That depends on the state and the section of highway. There is no Federal prohibition on walking or bicycling along an Interstate highway right-of-way. State laws vary.

In California, for example, bicycling or waling on freeway shoulders is permitted except where specifically prohibited and signed. Bicyclists and pedestrians are prohibted from most California Interstate highways where there are alterantives bicycle and pedestrian routes, but there are mnay California Interstates on which bicyclists and pedesrians are allowed, including sections of I-5, I-10, and I-80.

In California, state law expressly allows walking along a freeway in an emergency:

https://leginfo.legislature.ca.gov/faces/codes_displaySectio....