Presenting an impossible condition.

I was in a disagreement with someone about speed limits, I went into great detail about what's been shown to exacerbates speed limit-non compliance, as in what leads to people paying less attention to speed limits as in arbitrary placing of boundaries etc, shared speed and compliance data and also present evidence from Charles Marohn about the phycology of driving explaining that a lot of the non-compliance isn't down to wilful disobedience.

None of that seemed to matter to the person I was debating because I needed to explain what "physical force" was making people speed or else I could seemingly be dismissed.

Obviously there isn't one, I never claimed there was, he seems to have presented an impossible criterion, in order to dismiss everything I've said.

Another example would be Kent Hovind's $250,000 reward for "proof of evolution" he has likely given an impossible criterion otherwise Darwin's family would be able to claim it.

I can see this in an argument presented at an argument from ignorance . For example,

Kent: Can you present proof X for evolution?

Darwin: No.

Kent: Therefore, evolution is not true (and creationism is).

There are actually three reasoning errors here:

1. The impossible/irrelevant condition. As you point out, asking someone for impossible evidence is often a deflection. We don't need a sample of the moon's core to know that it's almost certainly not made of green cheese.
2. The argument from ignorance . Even if the request for proof was reasonable, being unable to provide it doesn't mean the claim is false.
3. The false dilemma . The claim that if your explanation is wrong, then mine must be right, when in fact, there could be other candidate explanations.

Since there is no real argument presented, I wouldn't call a fallacy. I wouldn't even claim their suggested condition is "impossible," because that can lead you down a rabbit hole. I would ask him to explain why "physical force" is the only thing that matters in this case. Put the burden of proof in his hands. Then by analogy, ask what "physical forces" make people do other things like murder, rob banks, etc. Through a reductio, you may be able to demonstrate that his request is, at the very least, unreasonable.

Obviously the physical force "making" them speed is additional foot pressure on the accelerator.  To be boring and unimaginative, I would suggest the simple cause of that additional foot pressure is usually just driver inattentiveness, nothing mysterious or deeply psychological about it.  I don't think anyone should try to make a big philosophical issue out of it.  People speed because they aren't paying attention.  To a certain extent, driving is a boring, rote activity, especially highway driving.  Barring unusual circumstances (accidents or severe weather etc) it can be done fairly efficiently without much concentration.  And so the foot pressure on the pedal can increase without a driver being aware of it.  Maybe they should use their cruise control.