Design traffic (equivalent standard axles)¶
All methods start from the design traffic: the number of equivalent standard axles of 8.2 t (ESAL) that will travel on the most loaded lane during the design life. RPD computes it automatically from the data you enter.
Why "equivalent axles"¶
Commercial vehicles have different numbers of axles, loads and types. To compare them, the standard axle of 82 kN (8.2 t) is used: each real axle is converted into the number of standard axles that produce the same damage. Damage grows with the fourth power of the load:
Ceq = (x / y)⁴ — with x the real axle load and y = 82 kN.
The mean equivalence coefficient (Ceq) weights this conversion over the traffic spectrum of the chosen road type (the frequency of the various vehicle types).
From AADT to cumulative axles¶
The starting figure is the AADT (annual average daily traffic) of the first year. RPD corrects it with:
- the annual growth rate (r) over the design life;
- the percentage of commercial vehicles;
- the split by direction of travel and the percentage on the design lane;
- the wheel-path dispersion coefficient;
- the equivalence coefficient Ceq (traffic spectrum + fourth-power rule).
The result is N₈.₂,Calc, the standard axles accumulated over the design life, then compared with the allowable axles in the AASHTO method. For the rational methods, instead, N is used — the number of standard axles per day in the last year of the design life, which enters the allowable deflection.
Data required in RPD¶
| Field | Meaning |
|---|---|
| AADT | Annual average daily traffic (vehicles/day), 1st year |
| Growth rate r (%) | Annual traffic increase |
| Commercial vehicles (%) | Share of heavy traffic |
| Traffic on the direction of travel (%) | Split between the two directions |
| Commercial vehicles on the design lane (%) | Share on the most loaded lane |
| Wheel-path dispersion coefficient | Reduction for wheel dispersion |
| Standard deviation S₀ | AASHTO only: error on traffic and performance (0.40–0.50) |
Road type
The road type (in General data) selects the traffic spectrum used for the Ceq calculation. Choose the one matching your infrastructure.
Preliminary sizing
In the preliminary stage the pavement catalogues are useful: in Italy the Italian Catalogue of Road Pavements (CNR, BU 168/95).
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