What the 85th Percentile Doesn't Tell Us: Engineering Safe Roads for Everyone Includes the Other 15 Percent

摘要

This paper provides insight about the characteristics of higher speed travellers on four-lanedivided highways on portions of Manitoba’s National Highway System (NHS) - insight which isoften lost in aggregate analyses. The 85th percentile speed (the speed at or below which 85percent of vehicles travel, hereafter V_(85)) is often used in civil engineering actions related to roadsafety: horizontal and vertical geometry, sight distances, roadside characteristics, and postedspeed limits have been set with reference to V_(85). This paper takes a closer look at thecharacteristics of groups of higher speed vehicle records (the groups of records at or above the85th, 95th, and 99.9th percentile speeds) and explores how these characteristics differ from thoseof lower speed records. The paper looks specifically at the effects of five different factorsinfluencing operating speed: (1) time of day, (2) lighting conditions, (3) season, (4) rain condition,and (5) vehicle type. This is done through: a literature review and an analysis of speed recordsfrom three weigh-in-motion (WIM) devices located on Manitoba’s NHS roads.