HEWITT, Texas -- Too high of a speed limit for road conditions and varied tread depths on the front and rear tires share the blame for a motorcoach accident in Texas two years ago that killed seven people and injured more than two dozen others, a federal investigation has determined.

The National Transportation Safety Board said Texas highway officials failed to consider a hill and possible rainy weather conditions when they set the speed limit on Interstate 35 near Hewitt at 70 miles per hour.

The accident in 2003 involved a motorcoach operated by Central Texas Trails that skidded on wet pavement when the driver topped a hill and braked for traffic that had stopped ahead. The slick conditions caused him to lose control, cross the median and collide with an oncoming sports utility vehicle before rolling over.

It was foggy and raining heavily at the time.

NTSB investigators said the probable cause of the accident was a decision by Texas to set a speed limit on the highway that did not take into account the roadway's limited sight distance or its poor drainage in wet weather.

"As a result, the bus driver was unable to detect the stopped vehicles as he approached the traffic queue and lost control of the motorcoach due to low pavement friction," the agency's investigators said their report on the crash.

The 70-mile-an-hour speed limit on the highway is 10 miles an hour above the highway design speed and 20 miles an hour above the speed the bus driver would have had to have been traveling to avoid the accident or reduce its severity, the investigators suggested.

Contributing to the accident, the investigators added, were the minimum tread depths on the motorcoach's drive-axle tires and differing tread depths on its front and rear tires. They noted that while the tread depths on the bus tires are allowed under federal regulations, they reduced the friction that was available to the motorcoach.

Part of the reason, they said is that the rut-depth threshold on the highway of 0.50 inch is insufficient because it exceeds the tread depths of most tires, and standing water in the ruts can lead to reduced tire and roadway friction situations in wet weather.

They said the lack of a median barrier and an occupant protection system also added to the severity of the accident, which killed five passengers on the bus and two people in the sports utility vehicle.

The NTSB recommended that the Federal Highway Administration issue guidelines recommending the use of variable speed limit signs in wet weather at locations where the operating speed exceeds the design speed of the highway and the stopping distance exceeds the available sight distance.

Additionally, it said the FHA should conduct research on commercial tires and wet-pavement surface interaction to determine minimum frictional quality standards. Following the research, it then should revise the tire requirements for commercial vehicles operating on wet pavement at highway speeds and develop minimum pavement coefficients of friction and maximum pavement rut depths as part of roadway maintenance requirements.

The NTSB also called on the National Highway Traffic Safety Administration to conducts tests on the effects of differing tread depths for the steer- and drive-axle tires and the Federal Motor Carrier Safety Administration to modify the tread depth requirements for each axle to reflect the results of the research.

Texas transportation officials also were advised to repave or repair roadways where speed and friction problems may exist and install variable speed signs where wet weather could be a problem.