I paced an in-line skater on the BWI trail once and he was doing 15mph, not a bad pace and it would put many cyclists to shame. The major problem with in-line skates is in many places they are considered play vehicles so you can’t use them on the road. But I found a legal brief that was successfully used to counter a ticket. So if you are going to commute on skates and they are play vehicle in your state I would go looking for that brief or ask me to go looking for it.