Dstraw 50 mph is a good average. The reason being it takes into account construction,accidents, pit stops,and such. Reefer companies quite often want their drivers to check on the reefer 3-4 hours. There are also medical reasons for stopping every 3-4 hours. The biggest one is a blood clot could form and cause problems. There have been a few news stories of where someone sitting for a long period has died from a blood clot forming and then breaking loose plugging up an vein.