My current research involves the impact of culverts on the distribution of a salamander species, and I am attempting to include road usage levels in the analysis. I cannot find any sources for good road use estimation methods.
This is an interesting question and one that I haven't really seen proposed anywhere. However, it does seem logical that "stream order" could be used to describe road usage, at least to some degree. You might be able to contact the department of transportation in your state and see if they have data on road usage from different "orders". They often collect this type of data when looking at widening roads, adding new stoplights, etc. So, any data they have and can share would allow you to look at possibly providing a proof of concept for this type of system.