L. R. Kadiyali, Chief Engineer (Planning)
Ministry of Shipping and Transport (Roads Wings), New Delhi
ABSTRACT
The paper evaluates the current standards and specifications for rural roads and sets forth suggestions on several aspects. These aspects include the classification of rural roads, geometric design standards and stages for development from an un surfaced gravel road to a metalled road with bituminous surfacing. The economics of different pavement types has been worked out on the basis of overall transport economy including initial construction cost, maintenance cost and road user cost.
Introduction
The highway engineering profession in the country has distinguished itself well in the past in adopting innovative and low-cost options in road construction. Stage construction has almost been the corner-stone of highway planning in India. While these policies will have to be pursued with continued vigour in the future years, an appraisal of standards and specifications for rural roads will be timely and useful. The present paper critically evaluates the current standards and specifications, keeping in view that any changes in them which are likely to result in economy should be welcomed. When talking of economy, one should consider overall transport economy, which should include initial facility cost, maintenance cost and road user cost.
Classification of Rural Roads
As per current practice, Rural Roads include Other District Roads (ODR) and village Roads (VR)²,³
These two classes of roads are the lowermost in the hierarchy of the roads in the country, first adopted in the Nagpur Plan4. The distinction between these two classes has been retained since then in the Bombay Plan5 and the Roads Plan for 1981-2001¹. The Road Plan 1981-2001 has proposed to designate these two classes as the tertiary system¹.
In the various plan schemes currently being implemented, viz., Minimum Needs Programme (MNP), Rural Landless Employment Generation Programme (RLEGP), National Rural Employment Programme (NREP) and the Integrated Rural Development Programme (IRDP), there is no distinction made between Other District Roads and Village Roads. Both these classes are considered as one, intended to connect villages. It is thus obvious that there exists no special reason for retaining the two classes separately and they can be combined into one class, to be called either "Rural Roads" or simply "Village Roads". This simplification will also do away with the slight differences in geometric design standards currently in existence between the two classes. When both the classes perform the same role of connecting villages and serving as the tertiary system of road communication, they should have the same standards.
Design Speeds
Design speeds for various classes of roads in India generally follow those in other countries. The values adopted for Rural Roads are given in Table 1.²
Research on free speeds of vehicles in India has recently been completed6,7. The results of this research
Table 1
Design speeds of Rural roads in India
| Road Classification |
Design speed (km/hour) |
|||||||
|---|---|---|---|---|---|---|---|---|
| Plain terrain | Rolling Terrain | Mountanous | Steep terrain | |||||
| Ruling speed | Min Speed | Ruling Speed | Min Speed | Ruling Speed | Min Speed | Ruling Speed | Min Speed | |
| Other District Roads | 65 | 50 | 50 | 40 | 30 | 25 | 25 | 20 |
| Village Roads | 50 | 40 | 40 | 35 | 25 | 20 | 25 | 20 |
have shown that free speeds of different vehicle types are governed by roadway factors such as pavement width, vertical profile and riding quality. The equations are given below:
Vc=52.42+1.76W + 0.16 RS - 0.12 FL
-0.0123 CV - 0.0025 RG
Vb=53.00+1.16W - 0.26 RS - 0.16 FL
-0.0142 CV - 0.0022 RG
Vt=45.92+1.19W - 0.21 RS - 0.14 FL
-0.0131 CV - 0.0017 RG
Vtw=46.51+1.03W-0.13 RS -0.08 FL
-0.0054 CV-0.0020 RG
In the above equations,
Vc¸ Vb¸ Vt ¸ Vtw = Free speeds of cars, buses, trucks
and two wheelers respectively in km/hours.
