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 parttime 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. 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.