrain. It is expected that Vd willnot be exceeded those proposed by the authorities. (AASHTO,1994) Proposed based on four guidelines on speed : Design speed - the free speed likely to be adopted, Speed corresponding to 85th percentile is used to charecterise the value , Design speed to to individual elements (Austroads, 1993). For speeds less than 100 Kmph the 85th percentile speed is used in the guidelines. Design speeds ivded in the bands of 125 kmph, 100 kmph and 85 kmph etc and is modifed to incorate the alingment contrain and layout constrain (Highway Link Design, 2006) Superelevation Considerable variation is observed in establishing maximum super elevation values. Each state sets their own limits due to large variations in both geographical and climatically conditions. The maximum values for the interurban intersections are are 6 %, 8 % and 10 %. The extreme values of 12 % as well as 4 % in the case of urban locations is also used The variations across different states in terms of geographical conditions and climate factors forces state administration to follow their own guidelines for fixing maximum super elevation. The recommended maximum superelvation values are 10 to 12 % for mountainous terrains and 6 to 7 % for flat terrains. Thr graphical chart to determine the required superelevation is available based on the radius of curveture ( 20 m to 200m ) and different design speeds ranging from 50kmph to 125 kmph. The superelevation is broadly classified into three classes - absolute maximum value of 7 %, desirable maximum of 5 % and favourable crossfall of 2.5 %. Superelvation greater than 7 % is permissible only on existing roads Side friction factor Based on the results obtained from the ball blank indicator the values of friction factor is determined. The side friction factor is established based on the driving behaviour on rural roads related to the 85th percentile driver. A detailed analysis conducted on the design guidelines for the road curves in majority of European countries shows considerable variation in the road design approaches (Brenac, 2007). The important aspects considered in these exercises were curve geometry and place of curve in the horizontal alignment. It is understood that the conventional approach of using design speed as an the important parameter is inadequate and a need for introduction of complementary rules in the design is highlighted. Further, the study also reveals that even in the countries where such complementary rules are introduced it has not been evolved from a knowledge base that have extensive accumulated information. This have resulted in the rules proposed being classified as non-homogenous and inconsistent( Brenac,2007). Further, a study undertaken on rural roads have revealed a strong relationship between the design speed and operating speed. A detailed introspection on the prevailing policies on the design speed is presented in addition to the observed differences between the design speed and operating speeds. The study also recommends the modifications for design polices being followed in United states, Australia and European Union (Krammes, 2007). The design of all horizontal curves involved the assumption that all the vehicles followed the curves exactly to the proposed curve geometry. The field studies undertaken have revealed that this assumption is hardly followed by vehicles maneuvering the curves. It was observed that path taken by most of the vehicles along the curves , irrespective of the speed at which they are traveling, often exceed the degree of the road curve. The authors identifies that atleast 100 percent of the vehicles are expected to cross 4.3 degrees when the curve has an angle of 3 degrees. Based on the results obtained, solution based on vehicle path percentile would help to evolve reasonable safety margin to account for the different type of uncertainties that lead to increase in the value of lateral friction or decrease the skid resistance . Proposed solutions Different nations have contributed to the to design of safe road curve considering the regional characteristics. In a study conducted in Australia on the identifying the safe speed limits on curves for high centre of gravity vehicles carrying live loads have given a set of guiding parameters on speed limits for the curves. The research results are separately presented for various radii and changing super elevation. The major aspects of this research is the specific situations considered in the study. The design speeds are proposed based on the observations made on different situations like , the live stock carrying vehicles having a lateral shift of stock by 25 centimetres towards outside the curve and the sloshing effects of liquids in the oil carrying containers etc ( Stewart and Chudworth , 1990). Conclusion. The results of effective safety designs have given very impressive results. In one such situations, the exercise conducted in Papua New Guinea showed an impressive reduction 40 percent reduction in the collisions (Kirk, 1997). The uniformity in design principles is necessary to give more consistent road geometrics across nations. In addition to designs, other factors like proper sign display units are also very important to ensure the safety of road users. Further, implementation of safety designs at the bends have also reported a 10 times return on the cost incurred on placing the chevron boards. Thus it is confirmed that implementation of proper safety measures not only makes accident free but also gives the economic advantages due to less disturbance to commerce and development. References: AASHTO (1994). A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials (AASHTO), Washington, D.C. AUSTROADS (1993). Rural Road Design-Guide to the Geometric Design of Rural Roads, Austroads, Sydney. Brenac, T (2007), Safety at Curves and Road Geometry Standards in Some European Countries , Transportation Research Record, 1523, pp 99-156 Department of Transport (1987) Highway Construction Details, Department of Transport, Scottish Development Department, Welsh Office, Department of the Environment for Northern Ireland, HMSO, London. Glennon, J C ( 2003), Thoughts on a New Approach for Signing Roadway Curves [Online] Available at [5 February 2009] Hauer, E (2001), Road grade and safety [Online] Available at [18 February 2009] Highway Link Design (2006), Design Manual for Roads and Bridges [Online] Available at http://www.standardsforhighways.co.uk/dmrb/vol6/section1.htm [18 February 2009] Motoring Policy, What goes wrong in highway deign [Online] Available at [19 February 2009] Kanellaidis, G (1999), Road curve superelevation design: Current practices and proposed approach Road & Transport Research [Online] Available from [13 February 2009] Krammes, R A (2007), Design Speed and Operating Speed in Rural Highway Alignment Design , Transportation Research record, 1701, pp 68-75 Kirk S (1997). Technical review of road accident counter measures and engineering design features. A comparative study of their relative effectiveness at reducing and preventing accidents. TRL Unpublished, Project Report PR/OSC/125/97. Transport Research Laboratory, Crowthorne. Roadway Design (1995), Roadway Fundamentals for Municipal Officials a workshop presented by the Maine Local Roads Center, Maine Department of Transportation, 1995 [Online] Available from < http://www.memun.org/SchoolsProject/Resources/Roads/Fundamentals.htm > [5 February 2009] Stewart D and Chudworth CJ (1990). A remedy for accidents at bends. Traffic Engng and Control. v31 n2 pp88-9,92-3. Printerhall Limited, London. Transport Research Laboratory (1988). Overseas Road Note 6 'A Guide to Geometric Design', Crowthorne. Read More