Okay, this is neat. Stanford has created what is effectively Google Maps for the Roman Empire.

What it shows you is the principal routes of the Roman World: the road network, the main navigable rivers, and the hundreds of sea routes that crossed the Mediterranean, the Black Sea, and the coastal Atlantic.

The tool then attaches both time and expense to these routes (which would have been used for the transportation of goods and people, but also for general communication across the Roman Empire).

So if, for example, you are curious about how many days and how many denarii it would have cost you to deliver an important dinner invitation from Roma to Alexandria during the summer months of antiquity, you now have an online tool. It's about 14 days.