Analyzing Influence of Geometric Design on Operating Speeds Along Low-Speed Urban Streets: Mixed-Model Approach

Studies have documented the significant effect of horizontal curvature on operating speed on two-lane rural highways. The geometric design of these facilities emphasizes the forgiving roadside to accommodate the higher vehicular speeds. High-speed geometric design is predicated on selecting design values for geometric elements that promote speed consistency and safety. The low-speed environment has different objectives in trying to provide access and accommodate multiple roadway users, such as bicyclists and pedestrians. The goal is to maintain lower speeds and thus to achieve the functionality of the roadway and improve overall safety. Too often, the speeds on these facilities exceed the intended target speed of the roadway. A study conducted by the Pennsylvania Transportation Institute supported several research efforts in the low-speed environment. Presented is a more sophisticated analysis of low-speed urban street data using mixed models. A mixed-model statistical approach with repeated measures is used to analyze the influence of geometric elements on operating speed. The power of a mixed-model approach is that it accounts for the random effect in the database (such as the data collection sites themselves) while modeling the fixed geometric effects. Because data were collected at several points along each roadway, the analysis also applies a repeated-measures approach that addresses the geometric elements effect on the same subjects traversing a roadway. The advantages and disadvantages of applying a more sophisticated statistical approach are presented.