The crash that killed the mathematician John F. Nash Jr. and his wife, Alicia, on Saturday on the New Jersey Turnpike remains under investigation, but preliminary findings by the police seem to make one thing clear: The Nashes, who were thrown from the back seat of their taxi, were not wearing seatbelts.

Their deaths come after years of increased ticket-writing by law enforcement agencies and educational campaigns by advocacy groups to get more people to buckle up. But such efforts have been undermined, in part, by a cultural mind-set that not using a seatbelt in the back seat is somehow safer and by what some see as mixed messages sent via a patchwork of state laws governing seatbelt use.

While nearly all states mandate wearing seatbelts in the front seat, only 28 states require it for those sitting in the back as well, according to the Governors Highway Safety Association, a nonprofit that represents state highway safety offices around the country. Moreover, in some states, drivers can receive tickets for a seatbelt violation only when they are stopped for another violation. The fines for a first offense can range from $10 to $200, according to data collected by the association.

In New Jersey, where state law requires all passengers to wear seatbelts, the crash involving the Nashes happened during the opening weekend of an annual summer campaign by the State Police to enforce safe driving, which included putting more than 200 additional patrol cars on the roads, said Sgt. First Class Gregory Williams, a spokesman for the New Jersey State Police. “Seatbelts save lives,” he said. “I’m a veteran of the force for many years and I can attest to that.”