Yes, but you don't want to re-litigate what constitutes recklessness in every case. Take drunk driving: a fixed threshold of blood alcohol content (0.08) is a rather crude threshold and takes little account of individual driving performance, but it would be expensive and time-consuming to objectively measure every motorist's baseline performance, degree of impairment, and so on in every case involving DUI. It's already difficult in that smart lawyers will challenge the accuracy and correct configuration of the breathalyzer equipment - as they should - and drivers who test just barely over the limit can often be acquitted or have their charges dropped. On the plus side, the arbitrariness of the threshold not only simplifies prosecutions where users are clearly very far over it, it also dissuades people from approaching the threshold by discouraging them from drinking and driving at all; thus the perception that a DUI charge is a big headache acts effectively to discourage the unwanted behavior. The fact that alcohol intake is objectively measurable, even if impairment is not, provides clarity about both the nature of the proscribed behavior and the risk of liability if one engages in it.

Interestingly, it appears that states where medical marijuana is legal see a lower rate of fatal traffic accidents; stoned drivers are thought to be more cautious than normal, and over-compensate for their psychomotor impariment by driving more slowly and waiting longer at stop signs: http://hometestingblog.testcountry.com/?p=17893