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    Mapping the futureThe National Highway System has designated 12 highway corridors as future interstates.Giving states the option of obtaining an interstate connector designation for routes designated as future interstates once they achieve AASHTO freeway standards would save billions over the long term with minimal impact on safety.Map: Federal Highway Administration

The desire for growth drives local governments to pursue interstate designation of highways, and since 1995 Congress has identified about a dozen — mostly rural corridors — as future interstates. Yet in the past eight years fewer than 100 miles have been added to the Interstate Highway System each year. During the 1990s about 200 miles were added annually, and about twice that many were added in the 1980s.

What happened?

The system has gone through many changes in its 50-year existence. In 1968 Congress decided that highways built with funds other than those set aside for interstate construction still could be designated as interstate highways. Even so, most were built through the Interstate Construction Program, which hasn't received an appropriation since 1996.

Congress had been identifying previously designated high-priority corridors as future interstates, and when the Safe Accountable Flexible Efficient Transportation Equity Act – A Legacy for Users (SAFETEA-LU) was enacted in 2005, it designated more than 4,000 miles of future interstates. The funding that would be required to build these roads is beyond the limit of any realistic restructuring of the federal highway program.

One cost-effective way to continue construction is to designate future interstates as “interstate connectors” and design them to American Association of State Highway and Transportation Officials (AASHTO) standards for non-interstate freeways, which are used for freeways on the National Highway System (NHS), rather than AASHTO's interstate standards.

The connectors would carry signing similar, but not identical, to an interstate shield. Other than that, there are three major differences between the two sets of standards. The interstate standard requires a higher vertical clearance for bridges, greater spacing between interchanges, and federal approval of all design exceptions.

There's a legitimate need for a solution that involves a reduction in cost as opposed to simply an increase in expenditures.

Consider State Route 99 (SR 99) in California between Sacramento and Bakersfield, which Congress designated a future interstate in 2005. In 2006, the California DOT (CALTRANS) developed a business plan for improving a segment between Bakersfield and Stockton totaling more than 90% of the corridor. Developed with local governments and municipal planning organizations, the $5.5 billion plan's priorities include widening critical freeway sections and adding interchanges.

In 2005 CALTRANS estimated that improving about 270 miles of the route to interstate standards would require an additional $10 billion to $15 billion. Although the agency expects more exceptions to the standard — particularly in regard to bridge vertical clearance and interchange spacing — the additional cost may still be $1 billion to $3 billion.

Similar situations exist in Kentucky, New York, Pennsylvania, Texas, and Tennessee, where designated future interstates could be “built” by upgrading existing highways.