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10 speed limits [were] generally set between 4 and 8 mph to design speed, and that the posted speed limit was highly less than the measured 85th percentile speed, and only correlated with the intercept variable and the number of lanes 23% to 64% of vehicles operated below the posted variable in their model. In addition, they found that several speed limit in urban areas in field studies)." significant variables in their tangent model became statisti- · "Change text to recognize freeways as a unique func- cally insignificant when posted speed limit was included. tional class. Encourage the recognition that the look Their major findings included the following: of a roadway (e.g., ramps, wide shoulders, and medi- ans) is associated with the anticipated speeds on the · The number of lanes per direction of travel had the facility." most significant influence on drivers' speeds at tangent · "Add comments in the design speed discussion to iden- locations. tify that the following may affect operating speed: radius, · On-street parking and sidewalks were "the second and grade, access density, median presence, on-street parking, third [most] significant variables that [affected] drivers' pedestrian activity, and signal density." speeds on tangent" sections of low-speed urban streets. · "Add information on the state of the practice for select- · Drivers selected lower speeds with an increase in the ing design speed values, [because] anticipated operating density of trees or utility poles, or with a decrease in speed and anticipated posted speed limit [were] being their offsets. used by a notable percentage of the states [surveyed]." · Drivers tended to select lower speeds with an increase · "Introduce the concept of speed prediction and feedback in density of driveways or T-intersections. loops, [with] reference to FHWA-[sponsored] work on the [Interactive Highway Safety Design Model] IHSDM." Donnell et al. (2009) employed another term for practi- tioners to consider, "referred to as `inferred design speed.' Garrick and Wang (2005) examined context-based alter- Inferred design speed is applicable only to features and ele- natives to the use of design speed as a controlling criterion ments that have a criterion based on [a] designated design for design of streets and highways. They concluded that there speed (e.g., vertical curvature, sight distance, superelevation)." were two main areas of concern--how to better define con- The inferred design speed of a feature will be different from text and how to design for appropriate operations (including the designated design speed when the actual value is differ- speed)--that must be addressed in developing a more coher- ent from the criterion-limiting (minimum or maximum) value. ent and context-based approach to design. They discussed the For example, the inferred design speed for a combination of need for an overarching design framework that integrates all radius and superelevation is the maximum speed for which facets required for good design; their framework consisted the limiting speed-based side friction value is not exceeded of a four-step process: define the context, characterize the for the designed rate of superelevation and the inferred design function, select the road typology, and determine the design speed; as such, it is determined through an iterative process. details. They believed that this framework would be essential The inferred design speed for a horizontal curve may also when designing truly context-based thoroughfares that facili- be limited by horizontal offsets to sight obstructions on the tate the operational and safety issues of all users and that also inside of a horizontal curve. The inferred design speed for address the issues of context and livability that affect how a crest vertical curve is the maximum speed for which the well streets or roads function as places. available stopping sight distance (SSD) is not exceeded by the required SSD. The inferred design speed may also be lim- Wang et al. (2006) investigated the relationship between ited by a combination of lane width and average daily traffic the speed choices of drivers and their associated low-speed (ADT). The inferred design speed can be greater than, equal (e.g., speed limits ranging from 30 to 40 mph) urban roadway to, or less than the designated design speed. environments by analyzing second-by-second in-vehicle global positioning system data from more than 200 randomly Design Consistency selected vehicles in Georgia. The authors developed operating- speed models for low-speed urban street segments on the Under NCHRP Project 15-17, Wooldridge et al. (2003) bases of roadway alignment, cross-section characteristics, "reviewed the domestic and international literature on geo- roadside features, and adjacent land uses; their goal was that metric design consistency and developed a comprehensive list the model could "help highway designers and planners better of geometric design features for high-speed, rural, two-lane understand expected operating speeds when they design and roads that can reduce geometric consistency or violate driver evaluate low-speed urban roadways." The authors concluded expectancy. They then identified the most critical roadway that the following variables were significant at the 95th per- features or combinations of features and considered how they centile: number of lanes, the density and offsets of roadside might affect driver performance. A data collection and analy- objects, the density of T-intersections and driveways, raised sis plan was developed to formulate relationships between curb presence, sidewalk presence, on-street parking, and key parameters of the features and driver performance." As land uses. They suggested that the posted speed limit not part of the research, the team recommended a definition for be included in the model because of its strong correlation design consistency: "Design consistency is the conformance