HELENA, Mont. (AP) - The Montana House is endorsing a plan to start testing drivers to determine possible marijuana impairment.

The chamber backed the bill Tuesday in a 68-31 vote before sending it to the appropriations committee to analyze a price tag of about $100,000 per year.

Supporters say it will crack down on driving under the influence of marijuana. The bill establishes a legal standard of allowable amount of THC - an ingredient of marijuana - that can be in a person's blood.

Republican sponsor Rep. Doc Moore of Missoula says the measure aims to make highways safer. He says many drunken driving crashes also involve marijuana use.

Opponents argue that testing is unreliable and can measure agents that don't cause intoxication, but remain in the blood stream long after impairment.