So many ideas; so little thought. I refer to that geyser of public policy wisdom known as Sacramento, from which the latest nostrum is the replacement of the per-gallon gasoline tax with a tax on miles driven, the latter to be monitored with a tracking device placed in autos.

This proposal is misguided economically and dangerous politically.

The economic idea is that the tax shift would raise additional revenues for road construction and maintenance, although that depends heavily on the magnitude of the new mileage tax and the ensuing responses of drivers.

More important, there is no guarantee that any additional revenues in fact would be spent on roads; Sacramento has circumvented the existing requirement that the revenues from the sales tax on gasoline be spent on roads by borrowing the funds and spending them on non-road programs. This practice did not end with the arrival of Arnold Schwarzenegger, and given the looming deficits of $7 billion and counting, it is likely to continue. Accordingly, the mileage tax "for roads" likely would become a general-fund revenue source.

Many seem also to assume that retail gasoline prices would fall with the elimination of the per-gallon tax. That is almost certainly incorrect because the California gasoline market is supply-constrained by refinery capacity and by various environmental and other regulations and capacity limitations. Were gasoline prices to fall substantially because of the elimination of the per-gallon tax, the quantity of gasoline demanded would exceed production capacity and the resulting shortages would drive market prices back upward.

In other words, the "market-clearing" price of gasoline - the price that equates supply and demand - would not change with the elimination of the per-gallon tax, unless the new mileage tax were sufficiently large to reduce gasoline demand sharply, a level of taxation unlikely to prove viable politically.

And so gasoline prices would change little, but the government revenues now yielded by the per-gallon tax instead would be collected by the gasoline producers in the form of higher refinery margins. As an aside, this actually would be a good thing - notwithstanding the views of some, "profit" is not a four-letter word - in that the increased profitability of gasoline production would improve market incentives to increase refinery and pipeline capacity.

This increase in refinery profitability would yield calls for price controls from the usual suspects, but anyone with memories of the gasoline lines of the 1970s knows what a disaster such policies are and must be, always and everywhere. Nor would the imposition of price controls be salutary for our efforts to acquire increased fuel supplies from outside the state.

At the political level, a mileage tax is likely to transfer wealth among geographic regions and across economic sectors, and in other complex ways driven by the details of the tax shift; thus will bureaucrats and politicians be given a new tool with which to bestow benefits upon favored groups at the expense of others.

The gasoline tax has the virtue of tying the tax burden not only to road use but also to peak congestion costs and to the weight and size of vehicles, however crudely. A mileage tax in principle could be structured to achieve such efficiencies, but there is little reason to believe that public agencies have net incentives to do so.

More ominously, do we really want a tax system under which government obtains license to track our movements? For all the certain-to-come political "guarantees" on privacy, nondisclosure to other agencies, destruction of records, etc., it simply is inevitable that rationales to use the individual driving information for other purposes will sprout like mushrooms after a rainstorm. Counterterrorism. Narcotics enforcement. Immigration restrictions. The battle against street crime. Deadbeat parents. Ad infinitum.

The basic rationale for this proposal is the decline in gasoline tax revenues relative to road use, caused in substantial part by the shift toward more fuel-efficient autos. Well, in the face of adverse shifts in market conditions the private sector finds it necessary to improve efficiency.

Let government do the same. Union pay scale restrictions on road construction and maintenance can be eliminated. Public sector pensions and other costs can be made more competitive. Outsourcing and privatization can be emphasized more heavily. The huge delays and cost increases caused by environmental fanaticism can be overcome with the application of political courage.

Instead, the Sacramento establishment is talking about a significant new tax to be imposed upon the citizenry writ large, without reductions in gasoline prices, but with more political gamesmanship and threats to our liberty. Amazingly, this is what passes for "new thinking" in Sacramento. Let us pray.