What interstate highway is most traveled?

Answer 1

Answering this might be splitting hairs:

The most traveled interstates in the US will be:

I-40 from Wilmington, NC to Barstow, CA.
I-70 from Baltimore, MD to Sulfurdale, UT.
I-80 from Ridgefield Park, NJ to San Fransisco, CA.
I-90 from Boston, MA to Seattle, WA.

By the location and the cities they pass through, I-40 and I-80 are likely to have the greatest volume of year round traffic.