Is there a "breathalyzer" for marijuana?

I was just curious if there was a breathalyzer-type device for marijuana (i.e. a device that could be used for roadside tests to determine the intoxication level of a marijuana user), or if such a device would be possible (assuming that blood and urine tests wouldn’t be used as roadside marijuana tests).

Well, actually, they’re working on it.

http://www.nlm.nih.gov/medlineplus/news/fullstory_10454.html

Cite forgotten, but I read recently that they may use simple urine test to disclose drug or MJ usage of a suspect driver. Problem is the residual effect can make it difficult to prove you smoked a joint 3 days ago.