W= Pavement width in metres
RS, FL= Rise and fall respectively in m/km
CV=Curvature in degrees/km
RG=Roughness in mm/km
3.3 Rural Roads are single lane in width such that W=3.75 m. They can at best have a thin premix carpet surfacing, such that RG=5000 mm/km. The total rise and fall (RF) for various terrains can at best be as under:
Terrain RF (m/km)
Plain; 0
Rolling 15
Mountainous 30
Steep 50
Curvature (CV) is related to rise and fall (RF) as per the following equation7:
RF=1.4011CV½
3.4 Thus, the values of curvature for various terrain type are as follows:
Terrain CV (degrees/km)
Plain 0
Rolling 115
Mountainous 450
Steep 1275
Substituting these values in the aabove equations, one gets:
Plain Rolling Mountainous Hilly
terain terrain terrain terrain
Vc 46.52 43.01 36.78 23.84
Vb 46.35 41.57 33.66 17.74
Vt 41.88 37.74 30.72 16.43
Vtw 40.37 38.17 34.78 28.23
The average composition of fast-moving traffic generally found on Indian roads is a under:
Per cent
1. Cars and light vehicles 10
2. Buses 10
3. Trucks 70
4. Two-wheelers 10
Using these values for weighting the individual free speeds, the freespeed of the traffic stream in various terrains will be as under:
Terrain Mean free speed of traffic
stream (km/hr)
Plain 42.65
Rolling 38.70
Mountainous 32.03
Steep 18.47
3.5 The speed adopted for geometric design is generally the 95th percentile speed. The standard deviation of speeds in India is in the order of 0.15-0.20 of the mean6. Thus the 95th percentile speed for various terrains is = Mean + 1.64 x Mean (0.15+0.20)/2
=Mean(1+0.287)
=1.287 (mean)
The Values are tabulated below:
Terrain 95th percentile speed (km/hr)
Plain 54.89, say 55
Rolling 49.80, say 50
Mountainous 41.22, say 40
Steep
3.6 These values are suggested for adoption instead of those given in Table 1. The distinction between Other District Road and Village Road is unnecssary. There is also no need for distinction between ruling speed and minimum speed.
4 For the low volume of traffic expected on the Rural Roads, a single lane pavement is sufficient. The current standard for the width of a single lane pavement in India is 3.75m. The Rural Road standards prescribe a width of 3.75m for ODRs and 3.0m for V.R.². It has been earlier recommended that the distinction between done, a common pavement width for Rural Roads is desirable. In view of the very low volume of traffic, a standard 3.75m land may not be needed and a width of 3.00m may do. This reduction will result in considerable economy. In any case, when local traffic conditions demand, the pavement can be marginally widened. it should be remembered that the future Rural Roads of the country will be needed to connect villages of smaller population than hitherto.
5 Roadway Width
5.1 The current standard for roadway width for O.D.R. and V.R. is 7.5m². The purpose of keeping the roadway width wider than the carriageway width is to provide shoulders to be used during crossing and over taking. For the low volume of traffic on Rural Roads, should be quite adequate. A research study in U.S.A. on design standards for low-volume rural roads also came to the conclusion that at traffic volumes in the range of 100 vehicles per day there were no economic arguments
favouring roadbeds wider than 5m (16ft)8. It may also be mentioned that keeping the roadway wider than absolutely necessary increases the cost of culverts and the recurring expenditure on maintenance.
6 Gradients
6.1 The current standards for gradients for Rural Roads are given below¹:
Table 2
Gradients for rural roads
|
Terrain |
Ruling gradient (per cent) |
Limiting gradient (per cent) |
Exceptional gradient (per cent) |
|---|---|---|---|
| Plan or Rolling | 3.3
(1 in 30) |
5
(1 in 20) |
6.7
(1 in 15) |
| Mountainous | 5
(1 in 20) |
6
(1 in 1.67) |
7
(1 in 14.3) |
| Steep
i) Upto 3000 m above mean sea level ii) Greater than 3000 m above mean sea level |
6
(1 in 16.) 5 (1 in 20) |
7
(1 in 14.3) 6 (1 in 16.7) |
8 (1 in 12.5) 7 (1 in 14.3) |
The current standards also specify that:
(i) ruling gradient may be used as a matter of course in design
(ii)limiting gradient may be used where the topography ofa place compels this course or where the adoption of a gentler gradient would add enormously to the cost.
(iii)continuous grades steeper than the ruling gradient should be as short as possible
(iv)exceptional gradients should be adopted only in exceptional situations and for short lengths, not exceeding 100m at a stretch; and
(v)in moutainous and steep terrain, successive stretches of exceptional gradient should be separated by a minimum length of 100 m having limiting gradient or flatter.
The above gradient standards are rather too liberal for low-volume rural roads and tend to make the cost of road construction too costly. It should be remembered that even on important stretches of National Highways (e.g. Kalka-Simla Road, Jammu-Srinagar Road and
Bombay-Poona Road), there are stretches which have gradients as steep as 1 in 8 even now. The hill areas of the country are extremely deficient in roads, and the next two decades will see a major thrust in road-building activities. Resources position will be extremely tight and the highway engineers will have to rethink on gradient standards.
