Last Updated 22 Jun 2020

Geometric Design Of Rural Roads Engineering Essay

Category Design
Essay type Research
Words 4178 (16 pages)
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Table of contents

It is a good known fact that route accidents in South Africa are serious causes of concern. Harmonizing to the latest accident statistics at that place has been an addition in the figure of human deaths.

Harmonizing to the article on the Arrive Alive web site ( Road decease toll 'unacceptable ' ) [ 1 ] dated 3 January 2010, Transport Minister Sibusiso Ndebele is disappointed with the route decease toll, stating that one human death on the state 's roads is one excessively many. Preliminary statistics released by the section on 2 January 2010 revealed that 1050 people had died in route related incidents during December 2009.


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Road safety is non merely a major concern for South Africa but so a concern that requires the immediate attending of other states worldwide. The Commission for Global Road Safety has during June 2006 presented a study titled `` Make Roads Safe '' [ 2 ] that could hold an consequence on the manner we approach route safety. The study aims to concentrate on political and public attending and relates to the planetary route traffic hurt epidemic that claims the lives of 1.2 million people and hurts around 50 million yearly.

Table: Gay period figures - December 2008








General practitioner



























































Beginning: Traffic Focus March 2008, p38, `` Fatal Road Accident Statistics '' by Cielie Karow ( RTMC )

The study besides indicates that unsafe roads have a important impact on developmental aims, particularly because of the huge economic and societal cost of route clangs to low and in-between income states. South Africa is in this class. It is of import that the cognition gained by high income states be transferred and implemented in South Africa.

The N3 path is a popular path during vacations and with high volumes of rider and cargo traffic on the roads, there is prone to be serious and fatal accidents. Road fury, velocity and driver behaviors are non the lone cause of these accidents. Holiday periods are by and large during the rainy season and route elements, particularly drainage, contribute to accidents.


There are changing point of views and sentiments among civil technology professionals on the use of superelevation values given in the TRH 17 [ 3 ] and SANRAL 's Geometric Design Guidelines [ 4 ] .

The geometric design considerations need to be reviewed particularly when it relates to route surface drainage at points where the route has no crossfall during development of superelevation. Presently the SANRAL Drainage Manual, 5th Edition Final Revision, p5-2 [ 5 ] recommends that the flow deepness during a 1:5 twelvemonth storm should non transcend 6mm. This is in contrast to the Highway Drainage Manual ( FHWA-TS-79-225 ) [ 6 ] recommendation of 4mm. Harmonizing to Highway Surface Design ( Transit New Zealand ) Manual [ 7 ] , the critical deepness for aquaplaning scopes from 4mm to 10mm depending on Sur and paving surfacing. The surface H2O deepness hence, should be restricted to 4mm for all but particular state of affairss where superelevation produces long, curved flow waies. There are three common particular state of affairss where surface H2O deepness may go critical, viz. , horizontal alliance curvature, intersections and inclines and superelevation development.

2.2 Why is the research being undertaken?

Standards for superelevation where steep perpendicular classs have a direct consequence on drainage on horizontal curves have non been developed harmonizing to the TRB [ 8 ] . Drain jobs associated with superelevation has resulted in an addition ( particularly at route broadenings ) in vehicular accidents and a thorough survey of the surface H2O flow waies on route surfaces due to superelevation demands need to be farther investigated, researched and analysed. The proposed analysis and context of the research proposal will place the factors such as velocity impacting the vehicle kineticss at crisp or decreased horizontal curves where flow waies of surface H2O are debatable and can take to aquaplaning.

Driver behavior during aquaplaning in concurrence with the geometric design of the roadway are factors considered in the research. This survey will be done under heavy vehicular traffic conditions during inauspicious conditions conditions. 'Context Sensitive Design ' ( CSD* ) [ 9 ] for the geometric design of safer roads will be addressed in the research survey.

2.3 How will the consequences add to the organic structure of cognition?

The TRH 17 papers was compiled to be a design guideline for the geometric design of rural roads in penchant to criterions. An aim of the TRH 17 was to continuously amend the bill of exchange papers, through audience between the governments, treatment with practicians and ongoing research. This has non happened. It is common pattern by geometric interior decorators to utilize the TRH 17 papers for the design of rural roads without oppugning the values obtained in the tabular arraies and graphs. The cogency of the guidelines should be judged by the geometric interior decorator in each specific state of affairs every bit good as the effects of going from the values suggested.

