Crashes are up 6 percent in Colorado, Oregon, Washington and Nevada, a joint study by the Insurance Institute for Highway Safety and the Highway Loss Data Institute shows.
"The bottom line is that there appears to be a negative impact of highway safety in legalized states, and states considering legalization need to be prepared to deal with this impact", he said.
"We know a lot of states are considering making recreational marijuana available", Harkey said.
IIHS estimates that Colorado, Oregon, and Washington combined saw a 5.2 percent increase in the rate of crashes per million vehicles registered.
Analysts controlled for differences in the rated driver population, insured vehicle fleet, the mix of urban vs. rural exposure, unemployment, weather and seasonality.
Driving high is illegal across the United States and Canada.
Although marijuana's role in crashes is not as clear as the link between alcohol impairment and crashes, Harkey said he thinks the public needs to be informed about potential collisions, especially as more states discuss legalization.
Other states where marijuana is either decriminalized or allowed for medical use were not included in the study. It points to a lack of driver drug-use stats in crash reports, in addition to the difficulty of actually testing to determine whether or not a driver was under the influence of cannabis in a collision.
Researchers know a lot about the direct link between alcohol and auto crashes but are still working on defining the nature and scope of how marijuana contributes to crashes, the report noted.
The studies used raw accident data that did not specify that marijuana involvement, due to inconsistent reporting on this factor.
Witnesses told authorities the driver had been driving erratically for more than 15 minutes before the crash.
In its report, the board says there has been an increase in the number of drug-impaired drivers across the country and that something must be done about it.
"The pick-up truck driver in this crash made awful choices with tragic consequences", said board Chairman Robert Sumwalt.
There are no national standards or standardized tests for weed-impaired drivers like there are for alcohol.