Watching public-transit ridership rise and fall with the price of oil is an interesting exercise, but not very helpful for long-range transportation planning.

Figures from the American Public Transportation Association show that between July and September last year — the most current figures released — bus ridership nationally dropped 8 percent. That was predictable, given that gas prices fell and the recession cost so many commuters their jobs.

But this shouldn't influence plans or weaken support for the city's $5.3 billion fixed-rail system, which is designed for the needs of O'ahu residents a decade from now and beyond.

Even in the short term, statistics provided by Oahu Transit Services, operators of TheBus, show riders here are pretty loyal customers. Job losses and cheap gas notwithstanding, ridership dropped only 2 percent in the last half of 2009.

And looking ahead, which is where transit planners have to look, only a fool would bet on gas remaining relatively cheap well into the future. (And have we conceded that $3.30 gas is cheap?)

China and India are slurping oil at a furious pace, just when global supplies appear to be waning.

The latest warning on this front came this month from a British task force on energy security. In its report, "The Oil Crunch," the business group warns that supplies may start a serious decline in as little as five years.

That, as well as the potential for recession-fueled inflation, will keep gas prices trending up — just when Honolulu's rail system could be nearing completion.

So we need to keep our eyes on the prize: building a reliable transportation system that will serve the needs of our children and grandchildren.