A recent news article reports that the cost for raising and widening the bridges over Interstate 66 at the U.S. 29 interchange near Centreville will be $17.5 million (43 percent) more than initially planned. Part of this extra spending will come from Fairfax County and part from the private companies that will collect money from the I-66 express lanes they are constructing.

The reason for this additional cost is to make room for a possible extension of the Metro’s Orange Line from Vienna to Centreville.

This extension is part of northern Virginia’s long-range transportation plan, but extending the Orange Line 11 miles further would likely cost more than $2 billion, and it’s not a cost-effective way to provide transportation to western Fairfax County. It doesn’t make sense to spend 43 percent more on the I-66 project in preparation for a possible Metrorail expansion.

Heavy rail transit service has a very high capital cost per mile. Paying such a cost can only be justified if there are enough additional riders who might use the Metro each day, which depends on how many people live or work within walking distance of each station. Stations located in densely populated urban centers can attract sufficient riders to fill frequent trains, but Fairfax County (particularly west of Vienna) has a population density that’s much too low to generate enough fare-paying riders to come close to covering the costs of expanding the Orange Line.

Besides the additional cost likely to be borne by highway users and county taxpayers, in order to make sure the bridges are high enough and wide enough, the capital costs of extending the rails further west—along with much of the operating cost for the additional distance traveled by the trains—would need to be covered by government subsidies. That’s money that could be spent on other priorities such as better maintaining the existing Metro rail lines, trains and stations.

Other types of public transit, like express buses, would be more cost effective in this growing low-density suburban corridor. By providing a congestion free commuting alternative, express toll lanes make it possible for buses to reach their destinations more quickly.

A much larger percentage of Fairfax County residents commute by car than by public transportation, and they do so for good reasons. Many live or work in places not well served by rail or bus transit. Even if they could get to the Metro by bus, the time it would take them to commute using transit is likely to be much longer than the time spent driving to their destinations.

Advocates of expanding the Orange Line argue that the areas around Metro stations can become more densely populated if the county encourages transit-oriented development (TOD) at those locations. TOD only works, however, if people want to live—or businesses want to locate—in densely populated neighborhoods close to transit stations. Fairfax County already has enough land close to Metro stations where TOD is being promoted to accommodate any new businesses and households that might be willing to locate in such areas for decades to come.

— Tracy C. Miller is a senior policy research editor with the Mercatus Center at George Mason University. He holds a Ph.D. in economics from the University of Chicago.