If there were an Interstate 1, it would likely pummel its way through California’s coastal regions.

To understand why, you’ll need to know a little about the clever numbering scheme that governs which interstate routes get which numbers. Each route number conceals coded information about its highway’s direction, geographic location, and function:

Direction. Odd-numbered highways (e.g., Interstate 15) run north-south and even-numbered highways (e.g., Interstate 80) east-west.

Odd-numbered highways (e.g., Interstate 15) run north-south and even-numbered highways (e.g., Interstate 80) east-west. Geographic location. Route numbers are also ordered from west to east and south to north; Interstate 5 is thus west of Interstate 15 and Interstate 8 south of Interstate 10.

Route numbers are also ordered from west to east and south to north; Interstate 5 is thus west of Interstate 15 and Interstate 8 south of Interstate 10. Function. Finally, a hierarchy of route numbers reveals each highway’s function. One or two-digit route numbers ending in 0s and 5s indicate trunk lines; a number ending in 1, 2, 3, 4, 6, 7, 8, or 9 indicates a lesser highway; and three-digit numbers indicate spur routes if they begin with an odd number (e.g., Interstate 105) or a bypass route if they begin with an even (e.g., Interstate 405).

Because 1 is an odd number, Interstate 1 would run north-south. And because it’s lower than 5, it would be located to the west of Interstate 5, the lowest odd-numbered interstate in existence. That would place it along North America’s western margin, where the Pacific foams against a fractured landmass.

1958 map of the interstate highway system. A hypothetical I-1 would run to the left of I-5. Courtesy of the Library of Congress.

1962 map of California's freeway and expressway system. As the map shows, long stretches of the California coast resist highway construction. Courtesy of the David Rumsey Historical Map Collection.

The problem for a hypothetical Interstate 1, then, is that the rugged character of the California, Oregon, and Washington coasts would resist the imposition of the Interstate Highway System’s strict design requirements – minimum lane widths, maximum curvatures, maximum grades, and so on. Interstate 5 accomplishes its trek between the Mexican and Canadian borders by taking an inland route and thus avoiding such difficult terrain. (It doesn’t sidestep challenging topography altogether, of course, as anyone who’s ever coaxed a groaning car up the Siskiyou Summit or the Grapevine can attest.)

A hypothetical Interstate 1 would trace North America’s western margin, where the Pacific foams against a fractured landmass.

In some places along the West Coast, constructing an interstate highway would be nothing short of an engineering miracle. Few highways dare to venture through California’s Coast Ranges, the Klamath Mountains, or the Oregon Coast Range. The ones that do, like California’s State Route 1, are notoriously slow, narrow, and curvy – and even Highway 1 bypasses the treacherous geology of northern California’s Lost Coast.

Perhaps the only plausible pathway for an Interstate 1 is already occupied, by U.S. Highway 101. In theory, U.S. 101 could become I-1 were it upgraded to interstate design standards. Where the highway now traverses oak-dotted hills or slices through redwood forests, a widened and straightened interstate would roar. Where it now serves as main street through small coastal towns, a new superhighway would bypass roadside businesses altogether. Where it zig-zags through San Francisco’s street grid, a new urban freeway would rise.

Why no Interstate 1? Numbering schemes and engineering challenges – not to mention costs – partly answer the question. But perhaps a better answer, and one that many residents of California’s coastal communities would offer, simply points to good sense.