40 km/h where? Boundary layer is not a well defined term relative to the surface of the Earth because there is no universal "free stream velocity". Everything is a complex system of currents depending on which layers of the atmosphere you look at. Also we tend to have trees and buildings that block out a lot of the wind close to the ground.

As it is commonly used in environmental and atmospheric science, the boundary layer of the Earth is approximately 1/4 mile.