Marijuana legalization poses challenge to police on lookout for driving while high

Smoking marijuana in Washington state is legal as of Thursday, but driving while high is not.
It's a fine line that police and prosecutors are hoping to walk.
"I don't feel we are in unchartered territory -- I know we are," said Seattle Deputy City Attorney Craig Sims.
Possessing pot is legal now in Washington and soon in Colorado. In 17 other states, marijuana for medical use is easily obtainable. Yet unlike alcohol, there is no national legal limit. Some states have a zero tolerance policy, while...