Spot ABB operation panel PP826

Spot ABB operation panel PP826

Technical data of product parameters

PP826A designs fall into two broad categories: experimental and observational. Experimental studies are
planned where sites are selected at random for treatment and control. In fact, it could be argued that
such experimental studies are the most rigorous way to establish causality (Elvik, 2011a). Observational
studies are not planned, and typically sites are not selected as part of an experiment, but selected for
other reasons including safety. Observational studies are more common because they consider safetyPP826A
improvements to improve the roadway system. On the other hand, experimental studies evaluate
safety improvements implemented solely for the purpose of measuring their effectiveness, and are not
very common partly because of potential liability considerations.
Observational studies can be broadly classified into before-after studies and cross-sectional studies.
Before-after studies include “all techniques by which one may study the safety effect of some changePP826A
that has been implemented on a group of entities (road sections, intersections, drivers, vehicles,
neighborhoods, etc.)” (Hauer, 1997, p. 2). On the other hand, cross-sectional studies include those
where “one is comparing the safety of one group of entities having some common feature (say, STOP
controlled intersections) to the safety of a different group of entities not having that feature (say, YIELD
controlled intersections), in order to assess the safety effect of that feature (STOP versus YIELD signs)”
(Hauer, 1997, p. 2, 3). Since in a typical before-after study, we are dealing with same roadway unit
located in a particular place used by probably the same users in the before and after period, thesePP826A
factors are less likely to confound a before-after study (Elvik , 2011a). Hauer (2005) also argues that
before-after studies are less prone to confounding compared to cross-sectional studies. However, there
are issues in both types of studies that need to be addressed and they are discussed below.
Following is a brief overview of different types of observational before-after and observational crosssectional study designs and the potential issues associated with such study designs. It is important to
reiterate that the scope of this discussion is limited to studies that make use of crash data to derive
CMFs rather than studies that use surrogate measures

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