"No known reason"? Seriously? I think you stated some of the known reasons in your post. The speed drops to 45 on the highway because you are entering a heavily used business area mixed with residential and there is a lot of cross-traffic from side roads and people entering/exiting all the businesses like the "food store, strip mall, car dealership" as well as the bank, barber and beauty shops, convenience stores, restaurants, multiple car dealers, houses, etc.