How did the Romans manage to effectively combine their whole empire into one unit?

In order for the Romans to combine the empire into one unit they had to have done a lot of things. One MAJOR thing was building roads. The saying "all roads lead to rome" had a greater truthfulness than it does today. The roads were the glue that held the city together. Without the roads the empire wouldn't have been able to travel from place to place. Another thing the Romans had to do was creating a passenger, freight, and express system that traveled on the roads. This way the Empire could carry out trades.