As suggested by Anton, here's a look at the Milan-San Remo race that's been using the same (or almost the same) route over the years:
... to give you a better idea of your original question, look at a race like Milan San Remo. Using the same route over all the years. (Or very close to the same route...) There you will see the average speeds have increased all the time over the years. Except the past couple of years it seems to have dropped a little. Maybe because riders are a little cleaner, although I doubt it is that.
Data from BikeRaceInfo:
All Italian racers dream of winning the most prestigious Italian single-day race, Milano-San Remo. It is the longest 1-day race on the pro calendar. Sometimes called La Primavera (Italian for spring) or La Classicisima (the most classic), it is held in mid-March.
Note the y-axis scales do not begin at zero, to make the differences more apparent. The distance has increased slightly over the years somewhat (except 2013 where it was shortened due to heavy snowfall and bad weather).
But the average speed increased in the first half of the 20th century but has levelled off in the 50 years since 1960.
A similar trend can be seen in the 'Five Monuments of Cycling':