State officials are promoting safety messages and urging drivers to use caution — and seat belts — after a string of fatal crashes on Massachusetts roadways.

At least 11 people have been killed in car crashes since Sept. 28, State Police and the state Highway Safety Division said, although that number includes only cases in which the State Police Collision Reconstruction & Analysis Section was involved.

“Sadly, it is not . . . unprecedented,’’ State Police spokesman David Procopio said in an e-mail. “The number is high but not an extreme outlier.’’

Jeff Larason, the Highway Safety Division director, echoed that. He said the preliminary number of 2015 fatalities is 345. That’s about 6.6 deaths a week.

Procopio said impaired driving and excessive speed are two major causes of serious crashes.

Larason stressed that seat belts can often save lives, adding that it appeared belts weren’t being used in some of the recent fatal crashes.

“Seat belt use alone is a huge factor in terms of surviving crashes,’’ Larason said. “Massachusetts is ranked No. 48 in the United States’’ in seat belt usage, according to the National Highway Traffic Safety Administration.

The string of fatalities happened between Sept. 28 and Wednesday.

“We urge motorists to be patient, to slow down, to only drive sober, and to always wear seat belts,’’ Procopio said.

Dylan McGuinness can be reached at dylan.mcguinness @globe.com.