Belt reminders can be just as effective as interlocks

April 25, 2019

Persistent audible belt reminders are no less effective at promoting belt use than interlocks, which drivers often circumvent, and may raise fewer concerns for drivers, new IIHS research suggests.

Encouraging belt use through this technology is a simple intervention that would help move the U.S. closer to the goal of zero fatalities. The researchers found that persistent reminders could save nearly 1,500 lives a year if all vehicles were equipped with them.

IIHS researchers conducted two studies that build off earlier research about the best ways to close the remaining belt use gaps. Seat belts reduce the risk of death among front-row occupants in crashes by 45 percent. About 90 percent of drivers and front passengers use seat belts, but nearly half of front-seat occupants killed in crashes weren't belted.

In the first of the new studies, 49 part-time belt users who had recently received a seat belt citation drove two vehicles with different seat-belt reminders or a speed-limiting interlock for one week each. The data were combined with data from an earlier study, also involving part-time belt users with recent citations, that compared a gear-shift interlock with an audible belt reminder from Chevrolet consisting of three seven-second periods of chiming, a minute or more apart.

In the latest round of data collection, some participants first drove a Chevrolet with the same fairly minimal belt reminder as used in the first study. They then drove either a BMW with a 100-second audible reminder or a Subaru with an audible reminder that lasted indefinitely, until the person buckled. A third group drove BMWs with the 100-second reminder first and then a BMW equipped with a prototype speed-limiting interlock. This interlock restricted vehicle speed to 15 mph if either the driver or front passenger was unbelted.

Comparing belt use with these four different technologies, the researchers found that the speed-limiting interlock, the indefinite reminder from Subaru and the 100-second constant reminder from BMW all increased belt use by 30-34 percent compared with the intermittent reminder from Chevrolet. The gear-shift interlock increased belt use 16 percent relative to the intermittent reminder.

Increasing belt use by 34 percent in all vehicles on U.S. roads would save 1,489 lives each year, the researchers calculated.

"We expected the interlocks to be more effective than any type of belt reminder, but that didn't turn out to be the case," says HLDI Senior Research Scientist David Kidd, the study's lead author. "Many people simply forget to buckle up, so a persistent reminder works well for them. For those who are really averse to using the seat belt, an interlock doesn't always help because they can find a way to get around it, for example by buckling the belt behind their back or sitting on top of it."

In a complementary study, some participants were invited back to give their opinions about three different reminder systems and three different interlock systems after experiencing each during a short drive. Based on their experience, participants felt the interlocks were more effective for increasing belt use.

Surprisingly, when asked how acceptable each technology would be to them in their personal vehicle, interlocks were no less acceptable than belt reminders.

Interlocks were previously so hated that Congress passed a law banning them in the 1970s. That was after the National Highway Traffic Safety Administration required them on all vehicles without airbags. The law also limited how persistent seat belt reminders could be.

Occupant sitting on seat belt

People who are determined to evade seat belt interlocks usually find a way, for example, by buckling the belt behind them.

A 2012 highway reauthorization law relaxed these restrictions. Now NHTSA can permit automakers to equip vehicles with belt interlocks as an alternative means to comply with a federal safety standard. The agency also can require belt reminders with auditory warnings that last longer than the prior eight-second limit.

"Attitudes toward belt interlocks seem to have softened as the culture surrounding seat belts has evolved," Kidd says. "However, participants in the study raised safety concerns about interlocks that were not expressed for reminders."

The main concerns people voiced were that interlocks could prevent someone from operating a vehicle in an emergency or that limited vehicle function could increase crash risk.

Some of these concerns are well-founded. In survey responses from the two-week on-road study, two participants described how the speed-limiting interlock suddenly slowed the vehicle because groceries or other objects were mistaken for an unbelted front-seat passenger. Another two participants felt that a sudden slowdown caused by the speed-limiting interlock almost resulted in a crash.