Of course, it isn't exactly straight.
For fun, I decided to try Jacksonville to Houston on I-10, which is pretty straight, and takes about 12 hours, so it should be a good average day length. Using the sunrise/sunset time on September 21, the sun would rise at 7:14 AM EDT and set at 7:19 PM CDT (vs 7:24 PM EDT for Jacksonville), nearly an hour extra daylight.