Interdependencies among elements of the technological networks are the most important source of concern for their reliability. Here we give a simple metric to quantify interdependencies in the framework of the classical Theory of Systems. We propose a data-driven method to quantify interdependencies among all technological networks (power grids, gas networks, water distribution, transportation, telecommunications etc), given a set of measurable physical, logical, topological features using the properties of reacheability and observability, that are still valid for (causal) nonlinear systems. We consider dependency as a consequence of reacheability and use the state vector to produce weighted indicators. Advantages of our method are simplicity, physical significance, independence from the measurable quantities, a reasonable computational cost, real-time implementation, and the possibility to reduce superior order dependencies.