Tbf, in 1969 I-80 (which would be the majority of the interstate route between Chicago and LA) wasnt remotely finished yet. It wasnt completed until 1986, with the last stretch being finished west of SLC. So in 1969 if you wanted to drive between Chicago and LA, at multiple points you would have not been on an interstate. And most of those points would have been in major urban centers that would have been fairly complicated to navigate if you didnt know them. This hypothetical nav system would have been much easier than whipping out a national road map to figure out how to get from one finished part of the interstate to another when passing through SLC
Tbf, in 1969 I-80 (which would be the majority of the interstate route between Chicago and LA) wasnt remotely finished yet. It wasnt completed until 1986, with the last stretch being finished west of SLC. So in 1969 if you wanted to drive between Chicago and LA, at multiple points you would have not been on an interstate. And most of those points would have been in major urban centers that would have been fairly complicated to navigate if you didnt know them. This hypothetical nav system would have been much easier than whipping out a national road map to figure out how to get from one finished part of the interstate to another when passing through SLC