It all depends on what the limit was before the change. I can certainly show that 55 MPH was safer than the (mish-mash but) higher speed limits we have now. That's ancient history at this point. The percent of speeders was higher at 55 (because drivers knew it was low-balled) but the fatality rate was lower.
Your own study proves my case. Average speeds all of them over the speed limit went up with higher speed limits:A survey of Indiana drivers in the fall of 2005 (a few months after Indiana Interstate speed limits were raised) found that under free-ﬂow conditions, drivers reported driving an average of nearly 11 mph over a 55-mph Interstate speed limit, about 9 mph over a 65-mph Interstate speed limit, and less than 8 mph over a 70-mph speed limit (27).