Common
Fourth power law
Rule in road engineering
Fourth power law
The fourth power law (also known as the fourth power rule) is a rule of thumb which states that the damage to the road caused by a motor vehicle increases in proportion to the fourth power of its axle load. This law was discovered in the course of a series of scientific experiments in the United States in the late 1950s and was decisive for the development of standard construction methods in road construction. A 1988 report by the Australian Road Research Board stated that the rule is a good approximation for rutting damage, but an exponent of 2 (rather than 4) is more appropriate to estimate fatigue cracking. A 2017 report commissioned by the New Zealand Transport Agency found a wide variation in the best-fitting exponents for a power law on 4T axle loads vs 6T axle loads, depending on the current condition and type of the roading. As a very rough summary of its highly detailed findings: A 9th-power law is most predictive when the road is barely able to withstand the 6T load; and the...
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