In a heavy rainstorm you cannot see farther than 100 feet ahead. According to the handbook, what is the fastest it is safe to drive?

Why

Visibility sets your speed. The handbook's rule for heavy rain or snow is that if you cannot see farther than 100 feet in front of your vehicle, it is not safe to drive faster than 30 mph, whatever the posted limit says. Forty-five and fifty-five leave you no room to stop for whatever appears at the edge of what you can see. Turn on the wipers, low-beam headlights and defroster, and stop from time to time if mud or snow builds up on the windshield, headlights or taillights. Remember too that the pavement is at its most slippery when rain first begins, especially after a long hot, dry spell, because oil and dust have not yet washed away.