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I am doing research on the impact of vehicles on roadways, specifically garbage trucks. In my research I am finding largely different "answers."

For example "Using American Association of State Highway and transportation Officials (AASHTO) vehicle load factors (VLF), a full, residential garbage truck is equivalent to 11,700 passenger cars." http://files.dep.state.pa.us/waste/Recycling/RecyclingPortalFiles/Doylestown_Township_504.pdf

However, when calculating based on EASL the number is 1,279 https://www.lrrb.org/PDF/201432.pdf

What is the difference and why are the answers so different?

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I think you are possibly concentrating on the wrong thing.

From your state.pa.us link:

The VLF of a passenger car (.0004) is so small that cumulative pavement impact is essentially moot

So, essentially, comparing a garbage truck to a car is pointless: the car may as well not be there. In other words: the car is so insignificant compared to a garbage truck that you can't use "a car" as your unit. It's like me looking out my window in London and saying "That building is about 1000 times the height of a person", and someone in New York looking out their window and saying "That building is about 10000 times the height of a person". Is the building in New York really 10 times higher? Or could I have been using a taller-than-average person, and rounded down, while the person in New York used a shorter-than-average person and rounded up. Maybe the difference in height is only 3 times.

Further, your state.pa.us link also says:

Roadway damage caused by vehicles is exponential (x4). For example, doubling the load for a given axle increases the damage 16 times.

So to get a vehicle damage ratio of 11700/1279 = 9.15, you'd need a vehicle weight ratio of 1.74. Is it inconceivable that the Pennsylvania investigation used a garbage truck that was 1.2 times heavier, and a car that was 1.5 times lighter, than the Colorado investigation? I suggest not.

In summary: I suggest you look at "damage done by garbage trucks" in terms of longevity of road surfaces used by them, and not in terms of relative damage done by cars.

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