and the network is well-designed network with security consideration, where if you are not priviledged, you are not able to see any other's traffic.

If you have the access to the distribution/core switch/router, you can use port mirror to "copy" the data to another port where you can have software/hardware network analyzer to calculate the thing.

If you have access, you also can use IP accounting to calculate the average bandwidth utilization in a certain period of time.