This incredibly detailed map shows the locations of Roman legions around the time of the death of Augustus. A standard legion may have 5,000 men and without such an effective road system, could not have provided the needed force projection and travel time.
A modern interpretation of the Roman road system. The saying "all roads lead to Rome" was not an exaggeration in the height of the Empire. Road improvement and expansion was always happening with legionaries providing most of the work on way to campaign.
The amount of resources traded throughout the Empire is mind-boggling. Rome gained near complete naval supremacy of the Mediterranean Sea with the destruction of Carthaginian fleet during the 2nd Punic Wars. This was brought up by Cato the Elder's speech of fruit that came from Carthage to Rome in under three days as justification of war.