[7MMmag]

I-30 does not cross the entire state (of Texas).

I-20 does not either, stopping (or beginning, depending upon how one looks at it) near Van Horn (or more precisely, Kent, Texas).

I-30 runs only from Texarkana to West of Ft. Worth. From there on (Westbound) the road designation is 1-20...

Distance of Interstate 20 across Texas = 636.1 miles

Distance of Interstate 10 across Texas = ‎878.6 miles.

[okie01]

I-30 does not cross the entire state (of Texas).

I understand your correction. And that is the way it is today. But, in 1967 (and aroun 1970), the milepost protocol must've been different.

There was a milepost 838 at Texarkana.

[lonestar]

I live in SE TX.

878 miles to El Paso.

985 miles to Chicago.