Roman Empire Unification

I think the Romans effectively unified their empire through roads. I think that the roads they built connected all the corners of their empire. It's a lot easier to move along a paved road then through a forest. Thus they could trade faster and travel faster and move their armies faster.
Because it's a lot easier to move along paved roads Roman armies moved quickly. This can come in handy when your invading a country and want to get your troops at the border as soon as possible. Also it can help in defense purposes, if someone is invading, the Romans could move thier armies to the border and provide backup for the armies already fighting faster than the other country(that doesn't have paved roads) can.
Also the roads helped the romans spread culture. The roads helped exchange goods faster and they also helped merchants and artisans travel more effectively. So farther parts of the empire were witness to the same art and goods as the closer and more civilized parts. And having the same art and goods results in having the same culture and having the same culture results in unity.
So I conclude that the biggest factor in unifiying the Romna Empire was the efficiency with which their roads worked. They made everytihng from soldiers to performing troops move faster and promoted communication, communication helps understand, and understanding causes unity.