You can't go all the way from Springfield IL to Las Vegas NV on I-70, because I-70 stops before it gets there (the western end of I-70 is in Utah, where it connects with I-15).
However, ignoring that, the midway point would be a little east of Denver. Limon, CO is probably the nearest town to the halfway point.
The halfway point between Monticello, IA and Springfield, MO falls in or around Perry, Missouri.
The halfway point between Las Vegas, Nevada and Marcellus, Michigan is Cornell, Nebraska.
The halfway point between Coldwater, Michigan, and Springfield, Virginia, is on I-76 (Pennsylvania Turnpike) where it meets I-376, just northwest of Pittsburgh and southeast of Youngstown.
Independence, California is about the halfway point.
my house
Indiana
What is the halfway point between Omaha NE and Springfield IL
Las Vegas
Springfield, North Carolina is the halfway point.
Wyandotte MO
Durant, OK
Gastonia, NC