DENVER - It's the zombie initiative of the Colorado Legislature: A proposal to create a blood test for drivers who are marijuana users came back to life for a fifth time as a House committee set about regulating the drug on Wednesday.The House vote to reconsider standards for impaired driving came just two days after a Senate committee decisively killed a marijuana driving bill on a 4-1 vote. Similar measures have failed three other times.The House committee also agreed to rules on how recreational pot should be sold, including purchasing limits for people from out of state. Non-residents would be limited to one-fourth of an ounce in a single transaction - enough for about six to 12 joints, depending on how they're made. Colorado residents and visitors alike could possess up to an ounce of marijuana, as long as they're over 21.Lawmakers also agreed to potency labeling and advertising rules for recreational pot business, which are scheduled to start next year.Colorado bans impaired driving, but state officials are struggling to decide whether to establish a blood limit for marijuana users. Drugged driving convictions now rely heavily on officer observations, though blood samples can be used as evidence.The 5-nanogram limit, in a regulation bill sponsored by Democratic Rep. Dan Pabon of Denver, would allow juries to presume drivers are impaired over that level."When it comes to a statement of public policy by this body, that's the appropriate line to draw," argued Assistant Attorney General David Blake.But some lawmakers again worried that because the body processes marijuana differently than alcohol, a simple blood limit isn't a fair gauge of impairment. Others complained that the blood limit died fair and square this week in the Senate."It's had its day in the Legislature, and it'll have its day next year," argued Rep. Joseph Salazar, D-Thornton.Pabon predicted a driving limit ultimately will be decided upon by a House-Senate conference committee to negotiate differences."There's something that everyone likes and everyone doesn't like in this bill," Pabon said of the regulation measure.The debate prompted a rare public interjection from Gov. John Hickenlooper, who sent a brief message on Twitter saying a marijuana DUI standard would "make our roads safer & give police more tools to stop mj-impaired drivers."The Democratic governor typically remains silent on pending legislation. He did ask lawmakers last May to approve a similar bill.The driving issue is just the latest involving Colorado's legalization of marijuana. A study released Wednesday by Colorado State University warned that the state can't expect a tax windfall from pot.Economic forecasters cited uncertainty about demand for a newly legal product and said that "the future holds more unknowns than knowns" about Colorado's pot industry. They estimated in their report that Colorado's demand for pot will hit 2,268,985 ounces a year - more than 70 tons.Depending on the price, the forecasters guessed that Colorado could bring in $130 million next year from pot taxes. That's a big hike from an earlier fiscal analysis prepared by the state, but not enough to significantly change Colorado's finances, economists concluded. Pot taxes would decline over time, especially if other states follow Colorado and Washington and approve pot, economists warned."The most productive marijuana tax revenue years will be the years just after legalization," the report stated.A bill up for committee review Thursday would ask voters to tax recreational marijuana about 30 percent.Still before lawmakers is a high-stakes vote on whether current medical pot shops should get preference when recreational sales begin. Lawmakers also must decide whether to ban drug felons from the marijuana business.