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

I think Romans managed effectively combine their whole empire in to one unit by roads. Romans built very helpful roads witch lead in help in so many positive ways. One of the good helpful things about roads were armies could travel faster. That meant that when the armies needed to get some where the roads helped the armies travel faster rather then traveling through hills, fields and forests.Another way might of been, that Romans had to trade and exchange goods. So roads helped them to travel more safe, easier and faster. And also here again if Romans just needed to travel some where they would travel faster and less dangerous.So i think the roads helped Romans in so many good ways.