Similarly SANRAL 's Geometric Design Guidelines have been developed to help design advisers. The inceptions of the guidelines stem from AASHTO [ 10 ] . Currently there is no criterions developed for SA and these guidelines are authority specific.

Literature reappraisal

The literature reappraisal will depict the current province of cognition on the component and will be researched. Applicable literature listed hereunder have been identified.

* '' Context sensitive design ( CSD ) asks inquiries foremost about the demand and intent of the transit undertaking, and so every bit addresses safety, mobility, and the saving of scenic, aesthetic, historic, environmental, and other community values. CSD involves a collaborative, interdisciplinary attack in which citizens are portion of the design squad. ''

3.1 Reference certification

Refer to list of mentions ( 11.3 List of Mentions ) .

How will the literature reappraisal nexus with the job statement and research aims?

The literature reappraisal gives a background and base cognition to develop in this research survey.

Case survey

As portion of the on-going committedness towards route safety, convenience and mobility, N3TC conducted a comprehensive route safety audit in 2006 [ 11 ] which highlighted velocity as the chief cause of accidents at assorted locations along the N3 between Cedara and Heidelberg. Sections where velocity bounds are posted necessitates forbearance, appropriate decreases in velocity and full concentration on the portion of drivers. Accidents in these countries frequently occur because drivers lose control due to rushing, every bit good as the high velocity derived functions between heavy and light vehicles. Mist and inauspicious conditions conditions have a important impact of driver conditions.

Van Reenen 's Pass is one of the most beautiful subdivisions of the N3 exceeding the escarpment between the Free State and Kwazulu-Natal renowned for its slippery and unstable roads ( peculiarly as a consequence of the frequent brumous conditions ) .The route is steep, really steep in topographic points, and turns and turns as it follows the cragged terrain. The conditions is risky at times with high air currents capable of blowing trains, light vehicles and light trucks onto their sides. Snow has on a figure of occasions wholly closed the Van Reenen 's Pass, whilst mist and rain on a regular basis cut down visibleness and render the route faithlessly unsafe. It is of peculiar involvement the visibleness of a figure of skid Markss on the route surfacing and guardrail replacing.

Traffic, in peculiar heavy traffic, increases yearly as the economic system grows. Trucks unrelentingly 'grind ' up and down Van Reenen 's Pass twenty-four hours and dark, while during extremum traffic periods every bit many as 3000 vehicles per hr use the base on balls. Using September 2004 to August 2006 statistics [ 12 ] , the day-to-day norm traffic was 9100 vehicles, of which 2600 were big trucks ( 5 axles or more ) , about a 3rd of all vehicles. During the month of December 2007 recorded at the Van Reenen 's Pass, the mean day-to-day traffic reached 11000. The velocity derived functions between trucks in low cogwheel and the powerful new coevals of light vehicles is highly awful, both up and down the base on balls. High velocity derived functions ( 76 % of drivers exceed the velocity bound on the base on balls ) and failure to accommodate velocity in relation to fortunes, have contributed to 70 % of all accidents on the base on balls over this period. The consequences do non portray a pretty image and from a route safety point of position, Van Reenen 's Pass is the individual most unsafe subdivision of the N3.

N3TC has identified and addressed route safety on Van Reenen 's Base on balls from three points of position, viz. technology, instruction and enforcement [ 13 ] . N3TC has implemented intercessions to cut down the figure of accidents on the base on balls but presently the greatest causes of accidents is by vehicle softness ( inoperative brakes in peculiar ) , rear-end hits due to high velocity derived functions, unsafe inter-lane tactics and driver freak out in utmost conditions conditions.

Elementss impacting safety at superelevation

The drainage conditions of the roadway in relation to the vehicle kineticss need to be farther investigated as superelevation along crisp horizontal curves with decreased sight distances presents a important degree of driver concentration. The driver tends to rectify the vehicular way. The surface stormwater run-off flow way along the horizontal curve effects natural braking forces which in bend can take to seaplaning ( or aquaplaning ) .

Aquaplaning occurs when H2O force per unit areas build up in forepart of a traveling Sur ensuing in an uplift force sufficient to divide the Sur from the paving. During high strength rainfall events, a H2O movie builds up on the surface on the route. The hazard of vehicle aquaplaning additions as the deepness of this movie increases. The loss of maneuvering and drag force produced during aquaplaning may so do the vehicle to lose control, particularly when a guidance Sur is involved. Rainfall strength is the most of import environmental factor in hydroplaning.

The hazard of dynamic aquaplaning is straight relative to the deepness of H2O in the route surface. This deepness is affected by a broad scope of factors that are contributed to by the environment such as the geometric design, paving design, drainage design and care and by the status of the vehicle.

