Are there any ways to directly measure impairment rather than using chemical trace detection as a proxy for the affect?

Maybe a driver could take a short impairment test on their cellphone or in car computer prior to starting their drive -- if they pass the test then they would have an argument against impairement should they become subsequently involved in an accident... If they fail the test and drive anyway then there's an even stronger argument that they made an irresponsible decision for which they must take responsibility.