st George aint 20 miles
The corridor is approximately twenty miles wide either side of I-15 from South border to North border. So if I-15 is the center point a twenty mile wide corridor would be ten miles on either side of I-15. All approximate as around Ogden you might run into lake water to the west within ten miles and the same in Utah Valley. The idea is the majority of the Utah population is within the corridor. You could probably add a short bit of I-80 from Coalville to Grantsville.