The number of vehicle thefts reported in the United States has fallen to a 20-year low even as the number of vehicles on the road has doubled.

Experts with the Highway Loss Data Institute point to more sophisticated anti-theft technology in cars and increased efforts by police to target organized car-theft rings for the decline.

The FBI estimates that 956,846 motor vehicles were stolen in 2008. That's less than half the rate in 1991, when a high of 1.66 million vehicles were stolen. Complete data for 2009 are not yet available.

The Alliance of Automobile Manufacturers estimates there are more than 245 million vehicles on the road today, up from 122 million in 1989.

Cars these days routinely include such things as ignition immobilizers, which makes it hard to start a car without the owner's key. Whereas only 5 percent of new cars included the device in 1989, today more than 86 percent have them, according to research from the Highway Loss Data Institute.

The institute also credits other anti-theft technology, such as alarms and GPS tracking devices, for helping cut down on auto thefts.