Depends on the route, but it is mostly between 21-25 kph (13-15.5 mph).

My personal best so far was 44.16 km in 1h:29m:28s, averaging 29.6 km/h (18.4 mph), all on pavement, with a slight tail wind; however, I was a bit hungover, so that probably cancelled some of that advantage out. The day before, going in the opposite direction, I was 1 kph slower at 28.5kph (17.7mph), but I did more stop/start city traffic, so on the open road it was pretty much the same speed.