Legalizing marijuana use in Illinois won't only lead to more vehicle crashes, but it will hamstring police and prosecutors by making arrests problematic, transportation experts said Tuesday at a AAA forum.

Illinois has allowed medical marijuana since 2014, and legislation is pending in the General Assembly to permit the general use and sale of up to one ounce for people 21 and older.

"We finally made so much headway with reducing fatal alcohol-related car crashes, and now this comes along and blows it out of the water," Carol Stream Deputy Police Chief John Jungers said at the Oak Brook conference.

"My concern is about putting a law in place without being prepared for the repercussions."

AAA has studied the impact of legalization in Washington state and found marijuana use and fatal crashes are increasing. The portion of drivers in fatal crashes who recently used marijuana spiked to 17 percent from 8 percent from 2013 to 2014, the motorist advocacy group reported.

"Illinois is not ready for this," Illinois Traffic Safety Resource Prosecutor Jennifer Cifaldi said, adding her greatest fear was "an increase in traffic fatalities."

Advocates for legalization say changing the law would take marijuana out of the hands of drug dealers, reduce related crimes and allow people to buy a legitimate product from regulated and licensed businesses.

A majority of Democratic gubernatorial candidates either support legalization or want a referendum on the issue. It's been estimated the move could generate $350 million to $700 million to boost Illinois' budget.

Authorities at the conference said it's difficult to get convictions for driving under the influence of cannabis because the state's crime lab does not conduct that test.

Illinois law now prohibits driving with 5 or more nanograms of tetrahydrocannabinol or THC in a person's system. THC is the active ingredient in marijuana that can affect a person's brain and behavior.

Colorado Traffic Safety Resource Prosecutor Jennifer Knudsen said many marijuana products, including gummy bears, drinks and cookies, have become available since that state legalized cannabis. Normalizing marijuana makes it difficult for Colorado police to make judgments on whether a person is impaired during traffic stops. And in court, "when we get our juries, many of them voted for it (legalization)," she said.

Alcohol use has been studied for decades, Jungers said, but the effect of marijuana on drivers is relatively unknown. "It affects people differently. ... There's not enough science to write a law," he said.

"It's something that we're hoping the legislature takes a really close look at," AAA spokeswoman Beth Mosher said. "There are other issues than whether or not this is good financially for Illinois."

Cifaldi thinks legalization isn't a matter of if but of when. "It's going to happen," she said.

• Daily Herald wire services contributed to this report.

Pot: 'It's going to happen,' prosecutor says of legalization