Traffic deaths soared 13.5 percent in the first quarter of the year compared to the same period last year, and the number of deaths per miles driven also rose significantly, according to preliminary government estimates released Friday.

An estimated 7,630 people died in motor vehicle crashes in the first three months of 2012, up from 6,720 deaths in the first quarter of last year, the National Highway Traffic Safety Administration said. It's not unusual for preliminary estimates to change, but revisions usually don't vary significantly.

If the estimate holds true, it would be the second largest year-to-year quarterly increase since the government began recording traffic fatalities in 1975. It would also run counter to historic declines in deaths over the past four years. In May, the safety administration, citing preliminary estimates, said total traffic deaths last year were the lowest in six decades, and the rate of deaths per miles driven was lower than at any time since 1921.

That rate was back up in the first quarter of this year at 1.1 deaths per 100 million vehicle miles traveled compared to .98 fatalities in the first three months of 2011. Less dramatically, the number of miles driven by motorists also increased by 1.4 percent during the period.

The safety administration didn't offer any explanation for why fatalities would have risen so markedly, but safety experts said a recovering economy and unusually mild winter may have been a factor. People drive more in good weather, particularly motorcyclists, and when the economy is thriving.

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"We are disappointed in the news, but not particularly surprised," Barbara Harsha, executive director of the Governors Highway Safety Association, said. "Unprecedented gains have been made since 2006 in reducing traffic deaths. So, from that low baseline, an increase is not unexpected."