State Highway 21 | ||||
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| Route information | ||||
| Maintained by TxDOT | ||||
| Length | 312.50 mi[1] (502.92 km) | |||
| Existed | April 4, 1917–present | |||
| Major junctions | ||||
| West end |  I-35 at San Marcos | |||
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| East end |  LA 6 near Milam | |||
| Location | ||||
| Country | United States | |||
| State | Texas | |||
| Counties | Hays, Caldwell, Bastrop, Lee, Burleson, Brazos, Madison, Houston, Cherokee, Nacogdoches, San Augustine, Sabine | |||
| Highway system | ||||
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State Highway 21 (SH 21) runs from the Texas-Louisiana boundary east of San Augustine to San Marcos in east and central Texas. SH 21 mostly follows the alignment of the Old San Antonio Road and the El Camino Real, except for the portion between Midway and Bryan, where the Old San Antonio Road took a more northerly route, and SH 21 follows a more direct route. That section of the Old San Antonio Road is served by Texas State Highway OSR.
- ^ Transportation Planning and Programming Division (n.d.). "State Highway No. 21". Highway Designation Files. Texas Department of Transportation. Retrieved August 30, 2013.