WASHINGTON — More than two-thirds of young drivers and passengers killed in nighttime car crashes aren’t wearing seat belts — deadly proof of what can happen when young people don’t heed parents’ pleas and authorities’ threats to ‘‘click it.’’

Though seat belt use actually is rising slightly nationwide, fatality figures published Monday offered a somber contrast as law enforcement launched its annual pre-Memorial Day drive to persuade Americans to buckle up.

Total belt use rose to 82 percent last year — from 81 percent in 2006 — the government said. Twelve states had rates of 90 percent or better, led by Hawaii and Washington. Only three were below 70 percent: Arkansas, Massachusetts and New Hampshire.

But the news was hardly all encouraging.

Sixty-eight percent of drivers and passengers between the ages of 16 and 20 who were killed in car crashes at night in 2006 were unbuckled, said the National Highway Traffic Safety Administration