The stirrup enabled Persian courier networks by making fast riding stable

Evidence

The stirrup made it possible to ride fast and far, without falling off. This revolution­ ized calvalry warfare. It also helped the Persians, who apparently introduced the stirrup in the Middle East, to build up their remarkable network of courier routes and relay stations, which is described with admiration by Greek and other visitors. — Bernard Lewis, From Babel to Dragomans, 789

Categorization

• XREF: Classic technological-adoption causation: an innovation in one domain (warfare gear) unexpectedly powers an information/communication system — echoes the broader theme of travel infrastructure.