The geometry of the route has a big consequence on the H2O deepness and is the factor over which the geometric interior decorator has the most control. The length of clip H2O is able to remain on the route will act upon the deepness it achieves. Longer flow waies intend more clip to roll up rainfall and consequence in higher movie deepnesss. Changes in superelevation, reduced horizontal alliance and droop curves are some of the job countries where the incline is low or where H2O has to flux a long manner over the paving before being intercepted by a drainage system or dispersing into the next terrain. Superelevation alterations can ensue in long curving flow waies which may be debatable. Steeper longitudinal inclines can besides increase the flow way length and ensuing deepness.

The paving texture deepness effects the H2O deepness by leting some of this H2O to flux between the sum or in channels supplying flow waies to let H2O in forepart of the Sur to be forced out under force per unit area. Porosity can besides be considered as some pavings such as open-graded porous asphalt allow H2O to run out through them, taking it off from the surface. Wheel path depressions have a important consequence on the drainage patterns increasing H2O deepness and concentrating flow. The N3 has changing pavement surfacing of which an analysis will be done sing the flow way on these surfaces.

Pavement drainage solutions is indispensable to guarantee that no H2O is able to pond on the trafficable route surface, peculiarly in sag countries. This is critical in order to cut down the aquaplaning hazard.

Vehicle features and behaviors are besides of import factors in aquaplaning. The velocity at which a vehicle needs to go to get down aquaplaning is determined by H2O deepness but besides by the vehicle 's weight and Sur features. The vehicle weight determines how much uplift force is needed to bring on separation and it follows that a lighter vehicle will aquaplane at a lower velocity. Higher Sur force per unit areas increase the aquaplaning velocity by cut downing the contact country between Sur and route, increasing the vehicle 's weight to country ratio. Tyre pace deepness besides affects aquaplaning the same manner as pavement texture, with deeper pace traveling the H2O off from the country of contact more efficaciously. While minimal tyre pace deepness and maximal velocity are both specified by jurisprudence, minimal weight and Sur force per unit areas are non. These are vehicle maker recommendations specific.

In drumhead, drainage demands versus vehicle kineticss are the cardinal factors to be considered in the development of superelevation standards at decreased horizontal curves to minimise aquaplaning on the N3. Practical considerations such as Sur deepness, paving features and drainage solutions will be evaluated along the path.

The assorted literature reviewed amplifies the demand for safer roads and the N3 is considered to be the most traveled path in South Africa.

Research background

Significant roadway debasement such as shining of sums, hemorrhage of bitumen and rutting depletes the clash supply available for cornering. This depletion consequences from the usage of a part of the clash supply to supply the necessary braking force required to keep velocity on the downgrade. The velocity of the vehicles on the roadway and the vehicle kineticss will necessitate to be analysed as differing vehicles have different clash forces exerted on the roadway. It can non be assumed that the relevant design standards for a auto is similar to that of a truck or frailty versa. As the Independent Engineer, my old audit studies [ 14 ] on the N3 has indicated the assorted surfacing failures. Skid Markss are significantly prevailing and N3TC/SANRAL/DOT accident studies will necessitate to be investigated as portion of the research survey.

It is noted from the TRH 17 that the design vehicle is a individual unit truck. This unwanted combination consequences in a important lessening in the border of safety ensuing from roadway class, particularly for heavy vehicles.

On long or reasonably steep classs, drivers tend to go faster in the downgrade than in the upgrade way. Additionally, research has shown that the side clash demand is greater on both downgrades ( due to braking forces ) and steep ascents ( due to the grip forces ) . Downgrades on horizontal curves may be debatable, and that accommodation for it may be desirable in some instances. There are no guidelines as to how this accommodation should be made for two-lane or multilane divided or undivided roadways.

Some accommodation in superelevation rates should be considered for classs steeper than 5 % . This accommodation is peculiarly of import on roadways with high truck volumes and on low-speed roadways with intermediate curves utilizing high degrees of side clash demand. The superelevation alteration proposition high spots that this accommodation be made by utilizing higher design velocities standards for the geometric design of the roadway.

More unequivocal counsel on this accommodation, every bit good as accommodation for other elements of the horizontal curve, is needed. The design velocity versus minimal curve radii of horizontal curvature needs farther probe to guarantee safety on crisp horizontal curves taking the other related factors like superelevation, etc into consideration.

