Before the commencement of any work authorized under the provisions of section five hundred forty-two of this act,¹ the city shall, upon notice from the secretary so to do, establish and thereafter maintain, at its own expense, an adequate detour for the accommodation of the traveling public, for such length of time as may be necessary or until the improvement contemplated has been completed and accepted and the street or sections thereof opened and made available to public travel. Such detour, when established as herein provided, shall be marked and posted by the department at the expense of the Commonwealth.