The width spec might relate to the width of the lanes on the nation's highways, which the government would be highly involved in. As stated in the Mopar Police Cars book, there's one Los Angeles freeway that is a little less wide than the others in that area, so they liked to use the Plymouth Belvederes (or similar) to patrol that freeway as their larger cars (basically as wide as an Imperial of the '70s) just seemed a little too "tight" for them on that stretch of roadway.
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