Thought consuming Marijuana is illegal in most states.
DUI is a bit redundancy if it is already illegal to consume.
If you are high but do not have any of the drug on your person then it would not be possible to charge you with possession/consumption as the government would not be able to prove that you consumed it.
If you are operating a motor vehicle while under the influence of Pot then they could get a search warrant to test their blood for levels of THC that would indicate they should not be driving.
This is a very big problem as it is much more difficulty to tell if you are "okay" to drive after smoking then it is to tell if you are "okay" to drive after drinking, as how much you drink is very measurable.