WHEN A PUBLIC agency’s job description, by necessity, involves socking it to the traveling public so dramatically that it might triple toll rates over the next six years, it had better make sure its own house is in order. People will be furious enough at the hit to their wallet without further inflaming them with stories of the agency’s fast-and-loose management practices.

Yet the Metropolitan Washington Airports Authority, the agency that runs Dulles and Reagan National airports as well as the Dulles Toll Road, has generated such a steady stream of low-grade scandal — baroque expense accounts; sweetheart contracts for those with insider connections; money frittered away by mismanagement — that it’s a wonder toll road commuters haven’t staged a general strike.

The latest outrage, detailed by The Post’s Cheryl W. Thompson, provides forehead-smacking detail to what was already known about another of the airports authority’s internal shenanigans: its anything-goes attitude toward nepotism.

In The Post’s account, members of the authority’s board of directors, as well as bureaucrats in the authority’s upper echelons, have used the agency as an ultra-convenient jobs program. This is particularly true of its student program, a sort of year-round internship that directors and officials alike have used to help their children and other relatives. But the habit of hiring relatives at the 1,400-employee agency has been so ingrained that it has produced employee rolls that more closely resemble family trees.

Take the Kulle family — mother Helen, her son and daughter, the children’s spouses and her daughter-in-law’s brother. All were employed by the agency — collectively pulling down more than $400,000 last year, not counting overtime or bonuses — and all were apparently hired without effective safeguards against nepotism.

In interviews, board members and employees suggested that they had done nothing wrong, that hiring relatives is the norm in the public sector and that relatives had been hired on their merits. But the fact remains that the agency turned a blind eye to the entire topic, treating nepotism as a non-issue. That is unacceptable.

Under pressure from Congress and the Transporation Department’s inspector general, the authority seems to have begun cleaning up its act. An ethics policy adopted this fall and set to take effect Jan. 1 prohibits employees from hiring or promoting relatives or doing anything to help relatives to be hired or promoted. A similar rule applying to board members has already taken effect.

Inside the agency, some heads have rolled. Several employees, including the authority’s most senior personnel official, have resigned or been removed over criticism in a report issued by the inspector general last month, which covered nepotism, among other problems.

The most worrying aspect of all this is that the authority, in addition to running the airports and toll road, is also building Metro’s Silver Line extension to Dulles airport. That project is half-built; finishing it hinges on financing and political support, both of which remain uncertain. Anything that jeopardizes its completion must be urgently addressed — and that includes mismanagement and nepotism inside the authority.