Warren Buffett's Berkshire Hathaway Inc. sees a green light at the intersection of low oil prices and record driving. 

In its most significant energy investment in two years, Berkshire has amassed a $4.5 billion stake in Phillips 66, making it the biggest shareholder in the largest U.S. oil refiner. Berkshire owns almost 58 million shares in Phillips 66, more than 10 percent of the total outstanding, up from the 7.5 million it reported at the end of the first quarter, according to a regulatory filing issued late Friday by the Omaha, Nebraska-based company.

The bet on fuel processing is a wager that an unexpected and significant rally by refiners during the shale boom will continue as low oil prices spur demand for gasoline, diesel and other petroleum products produced by the so-called downstream sector. Since oil fell by half last year, gasoline demand in the U.S. has surged to an 8-year high, as drivers see per-gallon prices fall below $3 and take to the road.

"As oil prices have dropped, it's obvious to people in the business that refiner stocks are your best bet," Carl Larry, head of oil and natural gas for Frost & Sullivan, said by phone from Houston. "When people say the oil industry is failing -- well, upstream might be, but downstream has never been better."

An index of four refiners on the Standard & Poor's 500 is up 12 percent this year through Friday, compared with the larger energy index, which is down 18 percent. Phillips 66 closed at $77.23 on Friday in New York, up 7.7 percent this year. The increase at Phillips 66 has trailed the results of some other U.S. refiners -- Valero Energy Corp. has gained 19 percent this year and Tesoro Corp. has climbed 26 percent. Marathon Petroleum Corp. is up 4.6 percent for the year.

The number of miles traveled on U.S roadways rose to the highest level ever in June, according to the Federal Highway Administration. Previously, American drivers facing higher prices, the 2008 financial crisis and a program that paid for replacing older cars with more fuel-efficient models were spurring forecasts that U.S. demand had peaked.

Berkshire has been interested in Phillips 66 since the company was formed after being spun out of ConocoPhillips in 2012. Berkshire held more than 27 million shares in the refiner for more than a year. About two-thirds of that holding was swapped for one of Phillips 66's businesses last year in a transaction designed to limit Berkshire's tax liability.

Since the spinoff, Phillips has more than doubled in value along with a number of other refiners, surging as new U.S. drilling produced an abundance of crude, allowing the companies to purchase oil domestically for less than it was selling abroad.

"Lower oil prices are precipitating an upwards revised forecast for world demand," Andy Lipow, president of Lipow Oil Associates in Houston, said by phone. "Refiners are going to see decent margins going forward."

The company still faces questions about what will happen with DCP Midstream, a pipeline and processing partnership it co-owns with Spectra Energy Corp. The two owners have disagreed on whether to put more money into the venture, which has suffered from low natural gas prices and high debt.