States are growing increasingly concerned about "drugged driving."

Yet, at the same time, many are offering avenues to legitimate marijuana use.

Twelve years after Colorado legalized the use of marijuana for medicinal purposes, more than 85,000 Colorado residents have been certified by the state health department to use it.

Now theres increasing concern about a rise in traffic accidents caused by people under the influence of marijuana.

Between 2006 and 2010, more than 300 fatal accidents involved drivers who tested positive for cannabis, according to the Colorado Department of Highway Safety.

Thats why a Republican state senator, Steve King, wants Colorado to set a legal limit for marijuana intoxication, somewhat similar to the 0.08 percent blood alcohol limit states put on driving under the influence of alcohol.

And in California, Democratic Assemblywoman Norma Torres wants to set a zero-tolerance ban on driving under the influence of any drug, including marijuana.

Bills put forward by both King and Torres have run into opposition from those who say the science around what marijuana does to the body and mind is not conclusive enough to set a legal limit.

In Colorado, critics also note that Kings approach would cost the state public defenders office about $600,000 per year to defend those accused of "drugged driving" charges.

Still, concerns over cannabis intoxication wont go away.

Voters in Colorado and Washington state will decide whether to legalize recreational marijuana use, while a dozen more states are considering legalizing marijuana for medical purposes.

Stateline.org, a nonpartisan, nonprofit news service of the Pew Center on the States that analyzes trends in state policy, has issued a new analysis of the research surrounding marijuana and road safety.

It also explored why its so difficult to say how high is too high to drive.

How does marijuana affect the body?

The active ingredient in marijuana is called delta-9-tetrahydrocannabinol, or THC. Whether marijuana is ingested through smoking or eating, THC rapidly goes through the bloodstream to the brain. In the brain, THC slows down receptors that communicate brain functions between synapses, throwing the brains natural information flow off balance.

Users experience diminished pain sensitivity, which explains marijuanas medicinal purpose.

But users also experience slowed reaction time, impaired memory function, impaired coordination and altered judgment.

How does marijuanas effect on drivers compare to alcohol?

Although the symptoms for THC intoxication are similar to alcohol intoxication, the effects on drivers are very different.

According to a study by researchers from Yale University, alcohol-impaired drivers struggle with complex tasks like merging onto a crowded highway, but can generally perform automatic functions like turning on the car.

Marijuana users, however, can better handle complex situations than simple tasks like following the curve of a road.

The Yale study also notes that the effects of cannabis can vary a lot more between individuals than with alcohol. Factors such as a users age, weight, tolerance and smoking technique all can make a large difference in how the body absorbs THC.

Users of alcohol and marijuana also differ in their perceptions of their own impairment. Alcohol users tend to underestimate their level of impairment and drive faster and more recklessly.

Drivers with a blood alcohol level of 0.04 percent - half the legal limit in most states - experienced impaired driving performance even though drivers themselves rated themselves as unimpaired, according to a frequently cited 1993 study from the National Highway Transportation Safety Administration.

By contrast, marijuana users tend to overestimate their impairment and dont display as many obvious impairment symptoms. In the same government study, drivers who were given about one-third of a joint to smoke rated themselves as impaired even though their driving performance was not.

How long does it take for marijuanas effects to wear off?

Marijuanas impairment effects usually wear off within about three hours from the time of smoking or eating it. But traces of THC stay in a users body for up to a month.

This can make testing for intoxication problematic because THC shows up in blood tests long after the symptoms that impair driving have gone away.

How big of a problem is marijuana-impaired driving?

No one is quite sure.

One measure comes from a voluntary national roadside survey conducted by National Highway Transportation Safety Administration in 2007. About 14 percent of drivers pulled over at night tested positive for the presence of impairing drugs in their system; the daytime figure was 11 percent.

The study looked beyond marijuana and included both legal and illegal substances such as prescription drugs, stimulants and narcotics. Of this group, the most commonly detected drug was marijuana.

In a more recent California roadside study, conducted at nighttime over a weekend, more people tested positive for THC (8.4 percent) than for alcohol (7.6 percent). Because of the long time THC can remain in the body, however, the study was not a clear measure of actual impaired driving.

How does the risk of traffic accidents following marijuana consumption compare with the risks of accidents after alcohol consumption?

Driving within three hours of consuming cannabis increases the risk of vehicle crashes by about two or three times, according to research published in February by Mark Asbridge, an associate professor at Dalhousie University in Canada.

Concerning as that is, it is much less of a risk than alcohol consumption, which increases vehicle crash risk by 5 to 16 times.

What laws do states have in place now? According to the National Conference of State Legislatures, 17 states have what are known as "per se" laws regarding drugs and driving. That means anyone driving with traces of an illegal or impairing drug in his or her system is breaking the law.

This is closer to a "zero tolerance" policy than the 0.08 percent blood alcohol content states have for drunk driving.

However, of those states that have per se laws, Arizona, Delaware, Michigan, Nevada and Rhode Island also allow medicinal marijuana, setting up an inherent conflict in their laws.

Marijuana is currently on unsure legal footing: Even as states are beginning to create a legal framework for allowing use of the drug, the federal government still classifies marijuana as a Schedule I drug alongside LSD and heroin and has recently begun raiding dispensaries and arresting cultivators.

The drugs tenuous legal hold has created "a mess for enforcement for impaired driving," says Chris Cochran, deputy director of public information for the California Office of Transportation Safety.

Do states try to measure marijuana intoxication like blood alcohol content?

Thats what Nevada has done. In an attempt to create something like the clear standard that exists for alcohol intoxication, Nevada set a limit for THC in the blood at 2 nanograms of THC per milliliter of blood. Thats about 2 billionths of a gram of THC - one gram is about the weight of a paperclip - in one drop of blood.

In Colorado, state Senator Kings bill would set a limit of 5 nanograms of THC per milliliter of blood. And in Washington State, the ballot initiative that would legalize recreational use of marijuana would also set a limit of 5 nanograms of THC per milliliter of blood.

Advocates of medical marijuana say the science isnt supportive of adopting such specific limits. They worry that this approach will cause drivers who arent impaired but have lingering traces of THC in their blood to lose their drivers licenses.

Research has not found a consistent intoxication standard similar to a 0.08 percent blood alcohol content.

How do police check intoxication levels?

There is no comparable test to a breathalyzer that can be used to test for drug impairment. A few roadside tests have been developed using cheek swabs to test a drivers saliva, but they have proved inaccurate and very expensive.

Roadside tests have been used in Australia since 2004, but according to Wayne Hall, a medical researcher at the University of Queensland, "there is no evidence that using the tests hasdeterred drug users from driving impaired."

For police, an alternative to roadside tests is the standard field sobriety test, which relies on observation as a driver is asked to walk in a straight line and balance on one leg, among other things. Currently, officers in every state but West Virginia are being trained to become drug recognition experts who can recognize the signs of drug impairment and make arrests for drugged driving offenses.

Once an arrest is made and a blood sample is taken, a prosecutor takes over the case. Although the driver has been identified as impaired, depending on the test results, securing a conviction for drugged driving has proven much more difficult than getting a conviction for drunk driving.