In Colorado, where America’s experiment with legal recreational marijuana began a little more than a decade ago, a team of federal scientists has been paying regular cannabis users to get stoned.

This unconventional line of research — which includes vans outfitted with hippie tapestries and a sleek car simulator — seeks to tackle what road safety experts regard as a serious blind spot as marijuana use grows nationally.

Law enforcement officials lack tools to detect cannabis-impaired driving as reliably as they can identify people who get behind the wheel drunk.

Only a few states routinely test the blood of drivers involved in serious accidents for marijuana, and as a result, little is known about how cannabis use is affecting road safety. Police officers generally need a warrant to compel a driver suspected of being impaired to provide a blood sample.

Even when blood samples are analyzed, tests cannot reliably establish whether a person last used marijuana hours before the accident or several days prior, making the tests an imprecise gauge of impairment.

Complicating matters, state laws on cannabis-impaired driving are inconsistent and confusing, which has made them difficult for the police to enforce and for motorists to understand.