The quality of the professional advice one can give depends on the quality of knowledge accumulated by the profession. In transport safety management, good professional knowledge is slow to emerge. This is partly due to the objective obstacles which make experimentation difficult. The other reasons for the sluggish emergence of knowledge are some of the methods used to study the effect of safety measures. The prevailing tendency is to draw conclusions from each dataset separately. It is left to the professional to make a mental amalgam of the diverse findings. Yet single studies are usually too small to settle an issue conclusively. Thus, while there may exist a multitude of studies, there is a scarcity of conclusions. We set out to do the obvious: to try a procedure which formally merges information accumulated from past studies. It is with this purpose in mind that we embarked upon the task of extracting information contained in data collected by others about the safety effect of the conversion from two-way stop control to four-way stop control. This report is one of several in a series of similar studies in which important datasets gathered in the past by other researchers are re-examined. The purpose is to extract from the data what reliable information it contains and to represent this information by a "likelihood function". The likelihood functions of different datasets will then be compared and perhaps combined to serve as an amalgam of the accumulated information - this time obtained by a formal and coherent procedure. (Author/publisher)
Abstract