MARIJUANA AND DRIVING
Wouldn't relaxation of the marijuana laws lead to more highway deaths ?
Not likely, and quite possibly less. In general, most studies have indicated that marijuana users drive as well or better than those who are drug free. 
- Inexperienced users will generally perform poorly under the influence.
- Heavy combined use of both marijuana and alcohol is more dangerous than use of alcohol alone
- There is no currently available technology that can quickly measure current impairment by marijuana itself. (This is another reason to favor general testing for impairment, whether due to fatigue, drugs or other factors.)
The graph below compares traffic fatalities in Texas and California after the passage of medical marijuana laws in California in 1996. When the laws passed, the rates were the same in both states, but as medical use grew in California, traffic fatality rates went down significantly. The gap became wider as medical use of marijuana has became more widespread in recent years. How much this may have been due to some substitution of marijuana for alcohol is unknown. It is consistent with what we know of driving behavior under the influence of both drugs. It is speculative but possible that a reduction in pain and other symtoms that distract drivers may play some protective role. In any case, allowing medical marijuana use clearly did not produce an increase in traffic fatalities.
- Laws against driving under the influence would likely be the same for marijuana as now exist for alcohol.
- Use of marijuana is now so widespread that no significant change in use is likely regardless of the laws; individual responsibility will govern as always.
- To the extent that marijuana use might replace alcohol use, the roads would be safer.
- Experienced users who have had a chance to practice various tasks do not appear to be significantly impaired when repeating those tasks. 
- A normal drug effect is for alcohol to promote reckless behavior and for marijuana to induce caution.