The article published in the Pretoria News, `` Wet conditions causes a spike in route accidents '' [ 15 ] refers to the important addition in roadway accidents during showery conditions. The drainage demands and vehicle kineticss in relation to superelevation design of the roadway will be investigated and researched.

I have consulted assorted professional applied scientists, engineers [ 16 ] and independent advisers [ 17 ] in the transit industry. The response received was favourable in footings of the demand for farther probes of superelevation in relation to drainage demands and vehicle kineticss for roadway geometric design.

Research jobs and purposes

I am presently the IE ( independent applied scientist ) and have audited the everyday route care points of the N3 for the past 3 old ages. As such, I have entree to some informations to analyze as portion of the survey.

Assorted subdivisions of the N3 are presently being upgraded or rehabilitated to better the quality and lifetime of the route due to vehicular traffic additions. There is important freight motion. This has resulted in an addition in vehicular accidents. My point of view on the design considerations adopted on the N3 is subjective as driver safety factors and other extenuating hazards of vehicular accidents need to be farther investigated.

From a geometric design analysis point of view, road-widening and general roadway rehabilitation are designed harmonizing to the bing roadway conditions and drainage jobs associated with superelevation is prevailing. Ponding has been recorded and ocular grounds is prevailing in certain countries. The general stormwater design standards should be viewed and analysed otherwise from the Kwazulu-Natal conditions as opposed to the Free-State and Gauteng conditions as the overflow rainfall strength and clip of concentration varies significantly.

This survey will sketch and place the undermentioned conditions with specific superelevation standards development on the N3 path:

Superelevation standards at steep gradients ( turn overing to cragged terrain ) with reduced/sharp horizontal curves ;

Drain jobs associated with superelevation ;

Flow waies on route surfaces due to superelevation ; and

Speed and Vehicle kineticss at crisp horizontal curves.

Steep classs at crisp horizontal curves presents a unsafe state of affairs for traffic.

The two scenarios where this status is prevailing is at broken-back curves on cragged terrain ( Van Reenen 's Pass specifically and other identified countries ) with multi-lane, 2-way roads ( whether it is divided or undivided ) and/or high velocity downgrade at/before perpendicular droop curves.

At these locations, the perplexing factors of vehicle `` off-tracking '' , pavement incline ( crossfall ) , and pavement clash tests the drivers ability to supply right vehicle positioning without compromising control of the vehicle. It has besides been recorded that air current has been a cause of accidents as the vehicles can non 'grip ' onto the roadway ( particularly at 'Windy Corner ' on the Van Reenen 's Pass.

From old design considerations, accident-related jobs have arisen where, as a consequence of Reconstruction, bing main roads have been rebuilt utilizing the 8 % -10 % superelevation rates in conformity with current guidelines. The rate of superelevation development is nevertheless non reviewed or adjusted.

Research methodological analysis

The research attack has a both qualitative and quantitative attack.

The theoretical research will consist of mathematical analysis with mold and simulation. The current geometric design guidelines for rural roads ( TRH 17 and SANRAL 's Geometric Guidelines ) will be used as base certification for values in the research input.

Case survey subdivisions of the N3 will be decided upon and N3TC, SANRAL and the DoT ( both national and provincial ) will be informed of the research survey. Data aggregation and statistical information will be sourced from the relevant governments with anterior consent.

This research would necessitate:

the reappraisal of current design guidelines ;

the development of an action program to accomplish the research objectives ;

the aggregation of statistical informations ( from SANRAL, N3TC, etc ) and other relevant information ;

elaborate ocular appraisal and rating of the roadway subdivision ;

Falling Weight Deflection ( FWD ) measurings will be performed at 50m intervals instead on the left and right exterior wheel paths along the subdivision identified ;

rut and siting quality measurings will be measured in both wheel waies every bit good as texture deepness along the outside wheel way as portion of the FWD measurings ;

Measurements of the stormwater sheet flow overflow deepnesss will be measured utilizing conventional agencies ;

vehicle kineticss will be physically and theoretically analysed ;

the rating of the effects of assorted options contextualized in geometric design guidelines and campaigner standards taking into consideration the CSD attack ; and

the readying of extenuating hazards, route safety steps and concluding geometric design standards for superelevation, drainage demands taking into consideration the velocity and vehicle kineticss at sharp/reduced horizontal curves.

