Reading your question both before and after the edit, I have a mental image which may not completely match your situation, and my understanding may be lacking.

I imagine that you use the testing method to take many measurements and get what looks like a normal distribution of Variance 5 and mean 10. You then want to update your understanding based on the documentation for the test method, which says that for any samples of known density x, the testing method's measurements will have a normal distribution with a variance of 3 and a mean of x+2. From $Var(X+Y) = Var(X) + Var(Y)$, you can then estimate that your road locations do not all have exactly the same density, and instead have densities normally distributed with Variation 2 and mean 8.

Also, I also until recently did not realise that, according to 'How do comment @replies work?', if more than one person has commented on your post and you comment on your own post without using an @, no one will get a notification and thus no one will know that you have said something.