An interesting point and comment on the used of noise contour from Dublin City Council on their website.
Important Note on Using Noise Maps
Noise maps are a tool intended for a specific purpose. They provide an insight into how environmental noise may be generated from a particular source such as traffic or rail transport. It is important to note that the results are not intended to be a precise calculation of the total noise exposure at any given point, and the temptation to rely too heavily on noise maps to predict the noise level at a property should be avoided.