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14 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Speed Data Speed data can provide insights about effective roadway conditions. For example, the meas- ured 85th-percentile speed, mean speed, and speed variance can be compared to the implied design speed based on the "Green Book" (AASHTO, 2004) intersection-sight-distance and stopping-sight-distance criteria. Comparing measured speeds to the posted speed could help identify differences between desired operations (as indicated by a posted speed) and actual conditions. Crash Data A high crash rate at an intersection may indicate any number of operational or geometric issues and does not, in isolation, indicate excessive speeds. A closer analysis of the crash types, locations, time of day, weather conditions, and other factors is useful for understanding the role of speed, if any, in the crashes. Crash patterns that may be associated with excessive speeds include the following: Frequent rear-end crashes--drivers not anticipating the location of the back of a queue, Frequent run-off-road crashes--drivers avoiding conflicts in the roadway proper, or Angle crashes--drivers accepting gaps that are too small. A close analysis of the crash data is necessary to determine if these patterns could also be attrib- uted to other factors such as driver inattention or impairment, geometric conditions (alignment, sight distance), or other conditions that do not meet a driver's expectations. 2.7 Summary Reduced speeds do not guarantee safety, nor do they guarantee operational or environ- mental benefits at a high-speed intersection. Speed is a product of many features, including the adjacent roadway segment, the user and vehicle type, and the general environmental con- text. Intersection approach speeds can affect a facility's safety and performance. Speed, when considered as a design criterion or consideration, can affect the roadway design while also influencing the environmental context. Understanding how speed affects intersection condi- tions and how those conditions affect speed is considered first to evaluate and select an appro- priate speed reduction treatment.