Icy winter roads and vacation-clogged summer highways might seem the most dangerous for motorists, but new research says fall is when driving is at its deadliest.

A new report has found that October has the year's highest death rate per distance driven. In fact, the danger of dying in a crash is 16 percent greater in October than in wintry March — the safest month of the year.

"Everything else being equal, inclement weather — snow and ice — should increase the risk of driving," said Michael Sivak at the University of Michigan Transportation Safety Institute in Ann Arbor. "However, because inclement weather also leads to general reductions in speed, the net effect is not clear."

Sivak analyzed monthly crash death figures for the U.S. from 1994 to 2006 and reported his findings in the July issue of the journal Traffic Injury Prevention.

October had a death rate of 10.2 per billion kilometers, compared with March's 8.8 per billion kilometers, he found.

November and December were next in deadliness, while February and April followed March in safety, Sivak said.

He said he knows of no single reason why October driving is the riskiest and March the safest.

"This pattern is likely the consequence of joint contributions of several factors," he wrote, saying future research could examine the specific seasonal effects.

A key factor could be daylight, said Peter Kissinger of the Washington-based AAA Foundation. October combines the relative warmth of early fall with the lengthening hours of darkness.

"The risks at night are substantially higher than during the day," Kissinger said.

In the 13 years covered by Sivak's study, traffic deaths in the U.S. fell from 10.7 to 8.8 per billion kilometers driven.

The National Highway Traffic Safety Administration announced this month that 7,689 motorists were killed in January through March this year, a 9 percent decline from a year ago and a low not seen in nearly half a century. Traffic safety advocates said there's little reason for pride or complacency.

"Setting aside the days with the most risk, we often lose sight of the fact that motor vehicle deaths happen like a constant drumbeat every day of the year," said Russ Rader, spokesman for the nonprofit Insurance Institute for Highway Safety. "Imagine if more than 100 people were dying in a plane crash every day. The country's air fleet would be grounded and the media and the public would be demanding that something be done."

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