Last year, Phoenix had no cases of injury or fatal crashes believed caused by people high on nothing but marijuana.
Pot was, however, involved in some of Phoenix's most serious crashes in 2012. Drivers who are drunk or high on other drugs sometimes also may have pot in their bloodstreams. The combined effect of several substances seems to have a horrendous consequence for some drivers. But figuring out whether pot contributed to the drivers' impairment often is like asking which snowflake caused the avalanche.
For instance, one driver suspected of causing a fatal crash in Phoenix tested positive for THC — but also had a .021 BAC and four different kinds of prescription sedatives. In another THC-positive case, the driver also was legally drunk and on cocaine.
In addition to crash stats, New Times reviewed all the DUI-drug cases involving marijuana in 2012. All told, Phoenix handed out 6,118 citations for DUI last year. Here's the breakdown, according to the Phoenix PD:
• 300 of the cases involved drugs and no alcohol.
• 111 of the 300 drug cases involved marijuana.
• 58 of the 111 involved just marijuana, while 53 people tested positive for pot, alcohol, and other substances.
• In 336 of the alcohol DUIs, drivers also tested positive for pot and/or other drugs.
Out of 15 fatal crashes involving suspected impaired drivers from November 2011 to November 2012, Phoenix police records show: Six were impaired by alcohol only; three had used booze, pot, and other drugs all at once; one tested positive for pot and meth; one was positive for alcohol and sedatives; and four tested positive for neither drugs nor alcohol, with no drug analysis having been performed on one of the four.
Information on fatalities in which the suspected impaired driver was killed were not obtained by New Times, though police predict that autopsy toxicology reports would reveal several more cases of drivers with alcohol, drugs, or pot in their blood.
An unknown number of alcohol-and-pot cases don't show up in the data, police say, because when a driver is found to be clearly drunk, police don't opt for a drug test, which costs more money.
Still, police in jurisdictions contacted by New Times say they always request either an alcohol or drug test for suspected impaired drivers. Phoenix, Scottsdale, and Chandler records show that, at least last year, police suspected relatively few drivers involved in crashes of being high.
"I'm not going to complain," says Detective Kemp Layden, supervisor of the Phoenix police Drug Recognition Expert program. "Obviously, alcohol is more of a problem."
County Attorney Montgomery declined to provide any evidence that his office has prosecuted a cannabis-only driver for causing a fatal or injury crash.
Records of pot-DUIs and crashes do suggest that marijuana can impair drivers. In the DUI cases, police reported pulling over drivers for weaving, speeding, driving too slowly, driving without headlights on, or other potential signs of impairment.
Blood tests revealed only the carboxy metabolite in some of the suspected marijuana-DUI drivers (precise THC measurements weren't available in all the cases). It's impossible to say for sure in these cases whether the investigating officer was incorrect about suspected impairment, the test was only for carboxy-THC and not active THC, or that levels of active THC and hydroxy metabolite (which has psychoactive properties) had dissipated by the time of the blood draw.
Layden denies that he or other Phoenix officers abuse the zero-tolerance law, which he says can't be applied without an accompanying suspicion of impairment.
In other words, Phoenix cops wouldn't pull over actress Amanda Bynes and cite her under the zero-tolerance law just because they'd seen a recent paparazzi picture of Bynes smoking pot in her car.
Still, Layden admits the obvious: Police can be mistaken about a driver's impairment. By his rough estimate, 90 percent of people who do poorly on field-sobriety tests are impaired.
But that leaves plenty of unimpaired people failing to count off 30 seconds while balancing on one leg. Some innocent drivers may have used marijuana a week before getting pulled over — in which case they still could be convicted.
Between the potential for overzealous enforcement, the scientific uncertainty about how marijuana affects driving, and the reality that pot is legal or almost legal for millions of Americans, enforcement of Arizona's zero-tolerance law is a ripe environment for injustice.
In the past two decades, studies have shown that the THC in marijuana may cause an elevated crash risk for motorists, especially in increased dosages.
Problem is, no one knows precisely why THC affects driving — or what levels of THC in the blood lead to impaired driving. And relatively few studies have been conducted on real-world crash data: State officials and police said no one ever had examined the actual blood-test data on drivers involved in Valley crashes before this year's request from New Times.