The short explanation is that Ohio uses a "clustering" system, in which groups of highways within an area are assigned sequential numbers. However, the numbering system in use today has evolved since 1924, when it was first put into use, and there have been many route numbering changes since then. Even so, it is very easy to see the clustering in many areas. For example, look in southwest Ohio, and you will see routes 131, 132, 133, 134, 135, 136, and 137. Similarly, in northwest Ohio, you will see routes 110, 111, 114, 115, 116, 117, 118, and 119.
The first highway numbering system in Ohio went into effect in 1912, when a series of "inter-county highways" were assigned numbers. This numbering system appears to have been for inventory purposes only, and there were apparently no signs marking these routes. Except in a few rare cases, the inter-county highway numbering system bears no resemblance to the current numbering system.
In 1924, the state route numbering system went into effect. The current route numbering system derives directly from the system put into place in 1924. According to the 1924 state route numbering system, the most important routes were given low numbers. For example, the National Old Trails Road was assigned route 1; the Yellowstone Trail route 2; the Three-C Highway route 3; the Scioto Trail route 4; the Lincoln Highway route 5; the Dixie Highway route 6; etc. Routes 7-11 were also assigned to important cross- state highways. Beginning with route 12, the clustering system took over. For example, routes 12-16 all radiated from Cleveland; routes 19-21 terminated or went through Columbus; and routes 25-28 could be found in or near Cincinnati. Route numbers 12-79 were given to routes of high importance, but those not as important as routes 1-11. Routes 80-229 were considered to be relatively minor. These route numbers were assigned using the clustering system.
During the time of the implementation of the new state highway numbering system, there was discussion of creating a new network of national highways. After much discussion, it was agreed that the new system would use numbers, and in November of 1926, the U.S. highway numbering system went into effect. This created something of a problem with the Ohio highway numbering system. Many of the newly created U.S. highways passed through Ohio, carrying numbers which conflicted with the Ohio numbering system. Ohio faced one of two choices -- 1) keep the existing numbering system, dually signing U.S. highways, and ignoring duplicate numbers -- those used by both a state highway and a U.S. highway within the state; or 2) restructure the state highway numbering system, removing duplicate numbers. Ohio decided to go with option #2. Ohio then considered U.S. routes to also carry an implicit state route with the same number. Click here for a list of all the routes which were renumbered or modified as a result of the creation of the U.S. highway system.
After the creation of the U.S. highway system, Ohio continued to add new state routes at a fast pace. The new routes were simply assigned sequentially, beginning with route 230, and going up into the 300s. Over the years, many routes were eliminated, rerouted, or combined, so that some numbers disappeared or were reused. There is no policy about reassigning unused numbers. The state simply maintains a list of unused numbers, and when there is a need to use a new number, the state picks one from its list as a proposed route number, subject to public comment.
In 1938, Ohio created a large batch of new state routes. For some reason, the entire 400 series was skipped. I have no proof of this, but I suspect that these numbers were being reserved for a new state freeway or toll road system, much like Ontario's 400 series highways. The new routes were assigned numbers in the 500, 600, and 700 series, with the 500 series in general being the most important and the 700 series in general being the least important. As with the earlier routes, over the years, many of these routes were rerouted, combined with each other, or eliminated; and numbers in these series were later reassigned.
In 1956, another system of cross-country highways was created -- the Interstate highway system. The arrival of the Interstate highway system presented the same problem as that of the U.S. highway system -- a new set of numbers resulting in numbering conflicts with existing highways. The first Interstates in Ohio did not appear until 1959, when sections of I-70, I-71, I-75, and I-90 opened to traffic. Ohio ignored the duplicate numbering problem for years, possibly to come up with a plan to eliminate duplicates. In fact, the state seemed to go out of its way to assign existing or new route numbers to new sections of Interstates. For example, US 40 was rerouted onto the new sections of I-70; and the new section of I-71 carried the new "state route 1" designation. The state finally decided on a renumbering plan in 1962, and the new route numbers went into effect rapidly. Click here for a list of all routes which were renumbered or modified as a result of the creation of the Interstate highway system.
Most highway numbering changes since 1962 have been relatively minor. The 400 series began to be used in the 1960s. The 400 series numbers were given mostly to routes which previously carried other numbers. Beginning in 1970, 800 series numbers began to be assigned, starting with route 800, a newly renumbered segment of the old route 8. For the most part, routes in the 400 and 800 series are not assigned sequentially. Instead, they are assigned according to the numbers of the routes they intersect or are near. For example, route 835 intersects US 35 on both ends.