It is 9 June, AD 68. Nero is dead in Rome, and a freedman named Icelus is hurrying to Spain, perhaps partly by sea, where the governor Galba does not yet know he is emperor. Pick a place, a month and a messenger, then send the news and watch it spread across the empire by road, river and sea, one day at a time.
Press Run 1,000 times to send the same message 1,000 times, each with its own weather, horses and ships. The map then shows the typical day each place heard, and this table the range.
Real messages whose travel time we know, against the same journey run 1,000 times in the model. Match: the typical run falls in the real range. Close: the real range overlaps the model's middle 80% (the 10th to 90th percentile), or the typical run is within 20% of an end of the real range. Off: neither. The parameters we had to guess were tuned against these cases; the sourced ones were not.
| Case | Real | Model: typical (middle 80%) | Verdict |
|---|---|---|---|
| Computing… | |||
The map is a network of 150 cities, ports and mountain passes around AD 100–150, joined by 300 roads (their lengths measured along the Itiner-e map of Roman roads), 68 sea lanes and the Rhine, Rhône, Danube and Nile. A message leaves on a date, and the model finds the fastest way to every place, as a messenger who knew all the roads and timetables would.
Each leg's time depends on the date and on luck. Riders keep their speed along post roads, with extra time at mountain passes. Ships only sail when one is leaving, fast with the wind and slow against it: Rome to Alexandria is a quick run, the way back a long beat against the summer winds, so the model doesn't sail Alexandria to Italy directly at all; ships go by Myra, Rhodes, Crete and Sicily, as Casson describes. Casson estimates 10–13 days out and 53–73 back. By ship only, the model gives the two runs in the calibration table: it shows the one-way street, but not its full size, because it doesn't keep ships waiting in harbour for a change of wind (see the table). The seas are safest from 27 May to 14 September and closed from 11 November to 10 March (Vegetius), apart from rare emergency sailings; in between they are risky. Vegetius's spring risk ends on 15 May; we count 16–26 May as risky too, as Ramsay does. Every leg also gets a random ±20% for weather, lame horses and floods, which is why 1,000 runs give a range rather than one answer.
Guesses, stated plainly (each marked ASSUMPTION in the code):
Units: miles first, kilometres in brackets. A Roman mile was about 0.92 miles (1.48 km).
The network is the empire of about AD 100–150. Castra Regina (Regensburg) and Lauriacum (Enns) are on it for the Danube line, but they became legionary bases only around 179–200.