Independence, California is about the halfway point.
Lincoln,NE
Lincoln in Nebraska
Detroit
Bettendorf, IA but anywhere in the Quad Cities would be fine.
South Holland, IL
Erie, PA
Williamsburg, MO
One halfway point between Lawton, OK and Chicago, IL is Lincoln, MO.
Assuming you're taking I-44, the halfway point is near Plano, Missouri but locations in the west part of Springfield are close enough.
The halfway point between 5700 and 5800 is (5700 + 5800) ÷ 2 = 5750
Huntingdon, in Cambridgeshire