Gov. Janet Napolitano has signed a bill to crack down on Arizona drivers who have neither licenses nor insurance and are involved in wrecks.

The measure requires the immediate impounding of the vehicle of any motorist found at the scene of a traffic accident without auto insurance and also lacking a driver’s license. It takes effect in August.

One study estimates as many as 40 percent of the state’s drivers are driving without insurance.

The new law, previously SB1420, makes a 30-day impoundment mandatory and imposes a $500 fine for a lack of insurance, up from $250. Motorists would have to pay a $750 fine for the second offense within three years.

Until now, state law allowed police officers to decide whether to impound a vehicle when a driver lacks insurance. Brian Livingston of the Arizona Police Association told The Arizona Republic officers routinely have cited drivers without seizing their vehicles.

Vehicles were not impounded when a driver lacked a valid license.

Carrying insurance and a driver’s license won’t necessarily prevent accidents, supporters say, but it could help ensure that drivers take responsibility when a crash occurs.

Though carrying proof of insurance is required under Arizona law, the Arizona Automobile Association estimates that about 40 percent of drivers in the state may lack insurance. In 2004, there were 5.3 million vehicle registrations in the state.