appropriate expressway speed?

As posted before, I see many people pass us multiple times in the same day due extra stops. I typically run around 60 mph and average 50 mph with stops by the end of the day. A long day for us will be 10 hours on the road with stops netting us 500 miles of distance. More than that becomes a job and we should never forget we are on vacation. We completed another 4500 mile 17-day trip this summer and stopped at a point of interest for 2-3 nights every 1000 miles. Earlier in the year we did a 2500 mile in 11 days following the same break and driving schedule.