The average minimum speed limit on US Interstate Highways is 40-50 mph with most being 45 mph. SMV triangles are for horse drawn vehicles as well as farm machinery that travel below 25 mph and are not allowed on the interstate in the first place. The use of 4 way flashers while moving in some states is illegal, they are referred to as "Emergency Flashers". If your vehicle will not, or you chose not to travel at or above the minimum speed then by all means stay on the back roads and stay off of the interstate. You are apparently in no kind of rush to get where you are going so enjoy the beauty of the back roads and get where you are headed when you get there. Most of the Interstates here now are 70 mph, Texas is 75 mph with some at 80 mph so driving 40 - 45 mph is not a very healthy mix.