Gaze out your window at the cars going by. Now pretend you will be paid a quarter every time one passes.

These days, the world's largest investors are making similar calculations, and they like what they see. Stable cash flow is something to cherish, with volatility whipsawing stocks and the debt market nursing the sting of billions in collateralized debt obligations gone awry. So the once-sleepy backwater of infrastructure investment—the buying up of public roadways, utilities, airports, hospitals, and even prisons—has become fertile ground for Wall Street titans.

The problem is that the chief benefit of the investment—a safe long-term inflation-adjusted return—becomes harder to reach as more investors pile into the market seeking it.

Experts worry that prices have already been driven too high for many of the best assets. As evidence, they note that more deals are being loaded up leveraged-buyout levels of debt. At the same time, yield-hungry investors are increasingly willing to build or buy infrastructure in riskier corners of the world.

Is infrastructure fated to become the next asset bubble? Ryan J. Orr, executive director of Stanford's Collaboratory for Research on Global Projects, says: "We're already there."

Such concerns aren't stopping the party. Financial-services companies continue to flood the market with new infrastructure funds and new ideas about putting these funds to work. Some of those new ideas involve less-traditional assets like lotteries, gas stations, and old folks homes.

Stanford's Collaboratory estimates that more than 72 new infrastructure funds have been introduced since the beginning of 2006 and that more than $160 billion has been raised during that period for infrastructure investment.

Robert Dove, co-manager of Carlyle Infrastructure Partners, said new funds are cropping up because "there is certainly an appetite in the investment community for more stable, predictable investments." He adds that Carlyle, which closed its first infrastructure fund in November after raising almost $1.2 billion in about 18 months, sees "great opportunities" in the sector.

It's an opportunity based on hard times, as governments around the world will have difficulty paying for what is estimated to be $53 trillion in needed infrastructure investment over the next 25 years. This leaves the door open for private investors to offer a helping hand—for a price.

At the same time, some of private equity's biggest clients, pension funds and other institutional investors, are looking at the possibility that one day they might not meet growing needs going forward. Underfunded pension plans in the United States currently have liabilities of more that $1.2 trillion but assets of only $875 billion. Infrastructure investment is seen as playing an integral role in helping to close this gap.