The Honolulu Authority for Rapid Transportation got a straightforward analysis last week from a federal consultant, and the news was basically good. Porter & Associates Inc. is vetting the city’s proposed rail project for the Federal Transit Administration in advance of the FTA releasing the full share of the $1.55 billion in federal funds. Its conclusion, that there is enough give in the HART financial plan for the project to make it sustainable, ought to boost confidence in the city’s capacity to handle the 20-mile rail system.

But it also projects that the finished transportation system, a network of buses complementing the core east-west alignment of the rail, is going to take a bigger bite of city tax receipts than the current bus-only transit service does now.

That figure is startling at first blush. Porter & Associates’ forecast is that transportation services will need 19.1 percent of revenues from city general and highway funds in 2021, up from 11.1 percent in 2011. That’s a peak figure: The share of revenues then will stabilize at an average of 17.5 percent through 2030, according to the report.

There are two takeaways for city policymakers and the taxpayers who fund their decisions, in reading this report:

» Any transportation system that will effectively serve the long-term needs of Oahu’s urbanization and suburban sprawl will cost more. Budget priorities need to be rebalanced to reflect that higher priority.

» The larger investment in the rail-bus network is worthwhile for reasons that go beyond getting commuters between Points A and B. It promotes a more efficient pattern of growth centered on that rail line, which means more land elsewhere on Oahu can be preserved for other needs and open space.

Taking the first point, the Porter study concludes that between 2011 and 2030, TheBus accounts for 67 percent of the operating cost, TheHandi-Van 15 percent and the rail 18 percent. So switching to a system that expands the driver-operated bus network, as former Gov. Ben Cayetano has proposed, is no real solution: It would still have a high operating cost while transporting far fewer people.

So the message here is that transit costs are inexorably going to rise, and the city must consider strategies for reducing other costs to accommodate that.

Ideas such as restructuring the bureaucracy to manage services more efficiently deserve more attention.

The proposed merger of the fire and emergency services departments is just one example.

Further, it may be time to consider privatizing some city services, such as parks maintenance, if that can be shown to be a cost-saver over the long term.

Second, the rail-centered transportation system will provide new opportunities for more compact urbanization along a limited corridor that, with careful planning, should yield more affordable housing units than sprawling developments do now. That, as well as the increase in property tax revenues that redevelopment should produce, make the transit system pencil out as a good investment.

Finally, it’s worth remembering that Porter’s job in producing its analysis was not to buff the case for rail to a gleaming shine. It was to take a hard-nosed look at a few worst-case scenarios and assure the federal government that its money would be well spent. And analysts found that even with a significant drop in general excise tax surcharge revenues or a 10 percent hike in costs, the city could still manage the construction and operation of the system through 2022.

Even in the midst of election-year pyrotechnics, such a clear-eyed assessment ought to defuse at least some of the criticism aimed at a project that will be an essential element of a well-run city.