All over the country, city and regional governments are writing “sustainability plans,” which are supposedly aimed at reducing greenhouse gas emissions. While the goal may be laudable, for the most part these plans won’t significantly reduce emissions. However, they will certainly impose huge costs on urban residents and taxpayers.

From Lafayette, La., to the Twin Cities, to the San Francisco Bay area, the heart of the plans consists of a one-size-fits-all prescription: make costly transit improvements in major corridors and then subsidize the construction of high-density housing in those corridors so lots of people will have access to transit. This prescription not only demands a huge change in American lifestyles, but also offers no reason to think it will help save the planet.

The Department of Energy, for example, has found that multifamily housing actually uses more energy (and therefore emits more greenhouse gases) per square foot than single-family homes. The only way multifamily housing would save energy would be if people accept smaller homes. A better solution is making single-family homes more energy efficient, which costs less and does not require the loss of privacy in multifamily housing.

Meanwhile, data from the Department of Transportation show that transit uses, on average, about the same amount of energy — and emits about the same amount of greenhouse gases — per passenger mile as the average car. Getting people out of their cars and onto transit won’t reduce emissions, but it will inconvenience a lot of people because transit is slow, expensive and inflexible.

Even if transit were truly greener than driving, the transit-plus-density solution doesn’t even reduce driving. Between 1980 and 2010, San Francisco Bay area population densities grew by more than 55 percent, and the region built more than 200 miles of rail transit lines and scores of high-density developments along those lines. Yet per capita transit ridership fell by a third while per capita driving increased by at least 5 percent.

Moreover, cars are rapidly becoming more energy efficient. It takes around 10 years (and huge amounts of energy) to plan and build a rail transit line, but 10 years from today the average car on the road will be at least 25 percent more fuel-efficient than cars today.

We can do a lot of things to emissions, but we have to ask whether they are cost-effective. It won’t do much good to reduce emissions if we bankrupt ourselves in the process, as our descendants will be too busy trying to survive to worry about the planet as a whole.

A 2007 report from McKinsey & Company suggests anything that costs more than about $50 per ton of abated emissions is a waste of money. Even using the optimistic assumptions built into sustainability plans, the transit-and-density strategy will cost thousands of dollars per ton — and it is more likely that it won’t reduce emissions at all.

While transit and density won’t significantly reduce emissions, it will have huge effects on cities. It will make traffic more congested and roadways less safe. It will make housing less affordable and increase other consumer costs. Besides, the increased tax burden will drive away jobs.

Population data clearly show that the fastest-growing urban areas are ones that have kept housing affordable by not using land-use regulation to impose lifestyle changes on their residents. For example, urban areas in Texas, which has some of the least restrictive land-use laws, are growing far faster than in California, which has some of the most restrictive laws.

Data also show that urban areas that spend more on transit grow more slowly. Of the nation’s 65 largest urban areas, the ones that spent the most on transit in the 1990s tended to grow slower in the 2000s than the ones that spent less. This doesn’t mean regions have to settle for poor-quality transit: in most places outside of New York City, buses can move as many people as fast and as comfortably as trains at a far lower cost.

In short, the transit-plus-density prescription imposes major costs on cities without significantly saving energy or reducing emissions. Nor does it cure obesity, end poverty, or bring about world peace, as some of its advocates seem to believe. Urban leaders need to be wary of people who propose policies that are anything but sustainable.