Basis for Mile Markers on Texas 130

[DevalDragon]

Anybody know what they (TxDOT) is using for a basis for the mile markers on Texas 130? They count up north to south; I-35 in Georgetown is 417 and 130/45 and US 183 is 458, and the southern extension to I 10 has no exit numbers at all.

[NE2]

http://onlinemanuals.txdot.gov/txdotmanuals/trm/reference_markers_coordinates.htm
Apparently Georgetown is 407 miles south of the north edge of the Panhandle (36° 30′ N).