In August of 2008 a 20-year-old man drove into a tree in Chilmark and was deemed intoxicated by police. In February of 2008 in Oak Bluffs another man failed five sobriety field tests and the chemical test at Dukes County jail. In November of 2008 in Oak Bluffs still another man, arrested a second time for driving drunk, blew above the legal limit once on a portable Breathalyzer and twice more on the chemical test at the jail.

None were convicted of operating under the influence of alcohol (OUI).

The three cases are an example of a marked trend of uneven prosecution of drunk driving cases on the Vineyard. Court records show that in the past year many cases of OUI have been either dismissed or downgraded to a lesser charge at the request of the assistant Cape and Islands district attorney.

Police reports attached to the cases show a pattern of driver failures in field sobriety and Breathalyzer tests that have not resulted in OUI convictions over the past year.

The driver arrested in Oak Bluffs in February 2008 failed all five field sobriety tests administered by an Oak Bluffs police officer, registered a 0.133 per cent blood alcohol on the preliminary breath test and then blew 0.11 per cent twice at the Dukes County jail. The legal limit is 0.08.

Though the driver was charged with not being in possession of a license or up-to-date inspection sticker, he was not charged with negligent operation on the scene. Ultimately though, the OUI charge was dismissed and the driver was convicted of negligent operation of a motor vehicle.

A negligent operation conviction carries a maximum fine of $200 as an alternative to jail time.

Drunk driving is among the most prevalent crimes on the Vineyard. In 2008, 210 cases of operating under the influence were filed at the Edgartown district court. The frequency of these cases follows a bell curve, with the majority filed in the summer months. In January there were 12 cases; in February there were 10; in March there were 13; in April there were 11; in May, 20; in June, 25; in July, 33; in August, 22; in September, 21; in October, 18; in November, 13; in December, 12.

Eight cases were filed in January. Five have been filed this month.

It is unclear if there are mitigating circumstances in any of the cases that were downgraded. In an interview yesterday, assistant district attorney Laura Marshard said she was unable to comment on specific cases, but she underscored that each incident is looked at individually.

“Every case warrants independent evaluation,” Ms. Marshard said, adding: “Each case is evaluated based on a variety of factors including criminal history, whether it’s an accident, an accident involving passengers.”

She said that many details included in police reports are not admissible in court, affecting the viability of a case. For example, she pointed to the preliminary breath test, administered at the scene on a voluntary basis. And she said that the chemical test administered at the Dukes County Jail is also optional for drivers.

“Ultimately it comes down to what is provable in court,” she said.

In the November 2008 incident, a 40 year-old man failed to come to a halt at a stop sign in Oak Bluffs. The arresting officer noted in his report that the driver continued approximately 10 yards past the sign before stopping in the intersection. He was also charged with a marked lanes violation.

According to the report, the driver had a pungent odor of alcohol and failed several field sobriety tests, including the Horizontal Gaze Nystagmus test, which tests the tracking ability of the driver’s eyes, and a balancing test.

The driver submitted to a preliminary breath test, registering a 0.112 per cent reading, and a chemical test at the Dukes County Jail where he registered a 0.09 per cent and then 0.10 per cent on the Breathalyzer test.

The charge of operating under the influence was listed as a second offense in a report made by Oak Bluffs police. Second offense OUI carries a mandatory 60-day prison term and a fine. The report lists the driver’s previous conviction in 2001.

The OUI charge for the case was dismissed. The driver was deemed not responsible for failure to stop and for marked lane violation. A second complaint was filed as a breakdown of the second offense of negligent operation.

In the case of the 20 year-old man who was arrested after crashing into a tree for charges that included speeding and operating under the influence, he blew 0.12 per cent on a preliminary breath test. At the jail he took three separate tests, which registered 0.09, 0.14 and 0.08 per cent.

The arresting officer also reported that his speech was rambling at times and that he failed several field tests.

The driver entered a plea and was convicted of negligent operation of a vehicle.