The most common cause of death in children under the age of 15 is unintentional injury, and the most common cause of unintentional injury is car accidents.

Between 2010 and 2014, 2,885 children died in motor vehicle accidents nationwide — an average of 11 children a week. That number excludes pedestrians, those who died in motorcycle or bicycle accidents, and those who died riding in an unenclosed cargo area or trailer.

Most of the children who died were not wearing seatbelts — nationwide, 43 percent were unrestrained or improperly restrained. Another 15 percent were sitting inappropriately in the front seat, and 13 percent were riding in cars driven by somebody under the influence of alcohol.

Researchers at Brigham and Women’s Hospital in Boston and at the University of Texas Southwestern Medical Center in Dallas carried out the analysis, which was published in The Journal of Pediatrics. The primary source of the data was the Fatality Analysis Reporting System, maintained by the National Highway Traffic Safety Administration.

The researchers found considerable variations in children’s deaths from state to state. In New Hampshire, for example, all of the five children who died during the study period were properly belted in. But in Mississippi, 56 of the 99 who died were not wearing seatbelts, or were not wearing them properly.

There were 0.25 deaths per 100,000 children in Massachusetts, compared with 3.23 per 100,000 in Mississippi.

Some roads were more dangerous than others. Rural roads were the worst: 67 percent of deaths among children occurred on roads classified as rural by the Federal Highway Administration.

There may be multiple reasons for this, including poor lighting, distance to trauma centers, and urban residents’ lack of familiarity with rural roads.

Many factors influence the number of fatalities, and the study could not cover all of them. It was difficult, for example, to determine the exact contribution to the overall risk of death of vehicle type, roadway characteristics, speed limits and red-light cameras. There was no data on the total number of miles driven in each state.

But at least one thing is clear: Child restraints are effective in preventing injury and death. “It’s amazing that many kids are not restrained,” said the lead author, Dr. Lindsey L. Wolf, a surgical resident at Brigham and Women’s Hospital.