This may seem absurd and unfair, but it's the law, in the state of Ohio at least. Mark Jenney, a 27-year-old motorist in Akron, Ohio tried to appeal his speeding ticket. The officer had made the charge based on a "visual speed estimate", as opposed to using a radar, to judge how fast he was going. The 8th District Court of Appeals in Cleveland ruled that this is not a sufficient basis for writing a ticket.
The case went to the Ohio Supreme Court, who overturned the decision in a 5-1 ruling. According to the majority ruling:
"Rational triers of fact could find a police officer's testimony regarding his unaided visual estimation of a vehicle's speed, when supported by evidence that the officer is trained, certified by (the Ohio Peace Officer Training Academy) or a similar organization, and experienced in making such estimations, sufficient to establish beyond a reasonable doubt the defendant's speed. Independent verification of the vehicle's speed is not necessary to support a conviction for speeding."