Pressurized pipelines of extruded ductile polyethylenes can, under severe conditions, fail by brittle rapid crack propagation (RCP). Critical pressures, below which RCP always arrests, have been measured for grades of medium-density (MDPE) and modified high-density polyethylene (HDPE) pipe, using a small-scale test. For 180-to 250-mm-diameter pipe of 11-to 24-mm wall thickness, HDPE performs much better than MDPE; but both materials show temperature, crack velocity, and thickness-promoted ductile-brittle transitions, beyond which their behaviors converge. Instrumented Charpy impact tests reflect the transitions, but only qualitatively. High-speed double-torsion tests for dynamic crack resistance across a spectrum of crack velocities yield only plane-strain data, even at 0°C and for specimens only 6 mm thick. They therefore do not help to locate the thickness-dependent fracture transition, but more efficiently characterize the worst case of a highly constrained crack in a thick pipe.