[TChris]

I was thinking about the much higher rate of mistakes that would occur with eyeballs only.

It's true that his visual estimate isn't going to give the accuracy of a radar lock. Any officer who gave anything more specific than a range ("The vehicle appeared to be traveling approximately 45-50 MPH") would be questionable.

But specifying an exact speed isn't required to show a vehicle was over the limit. If the officer can say that he's certain, based on his estimation, that the vehicle was not traveling at or below the speed limit, but some significant rate higher, then it would be enough.

So it would be a "mistake" only if the officer was far enough off that a vehicle he estimated was speeding was actually not. Given that most LEOs don't write citations until you're going well over the limit (10-15 MPH over is common), and that estimating a vehicle's speed to within +/- 5 MPH isn't terribly difficult for an experienced traffic cop, that's a pretty unlikely mistake.

[fr_freak]

But specifying an exact speed isn't required to show a vehicle was over the limit. If the officer can say that he's certain, based on his estimation, that the vehicle was not traveling at or below the speed limit, but some significant rate higher, then it would be enough.

Here in CA it makes a difference how much over the speed limit someone is going. 45 in a 35 zone is a minor ticket. 55 in a 35 would be a major ticket, and IIRC, 70 in a 35 would be automatic impound and jail. Precision would matter.