Upon completion in 1813, the Casselman River Bridge was the longest single-span stone arch bridge in the United States. (Robert Thomson/The Washington Post)

In far western Maryland stands a magnificent old stone bridge arching high over the Casselman River. The land around it is Maryland’s smallest state park, yet it’s easy to see the draw for many tourists.

It’s beautiful, and impressive. When built in 1813, the 80-foot stone arch span was the longest of its kind in the nation. The stone bridge was so cutting-edge that skeptics doubted the structure would stand once its supporting timbers were removed at the end of construction. What a tribute to American engineering.

What a waste of time.

The magnificent arch soars over a pretty little stream that never wound up carrying the boat traffic its planners foresaw when construction was authorized in the early 1800s.

Casselman bridge approach Today, no traffic crosses the bridge, a modern day tourist attraction. (Robert Thomson/The Washington Post)

The roadway that leads up to the bridge is another matter. That was the route of the National Road, or National Pike, an early transportation project that created a gateway to the West for the growing United States.

One nearby plaque reads: “During the Golden Age of ‘The Pike,’ 1842-52, traffic on the road was immense. Up to 14 stagecoaches per day, each way, used the road to carry passengers as far as the western frontier. … Traffic on the road included not only stagecoaches, freighters and Conestogas, but also droves of cattle, sheep, pigs and turkeys. Public inns, legally known as Ordinaries, flanked the highway, about one per mile.”

But the bridge itself was over-designed. It was built so big to accommodate river traffic on an extension of the C&O Canal from Cumberland, Md. But as the railroad network developed, the rationale for the canal diminished and the extension never was constructed.

Another plaque tells tourists of the span’s more recent role: “Restored in 1911, the Casselman Bridge is now listed on the National Register of Historic Sites. It has become a world renowned tourist attraction, a delight to photographers, artists and historians and is often prominent in photographic and art exhibitions.”

From infrastructure to entertainment.

Since I first saw the bridge last spring, I’ve been thinking about this relic as a lesson for today’s transportation planners. But what’s the lesson?

Your analogy here

The arch is a monument memorial to the lesson that growth patterns and technologies evolve, and even over a short period of history, they can take unanticipated directions that make project planners look foolish. But almost any project can be made to fit the historical analogy. In the D.C. area, we could make it work for highway construction: building the proposed Bi-County Parkway, expanding the capacity of Interstate 66, building a new Potomac River crossing. And we could make it work for transit projects: the Silver Line, the Purple Line, the D.C. streetcar network.

We could apply it to planning concepts, as well. Virginia has a new law that requires the state to prioritize many of its transportation projects by their ability to relieve congestion. At best, that’s going to be today’s educated guess about tomorrow’s traffic demand.

If you want to build support for a project, make an analogy to the National Road, an infrastructure project that enhanced mobility and spurred growth for decades. Want to raise doubt? Compare the project to the Casselman River Bridge, an engineering marvel way too magnificent for the stream it spans.

The one lesson that could work for everyone concerned with transportation projects is the need for humility. In the heat of battle over some of our local projects, supporters and opponents sometimes make projections that are way too optimistic or pessimistic. Many efforts will eventually be judged as a combination of National Road/Casselman Bridge. Some elements delivered on their promise, and some didn’t.

And there’s no comfort in thinking of the nation’s early history as a unique period of trial and error. Our vision remains clouded.

When Harriet Tregoning was director of the District’s Office of Planning and trying to see into the region’s future, she struck just such a note of humility in a discussion with other planners.

“People are changing modes [of travel] much more quickly than we ever imagined,” she said. A transportation project that looked smart when first proposed “may be a really dreadful idea 10 years from now.”

Or not.