Winter on I-90: Basic Speed Rule, a Crash Scene, and a Closing Semi
You are eastbound on I-90 between Butte and Bozeman at 6:40 a.m. in late December. It is still dark, the temperature reads 18°F, and light freezing drizzle is falling. The posted limit is 80 mph. Traffic is light. Ahead, a tow truck with amber lights and a state trooper's SUV with red and blue lights are stopped partly in the right lane, with an arrow board flashing. You are in the left lane. In your mirror a semi is about three seconds back and closing. Your all-season tires have 5/32 inch of tread, above the legal minimum but worn for ice. Your passenger tells you the limit is 80 so you should keep 80, and that the officers will see you coming.
1. What speed is appropriate, and how does Montana's basic speed rule apply here?
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The posted 80 is a ceiling, not a target. Montana's basic speed rule requires a speed that is reasonable and prudent for existing conditions, and 18°F with freezing drizzle in the dark is precisely the condition that rule is written for. Ice forms first on bridges, overpasses, shaded curves, and where the pavement merely looks wet. Stopping distance on ice can be several times the dry figure, so choose a speed at which you can stop within the distance your headlights actually show - often 40 to 50 mph or less in this weather. The crash scene ahead also has people on foot near live lanes. Your passenger is confusing the maximum legal speed with the safe speed; in winter the two are rarely the same number.
2. You are already in the left lane with a semi closing behind. What does the move-over law require, and how do you execute it?
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Montana's move-over law requires you to move out of the lane adjacent to a stopped emergency, tow, or utility vehicle displaying its lights, or, if you cannot move over safely, to slow below the posted limit and pass with caution. You are already in the left lane, the non-adjacent lane, so the move-over requirement is met; the remaining duty is to slow down. The semi changes how you execute that, not whether you do it. Do not stab the brakes with a truck three seconds back and closing - decelerate early and smoothly so its driver reads your brake lights, and let the speed difference build gradually. Had you been in the right lane, cutting left abruptly in front of the semi would be more dangerous than a controlled slow pass.
3. Midway through the pass, the car begins to slide. What do you do, and what should you have done differently beforehand?
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Get off cruise control immediately; it will try to hold speed the instant a driven wheel spins. If the car starts to slide, ease off the throttle, keep steering inputs small, look where you want the car to go, and avoid hard braking - a locked wheel on ice gives you no steering at all. Five thirty-seconds of tread is legal but marginal for ice, which is an argument for reducing speed rather than trusting the tires. If the slide becomes a spin, keep both hands on the wheel, keep looking and steering toward your intended path, and once the car is straight resume at a speed well below the one that produced the slide.