Traffic and loading factors controlling pavement design

Traffic and loading factors decide the thickness and the strength of the pavement layers. The main ones are:

1. Design wheel load and axle load - the magnitude of the load on one wheel or axle. Damage rises about as the fourth power of the axle load, so heavy and overloaded trucks control the design. Axles are converted to the standard 80 kN axle (IRC:37).
2. Axle configuration - single, tandem and tridem axles spread load differently; the number of wheels per axle (dual tyres) changes the stress under the surface.
3. Contact (tyre) pressure and area - the contact radius and tyre pressure (about 0.5-0.8 MPa) decide the stress at the surface and the upper layers; for rigid slabs the contact radius enters the Westergaard equations.
4. Repetition of loads - the cumulative number of standard axles (msa) in the design life, found from initial CVPD, growth rate, design period, lane distribution factor and vehicle damage factor. Fatigue cracking and rutting depend on it.
5. Traffic volume and growth - only commercial vehicles above 3 t laden weight count. Counts from the last survey are grown by the rate $r$: $A=P(1+r)^x$.
6. Speed of vehicles and wheel position - slow and standing loads (at junctions, climbing lanes) cause greater deformation of bituminous layers; lateral wander in the lane reduces the damage at one point.
7. Impact and dynamic effects - increase the effective load on rough surfaces.
8. Legal limits - the allowed maximum axle load and enforcement (axle-load control and weigh stations) fix the upper bound of loading.