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Publication Number: FHWA-HRT-06-125
Date: November 2006
Pedestrian and Bicyclist Intersection Safety Indices
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CHAPTER 3. APPROACH METHODOLOGY
The development of the Ped ISI and Bike ISI in this study followed the basic steps listed below. These steps are described in detail in subsequent chapters.
Each leg of an intersection can have different characteristics affecting pedestrian and bicyclist safety. Rather than rating the intersection as a whole, the Ped ISI and Bike ISI are intended to give an evaluation of the safety of a particular intersection leg—either a crosswalk in the case of pedestrian safety or an approach leg in the case of bicyclist safety. The core of the Ped ISI and Bike ISI development consists of four measures to gauge safety, illustrated in the concept of the pyramid shown in Figure 1:
Figure 1. Hierarchical order of safety measures.
The top of the pyramid is crashes, the most objective indicator of safety. In reality, pedestrian- and bicycle-motor vehicle crashes are so sparse that only one or two per year may cause an intersection to be considered a "problem" or "high–crash" location. Thus, even using multiple years of data per site, it is difficult to base the identification of intersection safety problems solely on pedestrian or bicyclist crashes. Furthermore, bicycle and pedestrian crashes are very random and a location with a high pedestrian or bike crash potential may have zero crashes for several years.
The next two tiers comprise the behavioral-based safety data. The first of these two tiers is conflicts, defined as a sudden interaction between a bicycle or pedestrian and motor vehicle, such that at least one of the parties has to suddenly change speed or direction to avoid the other. Such interactions usually involve hard braking or swerving for the motorist or bicyclist or jumping or abruptly stopping by the pedestrian. The next tier in the progression is avoidance maneuvers, defined as any change in direction or speed caused by an interaction between parties. These interactions often involve slowing, soft stopping, or non-sudden changes of direction by motorists and bicyclists and non-sudden stopping or maneuvering around stopped vehicles by pedestrians. Although these behavioral data are not necessarily direct measures of site safety, they can often be used as surrogate measures of safety. There are several advantages to this approach. First, pedestrian and bicyclist conflicts and avoidance maneuvers occur more frequently than crashes and therefore can provide more data on the potential hazard of a site. Second, crash history for an intersection may not fully contain all of the crashes that occurred at the site, depending on the reporting practices of the local authorities. A behavioral observation can capture all occurrences within the observed time period and can distinguish between various types of pedestrian, bicyclist, and motorist behaviors. Third, this research is focused on the safety of a single intersection leg. This leg-specific approach requires precise and reliable location data that are not always available or easily attained from crash reports. Using crashes and behavioral measures together can serve to confirm the safety of a particular leg.
The base of the pyramid is intersection ratings, a subjective scheme to have experts, practitioners, and experienced users view pedestrian and bicycle facilities at intersections and rate them according to perceived risk or degree of safety. The safety rating that a site receives is very similar to a safety index—the intended result of this research.
It was expected that the Ped ISI and Bike ISI would be based on one or more of the safety measures described here.