You are driving on a rural road at the posted limit when a fog bank cuts your view ahead to a few car lengths. What principle governs how fast you should now be going?

Why C is correct

The foundational speed principle is that you must never drive faster than permits stopping within the distance you know to be clear. Signs calibrate that principle for ordinary visibility; fog removes the ordinary conditions, and with them the number's authority over your behaviour. When you can see two car lengths, the safe speed is the one that stops inside two car lengths — which may be a walking pace. The alternative, maintaining speed because the sign says so, is how multi-car fog pileups begin: a wall of vehicles each travelling faster than their own vision could answer for.

Why the other options are wrong

A. The posted limit, because it was set with all conditions in mind

Posted limits encode ordinary conditions, and fog is their negation. The basic rule — stoppable within your sight distance — always outranks the sign when the two disagree, which is precisely why it is called the basic rule.

B. Twice the distance you can see, provided your brakes are good

Doubling your visible stopping distance is a wager that the obstacle will be kind enough to be visible early. The principle exists for the moment when it is not: the stalled car around the curve, visible at the distance you cannot stop from.

D. The speed of the vehicle ahead, because its driver can see better than you

The vehicle ahead sees the same fog you do and may be piloted by someone making the same mistake faster. Matching another car's speed delegates your judgement to a stranger's error, and in fog that error compounds car by car down the queue.

On the road

Ground fog pools in the low stretch of your morning commute, and the world shortens to forty metres. You drop to a speed from which forty metres is comfortably stoppable, and it feels absurdly slow for all of twenty seconds — until a dairy tanker emerges parked across the right lane with its hazards on, visible at exactly the distance you could, and did, stop in.

Similar questions

• School Zone Rules: Slow Down and Obey Guards

  You are driving through a school zone during arrival time with children walking nearby. What does the law require of you?

• Work Zones: Driving Past Workers Safely | CO

  You enter a work zone where workers are standing beside the travel lane and the lanes have been narrowed. How should you drive?

• Why Wet and Icy Roads Need Lower Speeds | CO

  Why must you drive well below the posted limit when the road surface is wet or icy?

• Merging Onto an Interstate: Use the Full Ramp

  You are merging onto an interstate using the acceleration lane. What should you be doing along that lane?

• Driving the Limit in a Blizzard | CO

  A driver holds exactly the posted 45 mph limit through a heavy snowstorm in which visibility has dropped to half a block. Which statement is correct?