I should have been a little more specific; the approach I described assumes that the driver makes some errors, so the speed is not actually constant in practice. That can be because the driver slightly errs in seeing the slope or gas, or because of other sources of error (gusts of wind, maybe). Without that variation, you're right, the system is unidentified. The larger the variation, the better the identification. But none of this is news to economists, and none fundamentally requires the econometrician to have better information than the driver (although measurement error can matter in some circumstances, it matters in a different way).
As I'm sure you know, these arguments (i.e. mainstream Economics is wrong and ignorant!) are more convincing when they're accompanied by a model. DSGE models have a lot of limitations, but they're pretty good for demonstrating failure of identification. Monetary policy is tricky to identify, and I'd be sympathetic to an argument showing that deviations from a Taylor-rule are bad for identifying the effects of monetary policy shocks (lots of people would agree with this, the interesting question is whether they're bad in empirically important ways or just conceptually bad) but your car+driver analogy seems like it's aiming to be broader than that.
Thanks psuet. I'm not one of those guys who goes around saying "mainstream economics is wrong and ignorant". Well, OK, we are often wrong and ignorant, but people who aren't in mainstream economics are usually even more wrong and ignorant!
This is just a particular beef I have. Because I keep on seeing examples where economists make mistakes by missing this point. Like when economists try to test whether headline or core inflation is better at forecasting future inflation, and use those results to give policy advice on whether inflation-targeting central banks should respond to core, headline, or both indicators. All they are modelling is the central bank's mistakes, in responding too strongly or too weakly to those indicators.
I think proper identification does require the econometrician be (in some sense) a "better" driver than the driver. If you can see the hills better than the driver can, then you can see the effects of a hidden hill that you know he doesn't see.
As I'm sure you know, these arguments (i.e. mainstream Economics is wrong and ignorant!) are more convincing when they're accompanied by a model. DSGE models have a lot of limitations, but they're pretty good for demonstrating failure of identification. Monetary policy is tricky to identify, and I'd be sympathetic to an argument showing that deviations from a Taylor-rule are bad for identifying the effects of monetary policy shocks (lots of people would agree with this, the interesting question is whether they're bad in empirically important ways or just conceptually bad) but your car+driver analogy seems like it's aiming to be broader than that.