More rest areas are needed, IMO. If there's more than 45 miles between rest areas along a major route, there's a need for a rest area in the middle.
It all depends...
Take 1975-ish for example on I-71 between Cleveland and Columbus. There were 4-5 sets of rest areas, but the number of exits with 1 or more truck stops was noticeably less than current day (usually a union 76, Sohio, or TA branded location back then).
When truck stops blossomed in the 90s and beyond, 76 disbanded, Sohio became BP, and you had new and larger Petros, Pilots, Flying J's and Loves occupying more exits.
They provided better overnight parking and more amenities for truckers and passenger vehicles alike -- not to mention a Mickey D's or similar popping up at nearly every or every other exit along I-71.
It made sense for Ohio to reduce the number of Rest Areas, as more drivers preferred stops with amenities the rest areas couldn't offer, and places that didn't look so desolate with no caretakers on duty.