With British children singing "The Wheels on the Bus" in the background, a new ad from the Amalgamated Transit Union Local 1181 shows a series of mangled and crushed post-accident school buses. "When inexperienced drivers take your kids to school," an ominous female voice warns, "sometimes, they never get there."

This fear-driven ad campaign, which borders on self-parody, is a last desperate push by the union's bus drivers and bus matrons, who have gone on strike and left about 113,000 Gotham schoolchildren without their normal means of getting to school. Never mind that New York City's union drivers also have accidents -- and that, in fact, members of the same national union drive the buses and trains in Washington and have proved somewhat accident-prone in recent years. Experience here shows that union membership does not prevent drivers from texting or even sleeping on the job.

The numbers behind New York City's story are striking. The schools there bus about 152,000 children to school each day, at the astounding cost of $7,000 per child each school year. That's $39 per day for each child -- enough, in many cases, to pay cab fare, and more than twice as much as the sprawling Los Angeles Unified School District pays.

Like many mayors before him, New York City Mayor Michael Bloomberg wants to rationalize this irrational system, in which many bus routes serve only a handful of students. Like his predecessors, Bloomberg is meeting stiff resistance. His attempt to put Gotham's notoriously inefficient bus routes out for a new round of competitive bidding provoked the strike, because his plan would not automatically guarantee jobs for the incumbent union drivers. The union has gone so far as to blockade the lots of bus companies that aren't on strike, prompting the city's police force to intervene.

The strike will probably be ruled illegal when it comes before the National Labor Relations Board this week, because it is victimizing the private bus companies to which the city contracts out its bus routes. These businesses are being forced to pay fines because their union workers won't work, even though they are not the target of the strike and in fact neither provoked it nor have any means of ending it.

But the entire episode provides a cautionary tale about unions for state and municipal governments as well as the contractors that serve them. Unions tend to resist efforts to reform a government's services to make them run more smoothly and serve taxpayers better. And for private companies -- even those willing to pay a modest union wage premium -- work disruptions and a loss of competitiveness are a constant danger. In this case, the striking drivers are stranding schoolchildren -- nearly half of them special-needs children -- in hopes of getting something that no other private employees have, namely a guaranteed job for life, even in the event that their company loses its contract.

It is well and good for workers at a company to demand better terms, whether or not they belong to a union. But in the event that those demands make their company structurally less competitive, so that it starts losing contracts to other firms, neither the city of New York nor any other government entity should shield them from the consequences of their actions. Mayor Bloomberg is right in principle -- he should stick to his guns and put in place the reforms that his predecessors failed to enact.