DESIGN SPEED, OPERATING SPEED AND POSTED SPEED LIMIT PRACTICES

NCHRP Project 15-18 evaluated how speed is used within existing practices and developed recommended changes. The strongest statistical relationship between operating speed and roadway characteristics on suburban tangent sections was with posted speed limit. Other variables that showed potential influence on 85th percentile free-flow operating speed included access density, median type, parking along the street, and pedestrian activity level. The findings from the field studies demonstrated than in urban areas, operating speed is insensitive to many geometric design decisions. Previous research showed that operating speed is sensitive to radius and grade on rural two-lane highways. When selecting the design speed value for a new roadway, the majority of the states use functional classification, with legal speed limit being used by almost one-half of the states responding to a mailout survey. A concern with the use of legal speed limit is that only between 23 and 64 percent of drivers operate at or below the posted speed limit, depending upon the roadway class. Data from 128 speed study zone surveys found that the speed limit at 10 percent of the sites was rounded up to the nearest 5-mph (8.1 km/h) increment. Suggested changes to the Green Book are included in this paper.