Thanks for your question.
According to this (rather old) site, the top speed recorded by an
in-line skater in 1999 was a rather scary 68.2 mph.
How In-line Skates Got Rolling
While this (evidently rather disapproving) skating site puts the
figure even higher for downhill skating:
"Skating downhill is also potentially dangerous. It doesn't take much
of a hill to get rolling at a speed well beyond your comfort level.
Skaters have been clocked at greater than 70 mph on highway grades--on
The Orbit: The Slopes - Let's Get Down!
Top speeds are apparently measured during competitive skating using
the kind of 'speed gun' which traffic policeman use to clock a
Other articles on the site come closer to defining 'normal' speeds.
"The math says I averaged just under 15 mph to complete the marathon
distance in 1 hour and 42 minutes."
The Orbit: My Summer Sabbatical As A Skate Pro
"According to studies done by Rollerblade in 1991, during a 30-minute
period, a skater going 8-10 miles per hour expends 285 calories at an
average heart rate of about 148."
Get Rolling FAQs
According to this site, a sensible average speed for keeping fit is,
similarly, about 10 mph:
Rollerblade.Com: Ten Week Workout
The consensus, then, seems to be that speeds for normal skating are
about ten -twelve miles per hour.
Some other links which you might find useful:
TeamRollerblade.Com: Speed Skating
I hope this answers your question, but please ask for clarification if
you would like it.