The results were tragic but not surprising last May when Suzanne Randa and her fiance, Thomas Donohoe, crashed while riding Donohoe’s Harley Davidson on Highway 79 near the Southern California city of Loma Linda.

In a highly touted safety achievement, deaths on the nation’s roads and highways have fallen sharply in recent years, to the lowest total in more than a half-century. But motorcyclists have missed out on that dramatic improvement, and the news for them has been increasingly grim.