Like the previous teacher stated, Roman roads were important to save time on travel. But here are a few other things that helped drastically.
1. Roman bridges. They are still in use, like in Trier, Germany. Commuters still use it everyday. This is all the more important, because when you read Caesar's commentary on the Gallic Wars, he makes this point repeatedly. The Romans were able to tame rivers through their bridges, while their enemies could do nothing. If you count the amount of words that he uses to describe bridge building, you would be impressed.
2. Romans had amazing military weapons. Josephus, the Jewish historian, who recounts the Jewish wars with Rome, writes about Roman military weapons, like the catapult. The ability to hurl missile weapons from afar was a huge benefit.
3. Romans also perfected the ability to set up and break down military camps. Again, Caesar speaks of his rapid ability to move from one place to another. This coupled with the roads made a huge difference.
4. Roman also were able to build a good navy. Historians (Livy) says that they were able to capture a Carthaginian ship and replicate and it and even make it better. The point is that Roman engineering was amazing.
5. Rome also had the ability to build the necessary infrastructure to support its troops from afar, not to mention the many allies that Rome possessed.
6. Closely related to point 5, the Romans had a great postal service with things like fresh horses set up at different places for effective travel.
If you put all of these together, then you have an impressive empire.
Perhaps the most important technology that helped expand the Roman Empire and made it more governable was the technology of good roads.
The Romans built well-engineered paved (cobblestones) roads throughout much of their empire. These roads allowed for a couple of things.
First, they allowed soldiers to quickly march to all areas of the Empire in all weather. Without the roads, it would have been hard to move when the ground was wet and muddy.
Second, they allowed goods and messengers to move quickly from place to place. This helped allow the Empire to be strong economically and it allowed messages to move quickly between the governments in the provinces and the central government in Rome.