Here's what stripped Americans of their last state without a daytime numerical speed limit 

March 10th, 1996 is a date that should live in infamy, for it marks the day Rudy Stanko started a series of events that would eventually strip Americans of their last state without a daytime numerical speed limit in 1998.  At the time of Stanko's first speeding citation in this series of citations, Montana had re-adopted the "Basic Rule" which cites the vague concept of "reasonable and prudent" driving as one of the yardsticks which Montana law enforcement officers interpreted to discern deviations from safe driving practices.  

The court documents are a mess of information, but it appears that Stanko was first cited for reckless driving after getting clocked at 85 MPH along a narrow/shoulderless stretch of MT-200; 5 months later, Stanko gets cited again for reckless driving after getting clocked at 117 MPH on a narrow road while cresting a hill on U.S. Route 87, then less than 2 months later, Stanko gets another reckless driving citation for speeding at 121 MPH literally 2 miles down the same stretch of U.S. Route 87 round a curve with a high crosswind.

All this hooning plus sundry charges led Stanko to then argue with the Montana Justice Court, resulting in a jury conviction & sentencing.  Stanko then appeals to the Montana District Court, and again gets convicted and sentenced by a jury of his peers.  Stanko finally appeals to the Montana Supreme Court contending that "the statute pursuant to which he was charged was unconstitutionally vague".

During deliberations, Montana Highway Patrolman Officer Scott McDermott testified:

that he is aware that § 61-8-303(1), MCA, requires that a number of factors be taken into account when determining whether a motor vehicle operator's speed is unreasonable, but stated that there were no guidelines for weighing those factors or for their specific application, and testified that the Highway Patrol policy manual basically leaves it up to each individual officer to exercise good judgment in his or her enforcement of the "basic rule."

It turns out that the vagueness once used to thumb Montana noses at Richard Nixon's Emergency Energy Highway Conservation Act and it's nationwide 55 MPH speed limit, resulting in 1970's speeding tickets  frequently written off as "an unnecessary waste of a natural resource" plus a $5 fine payable on the spot without points on one's record to push up insurance premiums; that basic rule vagueness was deemed unconstitutional and according to Justice Trieweller since it was:

 so vague that it violates the Due Process Clause found at Article II, Section 17, of the Montana Constitution.

Deliberations ensue, and 1999 sees Montana's maximum daytime speed limit set at 75 MPH, thus beginning the steady erosion of Montanabahn's glory.  To be fair, driving in a state that can take double-digit hours to go from the Idaho border to the North Dakota side has caused many to go Full Send, fully prepared to pay the butcher's bill Johnny on the spot should one get cited for speeding.   One of my favorite quotes was NYT Special Reporter Timothy Egan's statement:

Trying to drive the speed limit here in the Big Sky State is like trying to eat nothing but bread crusts at a banquet.
There is just so much road and so few cars, that the temptation to put the pedal to the metal is overwhelming.

Montana chapter of the Automobile Association of America lobbyist Mike Cronin eloquently co-signs with this quote:

It's not that we're all a bunch of crazed drivers with a disrespect for the law," said Mr. Cronin. "You can drive 200 miles on some roads here and not pass another car. So why do 65 miles an hour? It's like sitting at a stop light at 3 in the morning with nobody around.

Let's not mince words, empirically speeding kills, but driving in Montana is akin to Keiji Kiriya's answer to the Full Metal Bitch in Hiroshi Sakurazaka's All You Need Is Kill:

Food is like war. You have to experience it for yourself.

We humans in the IRL live in the grey morally contextual world where "it sounded like a good idea at the moment" can outweigh any impediment to getting where we want to go.  If you were born after 1990, be sure to blame Rudy Stanko for not paying his $70 ticket on the spot and instead starting us down the path that led to speed limits in Montana.