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Cippus
One interesting feature that seems unique to the aqueducts of Rome is the cippus, literally a gravestone. A cippus was a small stone marker set in the ground. It performed two functions; where the channel ran underground the cippi marked its location and since they were numbered like milestones, they gave the maintenance staff a convenient point of reference to any point on the line. No cippi have been found anywhere but on the aqueducts of Rome, and then not on all of them. Frontinus tells us that instituted by Augustus, who installed them on existing aqueducts and on new construction and renovation, see also under Administration.Hodge (2002:103) states that cippi were usually placed 240 Roman feet apart, about 71.3m. However, in practice, the placement varied. Not enough have been found in situ to make a definitive judgement on the matter. Hodge also notes that they may not have been used much, and were probably unique to Rome. See CIL 6.1250 for an example of a cippus.
From the thesis of Evan J. Dembskey on The aqueducts of Ancient Rome