The Georgia Regional Transportation Authority seems committed to building commuter rail, starting with a line to Macon. Chairman Walter "Sonny" Deriso recently spoke as if there were no alternative when he said, "It's cheaper to do this than it is to double-deck our interstate system." Well, not really.

Admittedly, it is more expensive to double-deck a 10-lane freeway than to build a commuter rail line. But such a comparison is misleading and irrelevant because the two are not the same thing.

Transportation is like potatoes. It is not the cost of the sack that is important, it is the cost per quantity. Because sacks come in all sizes, potatoes are sold by volume (pounds), and one expects to pay more for a 25-pound sack than a five-pound one.

And commuter rail is small potatoes indeed. The nation's new commuter rail systems are carrying passenger volumes less than 5 percent of that carried by a single adjacent freeway lane. As with potatoes, the cost of transportation is measured in volume, and miles tell us nothing. The appropriate measure of volume is total miles of travel (the technical term is person miles).

When measured in terms of total travel, commuter rail is generally more expensive than freeway projects, including those that are double-decked. For example, a major segment of San Antonio's freeway system has been double-decked. The cost of a double-decked mile of lane operating in both directions on this facility was under $20 million.

This is more than the $6 million-per-mile average cost of the nation's new commuter rail systems. But adding the dimension of travel volume changes everything. The full cost of San Antonio's double-deck freeways per person mile was at least 60 percent below commuter rail.

But even the most expensive highway projects can be less expensive than commuter rail. In recent years, considerable national attention has been focused on Boston's Central Artery. Also called the "Big Dig," this largest highway project in American history is one-half in tunnel or tube and is costing $14 billion. This is more than 10 times as expensive as building a double-deck freeway in San Antonio. Even so, assuming average Boston freeway travel volumes, the cost per automobile person mile is slightly below the average of the nation's new commuter rail systems.

Moreover, not all freeway expansions require expensive double-decking or even more expensive tunneling. Virtually all of the commuter rail lines GRTA envisions travel through long expanses of very low-density and rural territory, where construction costs would be considerably less.

All of this is not to suggest that Atlanta should double-deck its freeways. It is, rather, a call for transportation plans based upon reason, not tired, unproven, yet politically correct cliches.

GRTA recognizes that automobile use will continue to grow rapidly. Yet it has approved a transportation plan that spends too much on marginal strategies that produce too little. Its own data show that spending 55 percent of available funding over the next quarter century on transit will add less than one percentage point to the share of travel by transit.

A coherent transportation plan would also take actions to make the situation better. But the prospect for Atlanta traffic is worse, not better, with GRTA's plan.

Everyone knows that Atlanta has some of the worst traffic in the nation. But not many are aware that Atlanta's traffic density (traffic volume per square mile) is below average, and lower than that of Fort Wayne, Ind. (an urban area smaller than Augusta).

What gives the Atlanta area its world-class traffic is its backwoods arterial street system. Throughout the Atlanta area, once a freeway is exited, most travel is on winding two-lane country roads that are just not up to handling the traffic of a 21st century urban area.

Improving the arterial street system would both supplement the freeway system and provide alternatives to it. This and other automobile-oriented strategies need to be implemented to get traffic moving. GRTA needs to to recognize another reality: that people will not forsake their cars for transit --- such as commuter rail --- that either does not go where they are going or gets them there too slowly.

Wendell Cox, an adjunct scholar at the nonpartisan Georgia Public Policy Foundation, is principal of Wendell Cox Consultancy, a St. Louis-based public policy firm.