BLACKSBURG, Va. -- A new study underscores earlier evidence that the greatest risk of an accident occurs in the first hour of driving, and that the risk in the second through 10th or 11th hours is relatively flat.

The study by Richard Hanowski, director of the Center for Truck and Bus Safety at the Virginia Tech Transportation Institute, found that the risk of a critical driving incident is significantly higher in the initial hour of driving than in all other driving hours - and that there is no statistical difference in risk between the second through 10th or 11th hours.

In fact, a significant spike in the rate of critical incidents was found during the first driving hour and was found across many different analyses.

Additional analyses found a strong time-of-day effect which, upon closer examination, appeared to have resulted from hour-by-hour traffic density variations.

The study authors hypothesized that exposure to heavy traffic conditions, and possible sleep inertia and an increase in complex driving situations that may be typical in the first driving hour, may have influenced the increase in critical incidents recorded in the initial driving hour.

The Virginia Tech research underscores the first-hour spike found by the federal governments Large Truck Crash Causation Study, which reported that the first hour had the highest raw percentage (14.7 percent) of crashes, Hanowski said.

Still, the data on this issue is mixed.

A 2005 study, for example, found that risk increased as driving hours accumulate, Hanowski noted.

Hanowski said his analysis, which was completed in January and is now in peer review, is based on the largest on-road naturalistic study ever conducted. It includes 2.3 million miles of driving data and 190,000 hours of sleep data from wrist actigraphs worn by the drivers in the study.