Policy changes in safety enforcement for underground coal mines show mine-size-dependent effects

The objective of this study is to understand the changes in the safety measures of U.S. underground coal mines across different mine-size categories during the period 2005-2014. Empirical data were collected from the MSHA Address/Employment and Accident/Injury files, for the period 2005-2014. The differences in the means of two normalized safety measures, Non-fatal Days Lost Incident Rate (NFDLIR) and Severity Measure/100(SM/100) as well as the changes in their trends, were examined across five mine-size categories, Very Large, Large, Medium, Small, and Very Small, over the periods 2005-2009 and 2010-2014, before and after the institution of Mine Safety and Health Administration's (MSHA's) Impact Inspection program. Both NFDLIRs and SM/100s of the Very Large, Large, Medium, and Small mines were found to be significantly lower in the second period compared to the first period which can be attributed to MSHA's Impact Inspection program in 2010. No significant differences were found in the NDLFIRs and SMs of Very Small mines from the first period to the second period. A drastic decrease of more than 50% in the SM/100 mean of Small mines in the second period was observed which can be attributed to the closure of several small mines during that period.