Since Puget Sound voters rejected Roads & Transit last November, various transportation wonks have called for economizing on light rail at the expense of South King and Pierce County.

The argument: The proposal to extend rail from Sea-Tac Airport to Tacoma unnecessarily inflated the price tag of the transportation package. Why? Because the wannebe train commuters in this southern corridor are much less numerous than they are around Seattle.

There’s no quarreling with that latter assertion. Ridership would be lower on the south end of the Sound because the population is smaller. The potential demand right now may be too low to justify the expense of light rail.

Right now. But that’s going to change. The South Sound population will expand rapidly in the next 20 years and for decades beyond. Projecting the future using a snapshot of existing trendlines is always a risky business.

Sound Transit has just sprung a surprise on the region: Over the course of 2007, ridership on all its trains and buses jumped 12.3 percent – well beyond the agency’s own projections. The numbers get especially interesting upon closer examination.

Commuter rail – the trains that run from Tacoma through the Kent Valley to Seattle, and beyond to Everett – led that one-year surge with a 27.4 percent increase. And the Tacoma-to- Seattle commuter rail connection led the whole trend with 32 percent.

Sound Transit itself pushed the southern numbers up by adding two new trains to the route. That’s not cheating. Where the demand exists, ridership will go up if capacity is expanded and will go down if capacity is denied.

The growing demand for mass transit to King County is also evidenced by the 22 percent increase in riders on Sound Transit express buses from South Hill to Overlake and from Tacoma to Seattle, as well as the 24 percent increase from Lakewood to SeaTac.

It’s safe to surmise that some of the unexpected new demand has been driven by distressingly high gas prices. Another factor was last summer’s overhaul of Interstate 5, which drove motorists to the trains and buses – many of whom discovered they liked transit. Daily traffic backups on I-5 and Highway 167 also make a constant, aggravating case for switching.

None of this is going to go away in coming decades. The cost of driving an automobile will only go up until 2027 – when Sound Transit expected to reach Tacoma with light rail – and beyond. Congestion’s going to get worse, too. Growing concern over greenhouse gas emissions may also persuade people to use the greener alternative of mass transit.

This is a dynamic with multiple variables – any or all of which could drive up demand for transit at unanticipated rates. The South Sound could use light rail now. In 20 or 30 years, the region will desperately need it.