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A
Water is always more slippery than dry roads
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B
The first rain mixes with accumulated oil, rubber, and grime on the road surface, creating a slicker mixture than plain water — the first 15-30 minutes of rain on dry roads are often the most dangerous
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C
Road surfaces expand when wet
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D
Tires lose air pressure in rain
Why this is the answer
OIL AND RAIN MIXING is a phenomenon well known to experienced drivers but often overlooked by novices. During dry periods, vehicles drip motor oil, automatic transmission fluid, grease, and other petroleum products onto the road surface. Rubber abraded from tires also coats the pavement. These materials accumulate in the surface texture of the asphalt. When rain first falls after a dry period, water combines with these oil and rubber deposits to create an extremely slippery emulsion — more slippery than water alone because the oils reduce friction dramatically. The road surface essentially becomes lightly oiled. DURATION: The effect is worst in the first 15-30 minutes of rain. After sustained rainfall, the water washes the oily mixture away and roads return to wet-friction levels. SPECIFIC RISKS: Intersections are worst (most oil drip from idling vehicles); travel lanes are also affected by tire rubber; bridge decks are particularly dangerous (no oil below but rapid temperature change causes ice formation as well). DRIVER ADJUSTMENTS: Significantly increase following distance immediately when rain begins after a dry spell; reduce speed even on roads posted at higher limits; avoid sudden braking or steering inputs; test brakes at low speed when it's safe to do so to understand actual traction levels. The CDL exam includes this because new drivers often make the mistake of treating early rain as 'not that bad.'
Source: FMCSA CDL Manual, Chapter 4, Hazardous Conditions, Wet Roads