Berkshire Hathaway Chairman and CEO Warren Buffett(Photo: Nati Harnik, AP)

OMAHA, Neb. – Investor Warren Buffett says distracted driving appears to be a growing problem on American roads that contributed to an increase in deaths last year and hurt profits at his company's Geico insurance unit.

Buffett said on CNBC Monday that auto insurance rates are going up this year because of the increased occurrence and severity of accidents last year.

The National Safety Council recently estimated the number of traffic deaths in the United States rose 8% from 2014 to 2015, the largest year-to-year percentage increase in a half-century.

Geico's underwriting profit plummeted to $ 460 million last year from 2014's $ 1.16 billion.

Over the weekend, Buffett released his annual letter to Berkshire Hathaway shareholders.

Buffett's letter, which reviews the operations of the conglomerate over the previous year, is one of the most widely read business reports.

In other topics:

•  Buffett says the U.S. economy appears weaker than he thought it would be as recently as last fall, but that doesn't change his optimistic long-term view of the country's prospects.

Buffett says the collapse in oil prices will be good overall for the American economy because consumers will have more money to spend.

But he says the negative effects of falling crude prices hit the oil industry immediately in terms of lost jobs and reduced company values. Buffett says the benefit to Americans trickles in slowly every time they fill up their gas tanks.

• The billionaire investor also said he can't predict how having interest rates negative for a prolonged period will affect the economy because it has never happened before.

He said that Berkshire Hathaway's insurance units in Europe will lose money on the cash they have to keep in banks. He joked that Berkshire would be better off stashing cash in a giant mattress instead of banks if only he could find a trustworthy person to sleep on top of a billion euros.

Buffett says he understands why regulators cut interest rates to help Europe recover, but there's no way to know how that will affect business.

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