Seat belt interlocks can increase restraint use, IIHS finds

An unpopular safety technology which first appeared in vehicles in the 1970s—and was quickly nixed—could be effective in improving seat belt use if reintroduced, according to the Insurance Institute for Highway Safety.

IIHS sought to test the effectiveness of a gearshift interlock device, which prevents the vehicle from being shifted into gear until occupants are wearing their seat belts. The study also aimed to determine if this technology was more effective in improving seat belt use than an auditory reminder.

Study results

Researchers found 32 volunteers in Maryland who had recently been cited for failing to wear a seat belt and admitted that they did not always buckle up. All participants drove a Chevrolet Cruze with an enhanced seat belt reminder for one week. This reminder had three 20-second cycles where chimes sounded and a red icon appeared on the vehicle's instrument display panel when the vehicle detected an unbelted occupant.

In the following week, participants drove a Cruze with a different trim level. Half of the vehicles had the same enhanced reminder, while the other half had a gearshift interlock. The participants thought they were comparing the different trim levels and did not know that their seat belt use was being observed.

IIHS researchers found that a driver was 21 percent more likely to put on a seat belt at least once during a drive if an interlock was in place. It also increased the amount of time a driver was likely to wear the seat belt. Drivers in the interlock group were typically buckled in for 89 percent of their travel during the second week of the study, up from 85 percent in the first week. By contrast, seat belt use fell from 77 percent in the first week to 69 percent in the second week in the group that continued to use the enhanced reminder.

"We were encouraged that the gearshift interlock was more effective at increasing belt use than the enhanced reminder," said David Kidd, senior research scientist at IIHS and lead author of the study. "That said, some drivers in the study occasionally did things to circumvent the interlock. Six of the 16 part-time belt users who experienced the gearshift interlock sat on the belt, waiting for the system to deactivate, or unbuckled during the trip at least once."

Researchers estimated that the increase in seat belt use would have been 24 percent if participants had not been able to bypass the interlock.

Sixteen more drivers, who reported always wearing a seat belt, also took part in the study. Despite their habit of wearing a seat belt, all of these drivers were initially blocked by the interlock because they tried to put the car into gear before buckling their seat belt.

Approximately 80 percent of all study participants agreed that having a gearshift interlock in their vehicle was acceptable. Only one in five agreed that they wouldn't enjoy driving the vehicle if there was an interlock.

More than half of participants said they would support a gearshift interlock over other types of interlocks to improve seat belt use, such as systems that would not allow the entertainment system to be activated or the vehicle's speed to exceed 15 miles per hour until the occupants were buckled in. More than 80 percent were in favor of enhanced seat belt reminders, but only 32 percent favored an ignition interlock.

An unpopular history

Seat belt interlocks were first introduced more than 40 years ago. In 1973, the National Highway Traffic Safety Administration required all new cars without airbags or other passive restraints to be outfitted with an interlock preventing the vehicle from starting unless its front seat occupants were wearing their seat belts.

The system was soon met with popular backlash. Few Americans used seat belts at the time, and the requirement was seen as an example of government overreach. The move was also criticized as being too hasty, since automakers only had about six months' warning to install the device on 1974 model year vehicles.

Even if they buckled up, drivers sometimes faced bugs and nuisances with the system. A bypass button allowed drivers to start the vehicle in case of a problem with the interlock system, but this device sometimes failed as well. Weight sensors on the front passenger seat could also prevent a vehicle from starting if they incorrectly determined that luggage, bags of groceries, or other items were occupants.

Many drivers tinkered with their vehicles in order to bypass the interlock. In some vehicles, this could be done simply by unplugging the connection to the interlock system.

In 1974, Congress barred the NHTSA from requiring seat belt interlocks on new vehicles and allowed drivers and dealers to remove existing systems. The legislation also rolled back requirements to have a vehicle produce continuous auditory and visual reminders when occupants failed to buckle up.

Recent revival

As part of the Moving Ahead for Progress in the 21st Century Act in 2012, the NHTSA allowed automakers to equip vehicles with seat belt interlocks as a way to meet federal safety requirements. These interlocks might prevent the vehicle from starting if an occupant does not wear their seat belt, but can also cut off another feature such as the entertainment system. The legislation also allows the NHTSA to have seat belt reminders last longer than the current limit of eight seconds.

The NHTSA hasn't yet strengthened its requirements on seat belt reminders or seat belt interlocks, but some manufacturers have expressed interest in the technology. In 2013, the agency denied a petition from BMW asking to use a seat belt interlock system to meet some safety requirements. BMW argued that the technology would help reduce its vehicles' weight and make them more spacious, but the NHTSA said further research was needed to determine whether the devices would be effective in increasing seat belt use.

General Motors became the first automaker to offer a seat belt interlock system, although the technology was only available on certain 2015 models sold to fleets rather than mainstream vehicles available for purchase. GM's Seat Belt Assurance System, which was present on the Cruze vehicles used in the IIHS study, prevents the driver from shifting the vehicle into gear for 30 seconds after ignition or when parked unless the front seat occupants are wearing their seat belts.

Every state but New Hampshire requires front seat vehicle occupants to wear their seat belts. However, the number of unrestrained people killed in crashes increased 4.6 percent to 10,428 in 2016. This accounted for about 43 percent of all fatalities in passenger vehicles and large trucks.

Some people choose not to wear a seat belt due to discomfort or the belief that it is unnecessary for short trips, while others occasionally forget to put it on. IIHS estimates that drivers and front seat occupants who use a lap and shoulder belt reduce their risk of a fatal injury by 45 percent in a car and 60 percent in a pickup truck, SUV, or van.

IIHS estimates that gearshift interlocks could increase seat belt use by 16 percent in the United States and save at least 718 lives each year. Researchers also estimate that another 358 lives a year could be saved if the interlocks could not be bypassed.

"Interlocks should be intrusive enough to get the attention of unbelted drivers and front passengers, but at the same time they shouldn't aggravate the vast majority of people who always use belts," said Kidd.

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