In the present paper a statistical analysis of accidents data on hazardous materials pipeline failures has been carried out to identify the most common causes and consequences of release and to derive the distribution of their probability of occurrence as a function of a number of parameters. The analysis has included three classes of materials among those most commonly transported via pipeline (crude oil, natural gas and oil refined products): differences and similarities have been highlighted and, where possible, explained. Particular attention has been devoted to assessing the influence of the installation layout of the pipeline (underground or above-ground) on the probability distributions. As a matter of fact, both the accident scenarios and their evolution were found to be significantly different for the two layouts. It is expected that the presented results can be of help in the assessment of the risk associated with the transportation of hazardous materials via pipeline, as well as in addressing efficient prevention strategies, which could reduce that risk.