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Pennsylvania driving lesson · Space, Speed and Conditions

Fog, snow and snow squalls

A 3-minute lesson from the Pennsylvania course. Read the rules and examples, follow the handbook page references, and check your understanding before you move on.

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Read time
3 minutes
Course unit
Space, Speed and Conditions
Checkpoint questions
4
Source pages
46, 47

Fog and snow can cut visibility to almost nothing. Fog is probably the most dangerous driving environment: you can't see what you're about to hit, and others can't see you. Snow squalls can turn a clear, dry interstate dangerous in seconds.

Know the rule

Don't start a trip if fog is so dense you can't see. Caught in dense fog, pull completely off the road (a parking lot is best), turn on your hazard lights and keep your seat belt fastened.

Handbook p. 46 ↗

When you must drive in fog

  • Slow down enough to stop within the distance you can see, and check your speedometer often
  • Use low beams (and fog lights) day or night; high beams bounce off fog and create glare
Handbook p. 46 ↗

Six things to do in snowy conditions

  • Clear all windows, hood, trunk and roof before driving
  • Clean snow, ice and dirt off headlights, tail lights and turn signals
  • Let the engine warm up so the defroster can melt leftover ice
  • Keep the windshield clear and the washer reservoir full of non-freezing fluid
  • Keep the defroster on to clear steamed windows
  • Turn on your low beams, even in daytime
Handbook p. 46 ↗
More than 50%

How much a dirty headlight can reduce light output

Handbook p. 46 ↗

Snow squall

An intense burst of snow, often on an otherwise clear day, that cuts visibility to near zero and slickens the road in seconds.

Handbook p. 46 ↗

Snow squall tips

  • MONITOR: watch the forecast and transportation message boards
  • PLAN: change your travel time or route; interstates are often the greatest risk
  • AVOID: if a squall is approaching, exit the roadway and wait for it to pass
  • CAUTION: slow gradually, turn on hazard lights, and don't slam on your brakes
Handbook p. 47 ↗

Know the rule

If you're in a collision during a snow squall, stay in your vehicle whenever possible.

Handbook p. 47 ↗

Know the rule

The National Weather Service issues Snow Squall Warnings for specific roadways; squalls typically last less than 45 minutes on a road. If you get one for your route, delay or change your trip.

Handbook p. 47 ↗

Put it into practice

On I-80 on a sunny winter afternoon, a white wall of snow appears ahead. Take the next exit and wait for it to pass. If you can't, slow gradually, turn on your hazards and don't slam the brakes.

Check your understanding

Read the question, decide on an answer, then open it to check the explanation. These are Roadwise study questions based on the source handbook.

What is the safest action if fog is too dense to see safely while driving?
  1. Turn on high beams for better visibility
  2. Pull off the road and wait it out
  3. Follow closely behind another vehicle's taillights
  4. Speed up to get through the fog quickly

Answer: Pull off the road and wait it out

In dense fog, pull fully off the road, ideally into a lot, and wait it out with hazards on.

Handbook p. 46 ↗
Why should you avoid high beams in fog?
  1. High beams are illegal at all times in fog
  2. They can damage the headlight bulbs
  3. Light reflects back and cuts visibility
  4. They make oncoming cars look farther away

Answer: Light reflects back and cuts visibility

High beams bounce light back off the fog, creating glare that worsens visibility.

Handbook p. 46 ↗
A dirty headlight can reduce light output by more than what percentage?
  1. 10 percent
  2. 90 percent
  3. 25 percent
  4. 50 percent

Answer: 50 percent

A dirty headlight can cut light output by more than half.

Handbook p. 46 ↗
You're on the interstate and see a snow squall approaching. What does the handbook advise?
  1. Speed up to get through it quickly
  2. Stop in your lane and wait for it to pass
  3. Exit the roadway and wait for the squall to pass
  4. Switch on your high beams and continue

Answer: Exit the roadway and wait for the squall to pass

If you see an approaching squall, exit the roadway and wait for it to pass; if caught in one, slow gradually and use hazard lights.

Handbook p. 47 ↗
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