Glenrock, WY is exactly midway between Seattle and Dallas. You could also try Casper, Wyoming which is about 25 miles from the exact midpoint.
Sioux Falls, South Dakota
The state line of Louisiana and Mississippi is about the halfway point.
halfway points
Rochelle, Virginia is halfway between State College & Raleigh
North Dakota and South Dakota are about halfway.
Pennsylvania
NM
Mississippi
Tennessee
Kansas
Tennessee
Denver, Colorado is about halfway.