I was thinking about how to come with unit rates for infrequently used Box Culvert sizes recently, and I came up with the idea of plotting a graph of "cost-per-metre" vs "the square root of culvert area" for a range of culverts, and plotting a best fit curve through the points and using the graph to estimate the cost of installing other sized culverts. I came up with the graph below. I'd be interested to hear if others think this method would generate a reasonable unit rate for culvert sizes for which no local cost data is available?

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