Pretty often I’ve seen a witness answer a lawyer’s question with “I din’t know”. Assuming the question “How fast was the car going” Witness answers “I don’t know” the lawyer then asks “faster than or slower than 30mph?” to which the witness again says “I don’t know”. Now the judge threatens “Answer the question or go to jail”.
Is it not reasonable that a witness might not know the answer to a question?
Peace,
mangeorge
are you seeing this in fictional media, nonfictional media or real life?
Everything from Judge Judy through Law and Order to a little real life.
Are you a lawyer?
Judge Judy™ can’t send people to jail. She’s not a judge.
That was supposed to be a joke. 
True, although she was a judge of the New York judicial system, both in criminal part and later in family court. So she’s not unfamiliar with the concept. 
To the OP: your scenario is not realistic. The witness can say he doesn’t know without risking jail.
I’ve seen lawyers follow up with a request for an estimate, not a guess. I’ve never seen any bet slapped around by sticking to “I’m sorry, I really don’t know and am not able to estimate.”
I’m not recalling who this was, or many details, but in a similar vein some high level politico was convicted of perjury after repeatedly answering questions with “I do not recall”. His conviction was overturned. I would think you would need to prove someone doesn’t know an answer or can’t recall the answer to be rightfully convicted of perjury.
“I don’t feel that I’m qualified to guess that”.
An “estimate” is lawyerspeak for guess.
Maybe that was it. Estimates can border on being guesses.
Yes and no. They usually explain the difference something like this. “if I asked you how long this table is, you would have to say ‘I don’t know,’ unless you measured it. You don’t know exactly how long it is, but you could estimate. If I asked you how long the table in my office is, you’d have to guess.”
Yes. A witness may refuse to guess without penalty. I have even, not very often, cautioned witnesses not to guess if they were doing so too freely; the better answer far more often than not is “I don’t know.” Sometimes lawyers will ask, “Well, what is your best estimate?” or “What is the best educated-guess you could give us?”, but that’s not too common, either.
I can tell the difference between 20mph and 40mph (I think) but I can’t watch a car go by and say whether it was doing 60mph or 75mph with certainty. Maybe a policeman with extensive radar experience could, but then without his electronic crutch, maybe he’s not up to the challenge either.
Accuracy depends on real world experience. I don’t often stand on the side of the road and watch cars go by. I guess the question is what the lawyer wants to use your testimony for, and whether you want to help him…