First off, only a small percentage of the drinking water distribution lines are 'plastic.' These are the lines that have been placed into service in the last 25 years or so. (I assume you meant PVC.) PVC wasn't even invented when most of these lines were placed into service. (Remember they unearthed a wooden pipe recently in downtown construction?)
A lot of the lines are iron...which expands and contracts with heat and cold temperature fluctuations. This is not a CFPUA thing...this is a nationwide thing. A little research into watermain breaks causes would reveal such.
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