Several studies that used vehicle-based systems to measure street-level leaks in cities suggested that replacing leak-prone, cast-iron segments of the natural gas distribution pipeline network is effective in reducing emissions. This is a focus of mitigation efforts in U.S. east coast cities given their age and high prevalence of cast iron pipelines. von found that cities in the U.S. with advanced pipeline replacement programs (APRPs) had leak rates that were 95% lower (0.08 L/min/km) compared to cities without these programs (2.0 L/min/km). In Toronto, noted very few leaks per km of road surveyed, attributing this to a program (completed in 2012) that replaced 1800 km of castiron and steel pipeline. While these examples suggest pipeline replacement programs reduce emissions, other studies in the review indicate different results. For example, measured 61 distribution pipeline leaks in Boston before and within a year after repair, finding that 15 repairs eliminated the emissions, 22 partially eliminated the emissions, and 24 were ineffective. attributed the low repair success rate to temporary fixes, "super-failures" over time at the same location, or utilities only resolving one of multiple potential co-located leaks. The varying effectiveness of pipeline replacements reported across the studies indicates that follow up measurements to verify CH4 emissions reductions may be needed.