The accident studies sourced by SANRAL/N3TC/DoT 's archives will be of import for the research survey. The possible restrictions could be the hold in recovering these accident studies and ocular appraisals will be carried out as an option


Research aims

The aim of this research is to:

analyse the velocity and vehicle kineticss at crisp horizontal curves ;

develop drainage criterias for flow waies or deepnesss on route surfaces due to superelevation ;

develop superelevation standards for steep classs on crisp horizontal curves by placing and analysing drainage jobs associated ;

design safe roads from a geometric design point of view by taking factors such as clip, cost, quality, CSD into consideration ; and

develop an independent package tool to help geometric interior decorators and governments in the civil technology industry.

It is noted that other standards associated with the design of horizontal curves such as tangent-to-curve passages, the demand for paving broadening, and minimal curve radii would besides be considered in the development of the standards.

The standards will be based on quantitative informations obtained from theoretic considerations and simulations and verified by existent field observation.

The identified countries for the existent field observations will be done by going the path and monitoring of the N3. Accident statistics will be indispensable as fact-finding mention for the background of the research survey.

Plan of research activities


The activities to set about the research proposal will be to:

expand literature survey ;

collate statistical informations ;

get permission for design informations from confer withing technology houses and SANRAL/N3TC/DoT archives ;

coordinate with SANRAL/N3TC with regard to experimental countries identified ;

analyse the design guidelines ( geometric and drainage ) for rural roads ;

fix the roadway mold and simulation for the assorted superelevation standards taking into consideration the drainage demands and vehicle kineticss ;

Analyse the sheet flow way ( hydraulic analysis ) for the drainage demands ( perpendicular class versus route width - at sharp/reduced horizontal curves ) - the Rational Method will be used ;

Investigate the CSD of route rehabilitation undertakings in relation to geometric design considerations for future route rehabilitation and major building undertakings ( It is noted that the De Beer 's Pass will be constructed as an option to the Van Reenen 's Pass ) ;

Analyse research findings and observations utilizing relevant package ;

Synthesize the research findings, observations and consequences ; and

Write the study.


This research would take 18 months to finish.

Potential end products

The possible end products for the research survey will be to:

Reduce or increase the superelevation values or rates, dependent on research end products, with regard to vehicle kineticss and roadway breadths ;

Analyse the drainage flow waies as superelevation rates or values in relation to the drainage demands and vehicle kineticss will find the standards to be adopted for safe driver conditions ;

The velocity versus vehicle kineticss analysis at crisp or decreased horizontal curves will supply suited guidelines for future rehabilitation and road-widening undertakings ;

CSD findings and observations to be adopted in future route rehabilitation and major Reconstruction undertakings ; and

Produce an independent package plan which will be exhaustively researched, tested and developed as a tool for geometric interior decorators and governments in the civil technology industry. This package can be integrated utilizing current technology package utilised by confer withing technology service suppliers, authorities establishments and assorted organisations will be investigated.

Research results

The result of this research will help design advisers in finding a much easier and safer design attack to plan rehabilitation, road-widening and major building undertakings, by placing job countries and supplying appropriate design values.

The recommended standards would be documented in the concluding study and besides presented in a signifier that could be used by assorted governments.

Presentations will be made at national and international conferences, seminars or symposiums associating to geometric design of roads. Workshops and talks will be conducted or presented through educational institutes, CESA and other governments in the civil technology industry. The research survey will be published as an article in transit diaries and the research sum-up will be published in several magazines. I am of the sentiment that I envisage national acclamation for the research and part of the research outputs to constructing the cognition base in South Africa.

Key mentions and certification

11.1 Governments, Institutes and other beginning of mention

Department of Transport ( National and Provincial ) ( DoT )

Road Traffic Management Corporation ( Pty ) Ltd ( RTMC )

South African National Roads Agency Limited ( SANRAL )

N3 Toll Concession ( Pty ) Ltd ( N3TC )

Council for Scientific and Industrial Research ( CSIR )

Transport Research Board ( TRB )

American Society for Civil Engineers ( ASCE )

Consulting Engineers South Africa ( CESA )

Durban University of Technology ( DUT )

University of Stellenbosch ( SUN )

Aurecon SA ( Pty Ltd ( AURECON )

WSP SA Civil and Structural Engineers ( WSP )

3D Compu-Systems ( 3DCS )

11.2 Design Guidelines and Standards

Technical Recommendations for Highways ( TRH 17 - Geometric Design of Rural Roads - Draft 1988 )

SANRAL Geometric Design Guidelines

SANRAL Drainage Manual ( 5th Edition - to the full revised )

Design of Highway Drainage Manual ( FHWA-TS-79-225 )

AASHTO - A Policy on the Geometric Design of Highways and Streets 5th Edition ( 2004 )

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