When the Good Roads Movement started in the United States, the rush was on to create a system of highways connecting all points of the country. This led to a collection of rural roads being marked with multi-colored signage painted on utility poles, sometimes with large numbers of marking on some routes. When the Federal Government started getting into the road funding business, it was through the states be giving money to each state that had a government agency to control that money. In Indiana, this was accomplished, originally, in 1917. Constitutionality of the new State Highway Commission caused the agency to be recreated in 1919. The ISHC decided that it would be easiest to start the new state highway system with the already (somewhat) improved system of Auto Trails.
In 1917, five “Market” roads were created as the start of the state highway system. The first of these roads was a collection of different Auto Trails stretching from north of South Bend to New Albany. At the Michigan state line, original state road (OSR) 1 started along what was the Dixie Highway. At South Bend, the Dixie Highway was joined by the Michigan Road. This arrangement was used to Rochester. Here, OSR 1 would turn southeast along the Range Line Road, while the Michigan Road and Dixie Highway would veer to the southwest, using the historic route of the former. OSR 1 would continue through Peru and Kokomo on its way to Indianapolis. At what is now SR 18, the Range Line Road was joined by the Belt Line, an Auto Trail connecting Lafayette to Fort Recovery, Ohio, via Kokomo. This multiplex would continue to what is now SR 26 south of Kokomo.
At Indianapolis, where the Range Line Road officially ended, the original route of OSR 1 would leave the city southbound on the Jackson Highway. This would be followed to Seymour. A small section south of Seymour failed to follow any Auto Trail, but this would only last for a few miles, where OSR 1 began following the Pigeon Roost Route, which only ran from New Albany to Seymour. OSR 1 left Indiana as part of the Dixie Highway and the Jackson Highway.
The next two Market roads added to the state highway system, OSR 2 and OSR 3, followed Auto Trails for their complete routes through the state. OSR 2 followed the original route of the Lincoln Highway through northern Indiana. This road connected Valparaiso, Laporte, South Bend, Elkhart, Goshen and Fort Wayne. OSR 3 used teh National Old Trails Road, in Indiana known as the National Road, from Terre Haute through Indianapolis to Richmond.
One of the few new state highways that would not originally be part of the Auto Trails system, at least at the beginning would be OSR 4. The new state road would start in Evansville and follow a country road to Boonville. From there, it would continue to Gentryville to Huntingburg. At Huntingburg, the old French Lick Route would become part of OSR 4 through Jasper, French Lick, West Baden to Paoli.
At Paoli, OSR 4 left to the north following the Dixie Highway, the French Lick Route and the Midland Route. The Midland Route entered Indiana at Vincennes and left via New Albany via Mitchell and Paoli. At Mitchell, the Midland Route left OSR 4 to the west. At Bedford, OSR 4 would turn east, still following the French Lick Route. The French Lick would be part of this state road across Indiana to Lawrenceburg. At Vallonia, the Jackson Highway would join the road to Seymour. At the eastern end of the road, OSR 4 changed from the French Lick Route to the Terre Haute-Columbus-Cincinnati Trail to head off toward the state line.
The final original state highway, OSR 5, basically followed the Midland Route from OSR 4 at Mitchell west to Vincennes. While this is along the general line of what is now US 50, the original route bounced north and south quite a bit connecting Vincennes and Mitchell.