How does the road damage calculator work?

The road damage calculator is an easy-to-use implementation of the Fourth Power Rule, a formula developed in the late 1950s by the American Association of State Highway Officials. It was intended to help traffic engineers understand the impact of vehicles on wear and tear of the road surface.

The fourth power rule in traffic engineering is a principle used to describe the relationship between axle load and damage to the road surface.

It states that the damage caused to pavement by a vehicle's axle load is proportional to the fourth power of the axle load. This means that even small increases in axle load can result in significantly greater damage to the road surface.

The rule was developed from multi-year empirical studies in the 1950s and is crucial in the design and maintenance of roads and highways.

At first glance, it would make sense that a 2 ton car does twice the damage of a 1 ton car. However, during the various multi-year studies that were used to develop the rule, it was observed that doubling the weight on an axle would actually result in four times more damage; not a doubling. That means that a 2 ton car actually causes 16 times more damage than a 1 ton car (difference in weight, to the power of four) 2⁴=16.

This simple fact means that given a vehicle that has the same number of axles, the heavier the vehicle, it does significantly more damage to the road surface everytime it drives over that part of the road surface.

And you'd be correct. Weathering often is the initial cause of the damage to the road surface that becomes a pothole. Another initial cause is poor quality repairs to the road surface. However, neither of these causes are why potholes form.

Weathering is an incredibly slow process that takes decades, or even centuries outside of areas of the world with extreme climates. It simply doesn't explain how a small fracture can develop into a hole in the road surface tens of centimeters wide, and several centimeters deep in the space of months.

What the weather or poor workmanship creates, all vehicles (even the lightest of road bikes) contribute in turn into the potholes we all know and loathe. However, the greater the load per axle, the more those vehicles contribute to their growth.

It's also true that the heavier the vehicle, the more friction happens between the tyres and the road, and the more of the surface is degraded, also leading to the potential formation of potholes, but this is a much slower process.

Definitely not! It doesn't take into account the differences in wear and tear caused by variations in the weather, tyre size, and many other factors.

However, that doesn't mean that it doesn't show the impact that different sized vehicles have on the road surface given the same conditions. It doesn't show how much damage each vehicle does to the road; that's not what it was intended to do. But it does show that lighter vehicles cause significantly less damage to the roads; an incredibly important consideration as governments and the car industry pushes to replace existing internal combustion powered vehicles with electric equivalents that are significantly heavier for the same footprint.

It also helps advocates for modal shift demonstrate that a move towards smaller, lighter personal mobility vehicles like scooters, bicycles, and even micro-cars can help save governments significant sums on their road maintenance budgets by reducing the number of journeys made by heavier vehicles.