An attempt has been made to work out the economics of roads of different gradients9. The following assumption are made:-
The Vehicle Operating Costs from Road User Cost Study are adopted (Ref 7), updated to 1984 prices.
Time costs of passengers are neglected
Accident costs are neglected.
The costs are exclusive of taxes.
Unskilled labour for road construction is suitably shadow-priced (55 per cent)
The composition of traffic is as indicated earlier in Section 3. The cost of operation of one two-wheeler is taken as one-fifth that of a car.
A discount rate of 12 per cent per annum is used.
The maintenance cost is Rs. 0.10 lakh per km, and construction cost is Rs. 3.00 lakh per km.
The roughness of the road is 5000 mm/km.
The rate of growth of traffic is 7.5 per cent per annum.
A design life 10 years is considered.
The construction period is 3 years, with equal amount spent every year.
Table 3 below given the abstracts of the Net Present Value of the total transportation cost, including the facility construction cost, facility maintenance cost and the road user cost.
It is thus obvious that on a hill road where the purpose of a vertical gradient is to gain or lose height, economy in the total transport cost can be had by adopting steeper gradients. The disadvantage of higher operating costs on steeper gradients is offset by the saving in distance by adopting steeper gradients. Saving in distance results not only in savings to construction and maintenance costs but also to road user costs. This evident from Table 4.
Based on these results, it is advantageous, both from the cost of initial construction and from the cost
borne by the road user over the future years, to adopt steep gradients for Rural Roads. The present standards result in higher initial cost and higher road user cost. The only consideration needed for design of gradients should be the gradeability of vehicles, i.e. their capacity to negotiate grades. The present vehicles in the country can easily negotiate grades of 1 in 10 , even when over loaded. The present standards for gradients in mountainous and steep terrain for Rural Roads should be modified as under:-
Table 3
Net present value of total transport cost
Gradient Net present value of transport cost on
a road negotiating a height of 50 metres
(lakhs Rs.)
Traffic 50 veh/day Traffic 100 veh/day
1 in 20 8.12 13.72
1 in 16.7 7.39 12.53
1 in 14.3 6.73 11.69
1 in 12.5 6.39 11.23
1 in 10 6.13 11.04
1 in 8 6.01 10.99
Table 4
Cost of operation of vehicles per km and length of roads for negotiating a rise of 50m for various gradients
Gradient Cost of operation Length of road in
(Rs./km) at 1984 km to negotiate
prices exclusive of taxes
Truck Bus Car 50m rise
1. 1 in 20 4.97 5.76 2.16 1.000
(5 per cent)
2. 1 in 16.7 5.48 6.44 2.51 0.835
(6 per cent)
3. 1 in 14.3 6.05 7.20 2.91 0.715
(7 per cent)
4. 1 in 12.5 6.67 8.06 3.37 0.625
(8 per cent)
5. 1 in 10 8.17 10.08 4.53 0.500
(10 per cent)
6. 1 in 8 10.38 13.34 6.56 0.400
(12.5 per cent)
When an alignment has to be developed to connect two points at different elevations, it is advantageous, both from consideration of the cost of initial facility and its maintenance and consideration of the road user cost, to have gradients upto 1 to 10.
When the terrain is undulating and the choice is between a flatter up and down gradient and a steeper up and down gradient, the choice has to be made after an economic analysis. Generally a flatter gradient will be found to be advantageous.
Economics of Pavement Types for Rural Roads
The pavement specifications normally adopted for Rural Roads in India are:-
Granular soil or gravel (known as mooram)
Water-bound-macadam (WBM) which is a granular base
Light bituminous surfacing (surface dressing) over WBM.
A gravel surface is the most common to begin with, and is progressively turned into WBM and bituminous surfacing. This is the well-known principle of stage-construction, which is the corner-stone of road specifications in all developing countries.
The precise stage when a gravel road needs to be converted to a WBM road and a WBM road needs to be blacktopped is left to the judgment of the engineer so far. There are no definite warrants of these conversions, based on traffic volume.
It is possible to work out the warrants based on the results of the Road User Cost Study. It has been attempted elsewhere and is briefly described here.
The following assumptions are made in working out the analysis:
The total transportation cost consists of cost of initial road construction, its periodic maintenance and the cost borne by the road user.
