This is a very frequently-asked question. Now first of all, unlike in all other States in the Northeast USA, cannabis is not legal in NH. Possession of small amounts of cannabis has been “decriminalized”, but that doesn’t mean it's legal. Compare it to speeding – going in excess of the speed limit is also usually not a crime, but speeding is, nevertheless, illegal. 

More to the point, even if cannabis consumption was completely legal in NH, that doesn’t  mean that driving under the influence of cannabis is legal. Think about alcohol. It’s also completely legal in NH. But nobody thinks its OK to drive under the influence of alcohol. 

All that being said, the government still has to prove that the person’s ability to operate a motor vehicle was impaired by consumption of cannabis. That tends to be more difficult to prove than with alcohol, where the outward effects of intoxication are more obvious  to most people than with cannabis. We like getting the DWI-cannabis cases, because they present special challenges for the government, giving us more ways to get a good outcome for our client.