How do historians know the Indus Valley cities were carefully planned?

The historians know that the Indus Valley cities were carefully planned and organized because of the archaeological findings that they have on disposal.

Even though there's not much written material at disposal, still the archaeological findings are on an excellent level, and it can be seen very clearly that this cities were carefully planned, nicely organized. The architecture itself is on a very high level, the organization of the cities is in a very urban manner, where different parts of the cities that have different purposes are all set in a way that they do not get intermingled, and that every activity in the cities can go smoothly and not to be interfered by anything.