WASHINGTON—The Transportation Department will propose new rules on Thursday that would restrict truckers and bus drivers from sending text messages while they are on duty, part of a broader effort to curb the rising number of accidents caused by multitasking motorists.

Transportation Secretary Ray LaHood plans to unveil the rules on the final day of a two-day summit on the issue, attended by experts, safety advocates and industry officials. As the use of cellphones and other hand-held devices by drivers has grown, regulators have been seeking effective ways to curb such behavior.

On Wednesday, Mr. LaHood released data showing that 11% of drivers in fatal crashes had been distracted at the time of the accident in 2008, compared with 8% in 2004. Last year, more than 500,000 people were injured and nearly 6,000 were killed in crashes involving inattentive motorists, he said.

The proposed rules could effectively ban truckers and bus drivers from texting while operating a vehicle, said people familiar with the matter, following rules that many companies are beginning to adopt.

It wasn't immediately clear how the new restrictions would be enforced or what kind of penalties they might carry. Regulators also plan to step up outreach campaigns aimed at highlighting the hazards of distracted driving and encouraging driver's education programs to focus more on the issue.

The proposal comes nearly a year after investigators discovered that a commuter-train operator near Los Angeles had been repeatedly texting in the moments before his train collided with a Union Pacific freight train, killing 25 people. That accident led the Transportation Department's Federal Railroad Administration to issue an emergency order prohibiting the use of personal electronic devices while operating trains.

"Distracted driving is an epidemic and it seems to be getting worse every year," Mr. LaHood said.

Federal regulators don't have the authority to ban drivers of passenger cars from using handheld devices, but state laws are proliferating. At least 21 states and the District of Columbia ban all cellphone use by young drivers, and 18 states have passed text-messaging bans, according to the Governors Highway Safety Association.

The Obama administration has declined to endorse legislation sponsored by a handful of Democratic senators that would strip states of 25% of their annual federal highway funds if they don't pass text-messaging bans. Congress used similar tactics in the past to compel states to enforce a minimum drinking age of 21 and laws combating drunk driving. Ford Motor Co. and Verizon Wireless have joined some safety groups in advocating for passage of the federal text-messaging legislation.

The main trade associations representing the bus and trucking industries said they would support the concept of banning their drivers from using cellphones or sending text messages, and that many companies already have such policies in place.

"We believe and have agreed with Secretary LaHood that there really should be no cellphone usage or texting when a driver is carrying 55 passengers down the road," said Peter Pantuso, president of the American Bus Association.

The bigger question may be how to enforce such bans.

"That's a little tougher," said Dave Osiecki, vice president of safety, security and operations at the American Trucking Association.

Technology may be one answer. Companies are offering a range of solutions, such as devices that won't send text messages if they are in motion, and on-board dispatch devices for truckers that go blank once the truck's engine starts.

Write to Christopher Conkey at christopher.conkey@wsj.com

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