In the perfect system as the heat builds in the engine the thermostat opens and closes to maintain a set temperature. This keeps you going down the road just peachy. As the system becomes taxed, the thermostat just stays open. Your system is in trouble and there's no sign of it yet. When in truth it has been going bad for awhile.
Speed limits put cooling systems to the test.
Not overheated but hot. When towing a small travel trailer on a hot day it would overheat on a hill no matter what speed we went. Water bags hung on the front bumper of all cars on trips in those days. Now we expect older cars than that to go 80 all day and be just fine. Now that we are contending with air problems and will never be able to break away from gasoline, they will be constantly changing formulas.