The Romans would develop road systems as they went along with their expansion. Every time they'd conquest new territory, they'd make sure to keep the new colonies controlled by facilitating access to them through the construction of new roads. In addition, the development of these routes was crucial to containing attacks and ensuring a successful defense of the Empire, as the armies would move move faster and arrive quickly to the territories in conflict.
This strategic emphasis in roads and bridges was vital to the success of their army. The routes were built specifically with speed of movement in mind, designing them mostly in straight lines for efficiency.
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Cyrus' conquest was relatively humane compared to empires preceding his, such as the Babylonians and Assyrians. He allowed many areas of his empire a great deal of autonomy, rather than centrally controlling things. This distributed leadership allowed Persia to cover a greater geographical area than previous empires in that era and still be fairly stable, since most of its subjects were content to remain under Persian rule. Persian law was more codified and less arbitrary than previous empires; the king's seal declared approval of law which could only be reversed by further approved decrees -- not quite rule of law but going in that direction.
