He argues that the only way to prevent any Highway fatalities is to reduce all freeway traffic to 5 miles an hour. This however would not be realistic or conventionally efficient at all. It would be faster to ride a bicycle. What is a safe speed is a question that lawmaker, engineers and politicians have all tried to figure out. From an engineering perspective sagely effect of speed limits is largely unknown.
Design speed is what engineers refer to.
Speed at which on a good day with good weather and low traffic density, a driver with average ability is able to drive safely without losing control. The idea is to set the speed limit at the 85 percentile of the design speed. Accident risk greatly increases at speeds above and below this percentile rank. Roth argues that they have no proof of this 85t h percentile being the safest and that this speed is no longer relevant with the greater technology in car manufacturing. Speed variance and safety also seem to be an issue.