Standard Highway markers for Ohio
|Interstates||Interstate nn (I-nn)|
|US Highways||U.S. Route nn (US nn)|
|State||State Route nn (SR nn)|
The Ohio Department of Transportation (ODOT) is responsible for the establishment and classification of a state highway network which includes interstate highways, U.S. highways, and state routes.  As with other states, U.S. and Interstate highways are classified as state routes in Ohio. There are no state routes which duplicate an existing U.S. or Interstate highway in Ohio.
Ohio distinguishes between "state routes", which are all the routes on ODOT's system, and "state highways", which are the roads on the state route system which ODOT maintains, i.e. those outside municipalities,  with a special provision for Interstate Highways.  Besides the state highway network, there are various county and township road networks within the state.
This section needs expansion. You can help by adding to it. (June 2016)
The Ohio Inter-County Highways were created on June 9, 1911, with the passage of the McGuire Bill (Senate Bill 165, 79th Ohio General Assembly).  Main Market Roads, the most important of the system, were defined on April 15, 1913. 
In 1935 the Ohio General Assembly passed a law which added 5,000 miles of roads to the state highway system over a 12-month period.   These roads were assigned route numbers in the 500s, 600s, and 700s. 
|Interstate Highways: A list of interstate highways within Ohio.|
|U.S. Highways: A list of U.S. highways within Ohio.|
|State Routes: A list of all state routes within Ohio.|
|Ohio Turnpike: A toll road carrying Interstate 90, Interstate 80, and Interstate 76.|
- Ohio Revised Code 5501.11
- ORC 4511.01(II),(JJ); ORC 5511.01
- ORC 5521.01
- Ohio State Highway Department (1912). Map of Ohio Showing Inter-County Highways (PDF) (Map). c. 1:563,200. Columbus: Ohio State Highway Department. OCLC 13716556.
- "An act providing a levy and to create a fund for the purposes provided in the act passed May 31st, 1911, entitled, "an act creating a state highway department, defining the duties thereof and providing aid in the construction and maintenance of highways and to repeal certain sections of the General Code" approved June 9th, 1911. (102 Ohio laws, pages 333-349), and for other purposes defined herein". Act of April 15, 1913.
- "An act to amend section 1189 of the General Code, to authorize the director of highways to increase the mileage on the state highway system and the county commissioners to increase the mileage on the county highway system".
Act of April 15, 1935.
...during the period from July 1, 1935 to and including June 30, 1936, the director of highways shall be and he is hereby authorized and directed to take over and add to the state highway system in the manner provided by law five thousand miles of county and township roads and highways to be selected by him, provided that such roads and highways so taken over shall at such time have a duly established right of way of a width not less than fifty feet, and provided that not less than forty miles and not more than seventy-five miles shall be taken over from each of the eighty-eight counties.
- "State Ready to Take Over Secondary Roads". The Lima News. Lima, OH. March 20, 1936. p. 10. Retrieved December 17, 2018 – via NewspaperArchive.
- Ohio Department of Highways (1936). Official Highway Map of Ohio (PDF) (Map). 1:760,320. Columbus: Ohio Department of Highways. OCLC 5673562. Last map published before the additions.
- Ohio Department of Highways (1937). Official Highway Map of Ohio (PDF) (Map). 1:760,320. Columbus: Ohio Department of Highways. OCLC 5673562, 16960304. First map showing the additions.