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.

Try to dig out some other's traffic from NUS network? Can you make it?
(It's true in some cases, like some switches in some area are physically not well-protected, you can easily get access and do a port mirror and run away before get caught. :) But in most case I don't think you can.)