| I agree with your last sentence, but of course we can't instantly redesign any road at any time, and so there is a standard, orthodox method of setting speed limits which is the 85th percentile of the speeds people actually drive at. From a random document summarizing the process: "With the definition of 85th percentile speed, it would seem that the signed (posted) speed limit of a street would be highly influential in determining the 85th percentile speed, however the exact opposite is the case. A deeper dive into 85th percentile speed helps to reveal why it is a major consideration in determining a street’s posted speed limit.
As described above, the 85th percentile speed defines the speed that 85 percent of drivers will drive at or below under free-flowing conditions. Most people don’t drive according to the posted speed limit, but account for the visual aspects of the street and a ‘feel’ for the street. The visual factors that influence speeds can include: • Lane and shoulder configurations, widths, and presence of curbs • Presence of vertical and horizontal curves • Sight distance and obstructions • Presence of surrounding developments to the street • Access management characteristics and medians/turn lane configurations" "This uniformity of vehicle speeds increases safety and reduces the risks for vehicle collisions. When vehicles deviate from a standard speed, either faster or slower, the potential for crashes increase, whether caused by a slow car in a rear end collision or a fast car completing lane changes to maneuver through slower traffic. By setting the speed limit to the 85th percentile speed this uniformity is achieved and safety is increased." https://www.lincoln.ne.gov/files/sharedassets/public/ltu/tra... |