Driver's License Test Prep
Pennsylvania Practice Vehicle Equipment

The manual says a dirty headlight reduces light output by:

  1. A.
    More than 50 percent Correct
  2. B.
    About 10 percent
  3. C.
    About 25 percent
  4. D.
    About 75 percent
The Why

The answer is "More than 50 percent"

A dirty headlight reduces light output by more than 50 percent, which is why the snowy-conditions checklist tells drivers to clear headlights, taillights, and directional signals of snow, ice, and dirt before setting out. Test-takers who half-remember the manual's earlier line that dirty headlights may give only half of the light they should often pick 25 percent, splitting that loss in half again.

Why the others miss

The traps in the wrong answers

B. About 10 percent

The loss is far greater than a small trim off the top; the manual pins it at more than half. This answer treats a filmed-over lens as a cosmetic dimming and reaches for a token single-digit figure that sounds like normal wear rather than a real cut to the light reaching the road.

C. About 25 percent

A quarter cut is still well below the manual's more-than-half figure. This answer halves the manual's earlier 'half of the light they should' figure a second time when only the idea of splitting the loss is remembered.

D. About 75 percent

Three-quarters overshoots the manual's threshold, which stops at more than half rather than most of the light. This answer rounds any more-than-half loss up toward a near-total blackout once the figure crosses the 50 percent mark, instead of stopping at the stated threshold.

The Rule

This question tests your knowledge of Vehicle Equipment in Pennsylvania. Source: Pennsylvania Driver Manual, "Visibility Conditions - Snowy Conditions," p.42..

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