Road design, safety and Vision Zero

[jamincan]

From the WSDOT:

As congestion increases and traffic begins to move slower than 70% of the posted speed limit, overall productivity declines and the highway supports fewer vehicles, as highways are engineered to move specific volumes of vehicles. In addition, highways do not operate at their maximum efficiency when moving at 60 MPH (the most common highway speed limit in Washington State) because of the need for increased spacing between vehicles.

Maximum throughput (optimal flow) speed is not a static number for all highways, and can vary from facility to facility and from segment to segment depending on conditions such as lane width, slope, shoulder width, pavement conditions, traffic composition, and presence or lack of a median barrier for example. It should be noted that, as cars are equipped with more sophisticated devices and become easier to maneuver, maximum throughput speed should increase. Currently, maximum throughput speed on a typical freeway segment in the Central Puget Sound region is about 51 mph(roughly 85% of the posted speed). For surface arterials, maximum throughput speed is even more difficult to determine, as it is heavily influenced by conflicting traffic movements at intersections. Ideally, maximum throughput speeds for each highway segment would be determined through comprehensive traffic studies and validated based on field surveys. Due to resource constraints and for simplicity, 85% of posted speed is used as a surrogate for the true optimal flow speed for the purpose of performance analysis.