Route from Illinois to Disney World

Yes there is a flatter route from Illinois. Basically you need to get over to I-65 and depending upon where in Illinois you live that will taken you eastward or at least southeastward. Follow I-65 all the way south to Montgomery, Alabama. From there catch US231 down to I-10. You can take I-10 over to I-75 or you can follow US19 down along Florida's west side and catch another route over to I-75. You'll need to hook up to I-4 in Florida if you want to stay on the Interstate.

There are some hills in Tennessee and some rolling land in Alabama, but they are certainly not mountains.

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