For a while, the speed limit
After the repeal of the federally mandated 55 mph speed limit, Montana changed their speed limit in December of 1995 to what they called the "basic rule"... cars and light trucks were to drive in a careful and prudent manner based on road conditions. What it did was effectively remove the speed limit for automobiles driving on the interstate during the daytime.
Speed Limit signs read (it was really cool to see this):
Day -- Reasonable & Prudent
Truck -------------------- 60
Night --- All Vehicles --- 55
On May 28, 1999, Montana brought back the speed limit to 75 on the interstate (65 in urban areas), and 70 on other highways. The reason for the change was due to the increase in percentage of traffic accidents that were fatal (note that the number of accidents did not go up, just the % that were fatal), and because most people could not be trusted to drive in a reasonable
OR prudent manner.
Governor Marc Racicot remarked on the passage of the new speed limit bill, "Unfortunately, for a variety of reasons, no numerical daytime speed limit on major highways was interpreted by some as no speed limits at all. Perhaps they were reading too fast too. Whatever the reasons, such misinterpretations posed potential threats to safety."