As I bike through Portland I often catch a glimpse of Interstate 5 and think: For a region with such great transportation, this freeway is awfully crowded. Traffic is congested for about six hours a day. When I hunted up several people who use the freeway during daily commutes and asked about their experience, they tended to describe it in unprintable terms. Who knows what they would have said if I'd mentioned the likelihood of tolls in their future?

The I-5 bridge that connects Washington to Oregon is the source of much of this congestion. It's the region's major north-south link, and about 130,000 vehicles cross it every day: commuters, vacationers and freight trucks. The bridge as it now stands is actually two bridges, one for northbound traffic and the other southbound, which were finished in 1960. Back then the bridges carried barely a fifth of what is carried today.

The Columbia River Crossing Project estimates that traffic across the bridge is expected to double by 2030, leading to solid congestion in all daylight hours, which the Portland Business Alliance estimates would cost travelers $844 million per year. In other words, this bridge system needs to be replaced with something bigger.

Thus, a new bridge has been proposed. Though the bridge's final form is yet uncertain, it will certainly allow greater traffic capacity and have provisions for mass-transit systems, along with better access for bikers and pedestrians. The cheapest option on the table is estimated to cost close to $3 billion. Given the economic climate, neither the federal government nor the state governments are prepared to cover the cost of building and operating the bridge. Carly Francis of the Columbia River Crossing Project, the organization responsible for planning the bridge, says that while funding remains the biggest hurdle, it is clear that any new I-5 bridge will need to have tolls.

After the 2007 collapse of the Interstate 35 bridge in Minneapolis injured 145 people and killed 13, investigations and reassessments of transportation infrastructure were done nationwide. This brought public scrutiny to numbers that had been scaring experts for years:

Of the roughly 600,000 bridges in the country, 25 percent are more than 50 years old. Some 13 percent are rated structurally deficient and in urgent need of replacement. The gap between current highway funding and investment needed to update the system is, according to a 2006 US Department of Transportation study, roughly $61 billion annually. It would cost more than $32 billion to replace the structurally deficient interstate bridges alone.

Various methods have been used to raise funds for such updating. One way is to impose a tax; the gas tax (first started in Oregon in 1919) in particular is generally devoted to funding transportation infrastructure, and was instrumental in the construction of the Interstate Highway System. While both federal and state gas taxes continue to finance similar projects, the revenues they extract have been shrinking, and inflation cuts deeper into what each dollar can buy. The growing number of more fuel-efficient vehicles affects this source of funding too.

Proposals that tax all fuel purchased in the state are popular, but problematic. Most legislators realize that the more politically and financially astute move is to pass the cost of a major project only to the people who benefit from it.

Enter the toll road.

Tolls are nothing new to the Northwest. The first I-5 bridge, opened to traffic in 1924, charged a toll for the first five years of operation; the opening of the second span in 1960 came with a toll as well. Both tolls were removed by popular demand after they raised sufficient funds to cover construction costs. There are major toll roads in Washington and Oregon, and more being planned. The Northwest has fewer tolled miles than most other states, and it is increasingly clear that this won't last forever.

Though the impact of a toll road could be measured by the degree of frustration it engenders among drivers, communities tend to get used to them and even (gasp) sing their praises. And tolls usually bring substantial, measurable benefits: the most frequently cited is the assured repayment of construction loans and operation costs. (This also allows governments to free up more money for, say, hiring drivers for legislators, and writing long reports about zoning and traffic.)

Another clear benefit is the immediate, sustained reduction in congestion. Though demand for the transportation link is, as the economists say, relatively inelastic, even small reductions in the number of vehicles means more efficient travel, which brings many other benefits. Those include: greater mass-transit ridership and efficiency; better traffic flow and speed, fewer resources used and less air pollution; cost reduction for commuters, truckers and vacationers alike.

But, of course, there are downsides: Traffic slows down at toll booths; alternate routes will see heavier use; and people with lower incomes and/or fewer employment options are hit hardest.

There are a few standard and widely-used ways to pay for tolls, the most prominent being the flat fee, mileage fee, and congestion fee. These are exactly what they sound like. The flat fee is most often charged over short distances with limited exits, like a bridge or tunnel. The mileage fee involves giving the driver a ticket, which is paid upon exiting the road. The congestion fee is charged up front, with the price varying either by anticipated traffic or the actual vehicle count. Regular users are typically given devices that log their passage automatically, which keeps traffic disruption at a minimum, but still forces transient drivers to pay by cash or credit.

A newer idea, made possible by technologies developed for law enforcement, is to take a picture of every license plate that passes the camera; this means that traffic is not disrupted at all, and a bill is later sent to the car's owner.

Whatever the means, one thing is certain: The government's experts love the idea of tolls. Dr. James Moore, a noted expert on toll roads at the University of Southern California, emailed to me that economists believe "the only systemic solution to congestion is tolls. The more widely they are implemented, the greater the efficiency gains."

So, unless the opposition to tolls, which is now scattered and ineffective, gets much more organized — and does so quickly — expect to see the toll collector coming to bridge or freeway near you.