The Australian experience (as told in the "Metrication Matters" video) was that a "direct metrication" where everybody switches at the same time and doesn't try

The Australian experience (as told in the "Metrication Matters" video) was that a "direct metrication" where everybody switches at the same time and doesn't try to maintain two systems at the same time is fast (about a year) and cheap (often saving money). Road signs can be prepared in advance and covered. The covers can be removed quickly. Another option is to put stickers on the road signs on Metric Sunday.