Traffic in standard axles

Pavements are damaged by axle loads, not by vehicles as a whole, and heavy axles do vastly more damage than light ones. The AASHO road test in the 1950s showed that damage rises roughly with the fourth power of axle load. So a 16 tonne single axle causes about sixteen times the damage of an 8 tonne one. To design for a mixed stream of cars, buses and trucks, all traffic is converted into an equivalent number of standard axles, usually 80 kN (8.16 t) single axles with dual wheels.

The vehicle damage factor (VDF) is the number of standard axles per commercial vehicle, found by axle load surveys at weigh-in-motion sites or portable pads. Only commercial vehicles above about 3 tonnes laden are counted, because cars contribute negligibly. Overloading raises VDF sharply. A lane distribution factor allots the traffic to the design lane: for a two-lane single carriageway road it is a large share, for divided multi-lane roads less.

Design traffic over the design life is the cumulative number of standard axles, in million standard axles (msa). It grows each year with the traffic growth rate, so the calculation uses a compound growth sum. Design lives are typically 15 years or more for flexible pavements on national highways and 30 years for concrete pavements, with stage construction possible on low-volume roads.