Roadside sobriety tests are not at precise as breathalyzers, and I foresee no end of lawsuits and juries that have become accustomed to the idea that evidence should be precise and completely objective with nothing that can be seen as subjective.
Considering how prevalent pot use is, I think it is absurd that more of an effort has not been made to find a way of determining if someone is impaired by it. The need for a reasonably prices and usable device already exists. I don't really know if the technology is cost prohibitive, or if research into it is being obstructed by the DEA and FDA.
If it was legal, the person could say they smoked it three days ago. The test can't prove otherwise.