´╗┐The damage to road pavements has a - generally accepted - relation to the fourth power of the passing axle loads, while the damage to bridges is related to the total weights of the passing vehicles.
Organisations responsible for the construction and maintenance of roads and bridges need realistic information on the actual traffic loading on their roads. This loading information is an important input both for the design codes of the road infrastructure (new roads and bridges) and for the planning of the maintenance of the existing infrastructure.

Only a WIM system will measure the actual pavement loading. Additionally the information from WIM systems can be used for detailed analysis of transport flows over the road network and over time.

ESAL stands for Equivalent Single Axle Load and is used to quantify the damage to road pavements because of the passing traffic. This means that the total damage is expressed relative to the damage of one reference axle load (often 10 tons is used).
So when the ESAL value of a vehicle is 5, this means that the total damage of the axles of this vehicle is 5 times the damage caused by a single reference axle load. The following formula for calculation of the ESAL's of a vehicle is based on "AASHO Interim Guide for the Design of Rigid and Flexible Pavements, 1993":

ESAL = Σi (Li/Lref)4
With: Li = the load of the i-th axle of the vehicle in tons, Lref = the reference axle load = 10 ton.

Example: a 5 axle truck with 7.5 ton, 12.5 ton and 3 times 10 ton axles has an ESAL value of 5.6 that is more than 220,000 times larger of that of a normal car of 1,200 kg (2 times 600 kg axle loads, ESAL=2.6 10-5).