System safety refers to a diverse engineering discipline assessing and improving various aspects of safety in socio-technical systems and their software-intensive sub-systems. While system safety has been a vital area of applied research for many decades, its practice and practitioners seem empirically still not well studied. Beyond mainly anecdotal evidence (interviews, on-line discussions), incident reports, and surveys, we are missing open, large-scale, and long-term investigations that promote knowledge transfer and research validation. We explore means for work safety practitioners rely on, factors influencing their performance, and their perception of their role in the system life cycle. Along with that we examine observations from previous research. We build a construct of safety practice, collect data for this construct using an on-line survey, summarise and interpret the collected data, and investigate several hypotheses based on the previous observations. We analyse and present the responses of 124 practitioners in safety-critical system and software projects. Aside from other findings, our data suggests that safety decision making mainly depends on expert opinion and project memory, lacks evidence that safety is typically a cost-benefit question, does not exhibit the prejudice that formal methods are not beneficial, leaves it unclear as to whether or not standards and methods have become inadequate, and indicates that safety is not typically confused with reliability. Additionally, we contribute a research design directing towards explanatory empirical studies of safety practice. Empirical research of safety practice is still in an early stage, bearing the risk of undesirable mismatches of the state of the art and the state of practice. However, this situation offers great opportunities for research.