Why The Rules Of The Road Aren’t Enough To Prevent People From Dying | FiveThirtyEight: “The engineer will usually calculate the load a beam must bear and then design it to hold some percentage of higher load, for safety. When building roads, the 85th percentile calculates the speed the engineers hope or intend people will travel, but then it’s used to design a road to meet that speed at a minimum, with a factor of safety allowing for faster travel,” he told me.
In other words, by adding additional “safety” to the road, it is designed to make people comfortable going faster than the engineers’ intended speed. This is known as the interpreted design speed (the speed people actually feel safe traveling), which is often significantly higher than the intended design speed. Think of a subdivision with wide, flat roads. The speed limit may be 25 mph, but you feel utterly comfortable doing 40.