Truck drivers who push the limits of federal regulations by driving up to 10 consecutive hours more than double their risk of having an accident, according to UC Davis researchers. In a recent study examining the factors involved in truck accidents, consecutive driving hours was more important than experience, age, night driving and hours off duty prior to the last trip. For the first four hours of driving, trucks have the lowest accident risk. The likelihood of getting in an accident jumps 50 percent after five to seven hours of driving and up to 80 percent in the eighth hour. By the ninth hour of driving -- near the maximum allowed by the 60-year-old regulations -- truck drivers' accident risks rise 130 percent over those first few hours of driving. "In the safety business, 50 percent is a big increase," says Paul Jovanis, a professor of engineering at UC Davis. "It's a little scary to think about an accident risk that high and a trend that strong -- and in some ways we're looking at the best of the industry."