Big Island motorists, think twice about zipping around that vehicle ahead. You never know who might be behind the wheel.

For one driver, the decision to pass a silver Toyota 4Runner last month landed him a speeding ticket and a charge of driving under the influence, said Traffic Enforcement Unit Officer Joshua Lewis.

"I was on Queen (Ka'ahumanu) Highway between Makala Boulevard and the police station when I got overtaken by a silver Mercedes doing 112 mph in a 45 mph zone," Lewis said. "It led to a DUI. That guy probably would have killed himself, his passenger or another person."

Seeing a marked police vehicle or the blue dome light on top of a car often makes drivers think twice before driving unlawfully. However, without the markings or blue dome light, officers are able to find more people driving recklessly on Hawai'i County roads, said Lewis, who has patrolled in an unmarked vehicle for two years.

The department has eight unmarked vehicles, with three stationed in West Hawai'i and five stationed in East Hawai'i, Lewis said. A county test of the unmarked vehicles began in February 2006 with one Traffic Enforcement Unit officer. Six unmarked vehicles became part of the patrol roster by late 2007.

Honolulu police do not use unmarked vehicles for traffic enforcement purposes, said Michelle Yu, HPD spokeswoman. However, she said HPD does have a subsidized-vehicle program. Under that plan, officers who have been on the force for several years are allowed to purchase a personal vehicle from a selected list of department-approved cars.

HPD then pays the officers a subsidy for purchasing and maintaining the automobile, which can be used for both on- and off-duty purposes.

Traffic Enforcement Unit Sgt. Christopher Gali said that between July 2008 and June, the eight officers in unmarked vehicles issued 5,008 tickets for speeding, 1,175 seat belt and child restraint citations, 3,606 citations for vehicles without proper safety inspections or registration, and arrested 391 people for driving under the influence.