Mich. Comp. Laws § 254.20

Sec. 20. Every bridge which has a clear 2-way roadway width of less than 19 feet, but more than 17 feet at the narrowest part thereof, shall be posted as a narrow bridge; and every bridge which has a clear 2-way roadway width, as so measured, of 17 feet or less, shall be posted as a 1-lane bridge. Such posting shall be in accordance with the manual of uniform traffic control devices adopted pursuant to section 608 of Act No. 300 of the Public Acts of 1949, being section 257.608 of the Compiled Laws of 1948.