As more states legalize marijuana it's a growing concern. The insurance industry is leaning on Congress to come up with a standard for 'driving while stoned.' But experts say that won't be easy.

A House committee topic sounds like a bit on Saturday Night Live. 'Planes, Trains and Automobiles: Operating While Stoned.'

But John Bowman of the National Motorists Association says while a blood test is good for a DUI -- it may not be good for 'DWS.' He says pot chemicals stay in your system.

"It can remain in the bloodstream for, say, up to two weeks even after someone has smoked marijuana and those can be used to show impairment even though the person may not actually be impaired at the time the test was taken."

Bowman says whether a driver is stoned or drunk an officer should have to witness a violation before writing a ticket.

"I think it's hard to predict a person's behavior on the road. You have to actually see them doing something in order to really perform a legitimate traffic stop. That's our opinion."