Your Problem there is the population was roughly half the size back in 1956 the Original Plan completed in 1984 or 1985 under the "Reagan" Administration.
Also since then the "Enginering" and study got better remember "Clover-Leaf" interchanges and somebody took that sharp "25 MPH" curve way too fast they are rebuilding them now..
As the the Population has roughly "Doubled" since the "Original" Plan ya gotta alot more Commuters in cars, Big Trucks hauling the groceries & goods we all "Depend" upon..
Add to that the "E-Commerce" all these Amazon Delivery vans & Uber Eats out there too..
The old 4 lanes need 6 lanes each side even in many used 2 be "Rural Areas" and Farm Fields 20/30 years ago because thats all "Developed" today amd lots of traffic