Flexible pavement structural design in Nigeria have been based on one of the empirical methods of design known as the CBR method. The structural design of pavement involves the determination of the thicknesses of the overlying layers of the pavement. Two key components required to achieve this are the California bearing ratio (CBR –value) of the subgrade of the pavement and the design traffic in millions of standard axle (msa). While the CBR-value is determined from the laboratory CBR test, the design traffic of the pavement is determined from the traffic count or survey by incorporating many factors to take care of the road condition, the design life and expected future traffic etc. In this post I would show how to estimate design traffic based on HD 24/06 (Design Manual for Roads and Bridges (DMRB)) which is the most recent UK standard on that by the following steps.
Step 1
Determination of Annual Average Daily Flow (AADF)
The key element used here are the commercial vehicles. Commercial vehicles are those over 3.5 tonnes gross weight. They are the vehicles that cause structural wear to a pavement because the structural wear of lighter vehicles (bikes, cars and light goods vehicles) are negligible. Annual average daily flow (AADF) is obtained from traffic count by manual process or use of sophisticated equipment. The traffic flow is usually measured in one direction (1-way flow) over 12, 16 or 24 hour period. The result from the count shall be converted to an AADF using the principles given in the COBA manual (DMRB 13.1.4). If it is measured in both directions (2-way flow), then it should be converted into AADF assuming 50:50 directional split or any other split ratio if directional bias indicates that. The traffic count is obtainable for existing road schemes (maintenance or realignment). For new road schemes that have not had traffic before, the commercial vehicle class/category count data shall be determined from traffic studies using the principles described in the Traffic appraisal manual (DMRB 12.1.1).
To determine the design traffic, the AADF of commercial vehicles per day (cv/d) in one direction, at scheme opening (or for existing roads, the current flow) and the proportion in the OGV2 category shall be used. Table 1 shows the commercial vehicle classes
Table 1: Commercial vehicles classes and categories
As noted earlier, the proportion of OGV2 category shall be used. For new road designs, the percentage of OGV2 vehicles shall be obtained by calculation and modelling but shall not be less than the percentage given in Figure 1 below.
Step 2
Determination of design period (Y)
The number of years over which the traffic is to be assessed shall be selected. For past traffic, this will generally be the number of years since last major structural maintenance. For future design traffic, it shall generally be 40 years. Other design periods may be used if proven to be economic and agreed with Overseeing Organisation (In Nigeria, 20 years is normally adopted for flexible pavement). Where additional information in the following steps are not available, the design traffic can be estimated based on Figure 2 for all lanes in one direction for a design period of 40 years only.
Percentage of Commercial Vehicles in Heaviest Loaded Lane (P):
The heaviest loaded lane for carriageways with 2 or 3 lanes is typically the nearside lane referred to as Lane 1 (Figure 3). For carriageways with 4 or more lanes, it is not always Lane 1. It was observed that commercial vehicles use a particular Lane on road more than small vehicles. The reasons for this include but not limited to choice, necessity of overtaking by smaller vehicles, fear, and speed limit. This is strictly considered in design and it is based on the number of commercial vehicles per day (cv/d). Junctions with lane drops and lain gains will considerably influence the flow of vehicles in each lane. The percentage of vehicles in this Lane, gotten from Figure 4 is adopted in the design.
Note: For new roads, all lanes, including the hard shoulder shall be designed to the same standard as the heaviest loaded lane. The actual traffic in other lanes is not considered.
Step 3
Determination of Growth Factor (G)
The number of vehicles used on a road continues to grow each year. It necessary to determine the growth factor so as to incorporate it in the design. The following points should be noted here
- The National Road Traffic Forecast (NRTF) is published in eight year intervals and predicts future traffic trends. The 1997 NRTF growth lines shown for OGV1+PSV and for OGV2 (the bold lines on Figure 5) shall be used unless specific alternative and more reliable local data are available. This is applicable in the UK. It is important that each country have their own traffic forecast. That is what necessitated the second clause.
- Past growth, where known from traffic counts, can also be used to give an indication of future trends in a particular situation, but only where data over at least a 10 year period are available, since averaging over a shorter period may give misleading results.
- For each cv class or category, traffic growth can be calculated which is dependent on the selected design period and the growth rate. The growth factor represents the proportional difference between the average vehicle flow over the entire design period and the present flow (or flow at opening). The growth factor for future traffic shall be found by using Figure 6.
- If past traffic is being calculated, the applicable growth factor is given in Figure 6. Bold lines are shown for OGV1+PSV and OGV2 which represent national trends. These bold lines are to be used unless actual growth rates are known for a specific cv class or category.
Step 4
Determination of Wear Factor (W)
The structural wear to a road associated with each vehicle that passes increases significantly with increasing axle load. Although alternative methods are available, structural wear for pavement design purposes in the UK is taken as being proportional to the 4th power of the axle load, i.e:
Wear/axle α L4
(L = axle load) A ‘standard axle’ is defined as an axle exerting or applying a force of 80kN. The fourth power law is used to equate the wear caused by each vehicle type to the number of equivalent standard axles, to give the structural wear factor of that vehicle. The wear factors for the new design case are higher than for the maintenance case in order to allow for the additional risk that arises from the additional uncertainty with traffic predictions for new designs.
Thus, a 50% increase in axle load results in a five-fold increase in calculated structural wear. Table 2 shows the wear factors of different schemes.
Table 2; Wear factors for cv classes and categories
The wear factors for the new road design case, WN, shall be used to calculate design traffic for all new road and pavement construction projects including road widening.
Step 5
Determination of Design Traffic (T)
The future cumulative flow, in terms of million standard axles (msa) for cv class Ti can be determined according to the following equation: Ti = 365×F×Y×G×W×P×10-6msa
Design Traffic (T) = Σ Ti
Where:
F = Flow of Traffic (AADF) for each traffic class at opening
Y = Design Period (Years)
G = Growth Factor (Figure 5)
P = Percentage of vehicles in the heaviest loaded lane (Figure 4)
W = Wear Factor for each traffic class (WM for Maintenance or WN for New Design Case) from Table 2
Note:
- For past traffic, Y = years since opening;
- If the calculation of traffic in other lanes is for maintenance purposes, P shall be the percentage of the commercial vehicles determined to be in each lane.
Table 3; HD 24/06 Traffic Assessment model
Example
Assuming we are designing for maintenance scheme on an existing dual carriageway with two lanes on each carriageway. No widening is involved. The heaviest traffic lane is Lane 1. The design period is 20 years. A classified count has been carried out and converted to AADF values as shown in Table 4, Determine the design traffic (in msa) if the growth factor for OGV1 + PSV fall within 2% and that of OGV2 fall within 3%.
Table 4; HD 24/06 Traffic Assessment model data for a given road
Design Traffic (T) = TWAT x (P/100) x Y = 2.327 x (90/100) x 20 = 41.886, say 42 msa
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