This is an astonishing new tool developed by Stanford University, seemingly designed for those wishing to run wargame campaigns set within the Roman Empire:-
The really useful tab is the "Mapping Orbis" one. Using this, one can calculate journey times from any place in the Roman empire, to any other, and choose appropriate means of transport. It even takes into account season and wind direction!
One needs to be a bit careful about the suggested routes, which can be a little eccentric. Also I'm cautious about land travel in winter, which (unlike sea travel) doesn't seem to take longer than land travel in the summer. However, the potential for use as a tool to work out travel times is immense; the map is pretty handy, too, and could be made available to players. The GM could calculate travel times for armies and modify with random factors or according to the generalship of the commander.
Hopefully I'll get a campaing running late in the year, and this will be the engine that I'll use to calculate the speed of movement of armies. Do have a go, it is very easy to use and rather fun!