 East of I @-@ 35 , I @-@ 80 meets up with US 65 on the outskirts of Altoona . The two routes only share the highway for one mile ( 2 km ) as US 65 splits away at the next exit . Here , US 6 rejoins I @-@ 80 for the second time and the interstate returns to its four @-@ lane configuration . After a third exit for Altoona , the interstate resumes its 70 mph ( 110 km / h ) rural limit . Near Colfax , the highways cross the South Skunk River . After an interchange with Iowa 117 , the highway is forced to the north to avoid crossing the river multiple times . As it returns south to its original line , it meets CR F48 , which was another former alignment of US 6 . At the Iowa 14 exit in Newton , US 6 exits off the interstate . East of Newton is an interchange that serves the Iowa Speedway . 
