Building and maintaining roads, bridges and tunnels is an expensive proposition, and rising costs mean that municipal, county and state budgets often fall short. Raising tolls, or raising the taxes levied on gasoline are both ways to address revenue shortages, but neither are popular with drivers.

A nine-county regional agency in Northern California, the Metropolitan Transportation Commission (MTC), is considering a plan that would tax motorists by the miles actually driven. While the plan sounds fair on paper, it relies on technology than most drivers would object to having in their personal vehicles.

In order to measure miles driven, cars would likely be equipped with a GPS tracking device, and as Wheels Blog points out, that raises all kinds of privacy concerns. After all, couldn’t such a system also track where a vehicle was driven, at what time and at what speed?

The MTC is quick to point out that, “tracking people is the last thing we care about,” according to spokesman Randy Rentschler. The vehicle tracking scenario is also just one of many under consideration, and Rentschler further clarifies that, “’s way too early to say if any plan would track people’s mileage with GPS. That’s one way it could work.”

Critics say that taxing by miles driven would unfairly impact lower-income drivers, requiring any such fee to take income into consideration. Still, a user fee is seen as more relevant than raising sales taxes, as doing so would impact those who opt not to drive.

The MTC is hardly the first agency to propose such a plan. In 2009, a bipartisan Congressional commission, the National Surface Transportation Infrastructure Financing Commission, recommended a switch away from taxing gasoline to taxing miles driven by 2020.

No detailed discussion of how to implement such a system was provided, and the group’s ideas were never implemented. As this idea keeps turning up in discussion, it seems to us that a road tax based on miles driven is inevitable.