Highway Superintendent Thomas Tiernan has resigned after nearly four decades working for the town of North Hempstead, which is investigating the highway department’s overtime use.

The resignation goes into effect Nov. 1.

A March Newsday investigation showed that over the past five years Tiernan, who became highway chief in 2000 and is the town’s highest-paid employee, collected more than $134,000 in overtime. In 2015, he was paid $178,537, including nearly $39,000 of overtime. Tiernan is the only town highway department chief on Long Island to receive overtime.

Tiernan, 54, is a member of the Civil Service Employees Association, which made him eligible for overtime, Town Supervisor Judi Bosworth told Newsday in March. He has worked for the town since 1980.

Tiernan’s resignation sets in motion a series of other shifts for the Highway Department, which maintains about 275 miles of roadway. After a unanimous board vote Tuesday night, Joseph Geraci, previously a salaried employee of the Buildings Department, will act as superintendent while the town does a full search for a replacement, says Deputy Supervisor Aline Khatchadourian. Geraci, who will also be promoted to deputy commissioner of public works with a salary of $140,000, is also a union member. He has worked for the town since 2008.

Douglas Schlaefer, previously a highway maintenance supervisor making $98,488, will get a $12,000 pay cut as a highway construction supervisor.

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Tiernan has several family members who are also town employees. His brother, John Tiernan, is a highway construction supervisor who made $115,751 last year, including nearly $27,000 in overtime. His son, Thomas Tiernan II, made $55,990 as a laborer handling parking in Port Washington, and his wife, Jill Guiney, made $128,366 last year as the deputy commissioner of public works.

In March, Tiernan’s sister, Helen McCann, was arrested by Nassau prosecutors and charged with embezzling $98,000 from the town’s Solid Waste Management Agency. She was fired in January from her position as an administrative assistant for the agency.

Nassau district attorney spokesman Brendan Brosh said Tuesday that the case is pending.

McCann’s departure was followed in May by the resignation of the executive director and deputy director for the Solid Waste Management Agency.

Bosworth previously told Newsday that she was unaware of the number of Tiernan’s relatives employed by the town and that the board was exploring a nepotism policy.

In March, the town formed an ethics committee, which is considering a nepotism policy, and the town board is discussing prohibiting town employees from supervising family members.