TURNPIKE TRAFFIC OPERATIONS ENGINEER

Traffic engineering practices support the use of 85th percentile speed as a reasonable measure for posting speed limits on roads. Design elements are based on a predetermined speed called the design speed. On some roadways, motorist operational speeds tend to be high enough to result in 85th percentile speeds that may be much greater than roadway design speeds. Because posted speed limits are generally close to design speeds, marked discrepancies may exist between posted speed limits and motorist operational speeds as measured by 85th percentile speed values. In essence, if roadway and vehicle elements create a comfort level for motorists to drive at high speeds, their operating speeds are likely to be much higher than posted speed limits. Thus, use of 85th percentile speeds for posting speed limits may lead to a discrepancy between the roadway design speeds and posted speed limits. This paper illustrates the design and traffic view points, evaluates them with 2 case studies, and suggests using criteria such as crash reviews for resolving the 85th percentile speed vs. design speed dilemma in determining proposed posted speed limits. Current design standards and traffic engineering practices are reviewed to determine the appropriate measure for posting speed limits. Motorist perceptions about speed limits and challenges they pose due to use of technology such as radar units are explored in addressing the human element of speed limits.