He wanted more land for the new coming settlers
With the rise of a Nuclear Soviet State, the United States was auditing it's own internal defense capabilities.
One critical point was the size of the United States and the time it could take to move forces from one point to another in times of war and emergency.
Even within the United States, there was a rise in communist parties which were slowly becoming a threat.
The Interstate highway act not only produced economic benefits to stop the rise of communism in the United States, but also provided nation-wide infrastructure to smoothly and quickly move forces across the country.
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I would go with the first one