The fast-moving traffic is supposed to have the following composition:
Cars and light vehicles 10 per cent
Trucks 70 per cent
Buses 10 per cent
Two-wheelers 10 per cent
The Vehicle Operating Costs from the Road User Cost Study are adopted, updated to 1984 prices.
The cost of operation of a two wheeler is taken to be one-fifth of a car.
Time costs of passengers and accident costs are neglected.
All costs of economic costs, exclusive of taxes.
All costs of economic costs, exclusive of taxes.
The traffic is expected to grow at 7.5 per cent per annum.
The typical construction costs of different specifications, net of taxes, are as under:
Rs./km a) Gravel road 67,500 b) Addition of WBM layer over gravel road 84,500 c) Additional cost of providing 56,250 d) Additional cost of providing a premix carpet of 20 mm thickness 75,000
The maintenance costs are as under:
Rs./km/year a) Gravel 10000 b) WBM 6000 c) Blacktopped surface 4,750
Roughness values of various specifications are as under.
Roughness (mm/km) Gravel 10000 WBM 8000 Surface dressing 6000 Premix carpet 5000
Design period is 10 years.
The analysis yields the following equations for Net Present Value (in Rs. lakhs):
Gravel road: NPV = 0.859+0.080 ADT
WBM road : NPV = 1.919+0.076 ADT
Surface-dressed road : NPV = 2.298+0.070 ADT
Premix carpet road : NPV = 2.486+0.068 ADT
In the above equations, ADT is the average daily traffic, in terms of fast moving vehicles. The solution of these equations gives the following results:
A gravel road can be converted to a WBM when its present ADT is 250 vehicles. This shows that Rural Roads, the majority of which have a traffic of less than 250 vehicles per day need only be of gravel and need not be paved with WBM
A gravel road can be straight away paved with WBM and surface dressed when its present ADT is 150 vehicles.
An existing WBM road should be surface dressed when its present ADT is 50 vehicles. This clearly shows that even for a very low volume such as 50 vehicles per day, the blacktopping of WBM is economically justified.
An existing gravel road can be paved with WBM and provided with a premix carpet when its present ADT is 135 vehicles.
Conclusions
8.1 The present geometric design standards for Rural Roads are tending to be liberal. Under the resources constraint, there is a need to review them and make them cost-oriented.
There is no need to differentiate between ODRs and VRs, as both serve the same purpose.
The design speeds for Rural Roads should be fixed on the basis of recent research findings.
The pavement width of Rural Roads need only be 3.0m and the roadway width 5.5m.
The gradients of hill roads can be made as steep as 1 in 10 when negotiating height, without causing extra road user cost.
Rural Roads can be left as gravel roads till the ADT is 150. At that stage, if funds permit, they can be paved with WBM and blacktopped. If sufficient funds for this improvement are not available, the gravel road can be left as it is and paved with WBM when the ADT is 250. If the road already has a WBM pavement, it is economically worthwhile to blacktop even if its present ADT is as low as 50.
Acknowledgements
The paper is published with the permission of the Director General (Road Development).
References
Road Development Plan for India (1981-2001), Indian Roads Congress, New Delhi.1984.
Manual on route location, design, construction and maintenance of Rural Roads, Special Publication 20, Indian Roads Congress, New Delhi, 1979.
Report containing recommendations of IRC Regional Workshops on Rural Road Development, Special Publication 26, Indian Roads Congress, New Delhi, 1984.
Chief Engineers' Conference on Road Development Post-war Road Development in India (Nagpur Plan), Indian Roads Congress, New Delhi, 1943.
Report of the Chief Engineers on Road Development Plan for India (1961-1981), (Bombay Plan), New Delhi, 1958.
Kadiyali, L.R., et al, Free Speeds of Vehicles on Indian Roads, Vol. 42-3, Journal of the Indian Roads Congress, New Delhi, 1981.
Road User Cost Study in India, Final Report, Central Road Research Institute, New Delhi, 1982.
Economics of design standards for low volume rural roads, NCHRP Report 63, Highway Research Board, Wahsington, 1969.
Kadiyali, L.R., A simplified framework for economic analysis of highway projects in India, PhD Thesis under submission to the Kakatiya University, Warangal, 1984.
Kadiyali, et al, Roughness measurements as an aid to assess maintenance needs, IRC Seminar on Maintenance and Drainage aspects of Road Pavements, Bangalore, published by the Indian Roads Congress, New Delhi, 1982.
