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Status Report, Vol. 52, No. 8 | November 21, 2017 Subscribe

Gearshift interlocks could get more people to buckle up

An old idea is getting a new look to try to get everyone in front seats buckled up. About half of those killed in crashes are unbelted, despite laws in 49 states requiring safety belt use. An IIHS study of consumers finds that preventing an unbelted driver from shifting out of park increases the likelihood of belt use by more than 20 percent among people who don't always use belts relative to an enhanced belt reminder.

In-vehicle technology to spur motorists to use belts has had a problematic history in the U.S. In the early 1970s when few people buckled up, the National Highway Traffic Safety Administration (NHTSA) required new vehicles to have auditory and visual belt reminders lasting at least a minute if front-seat occupants were unbuckled at ignition. Regulators followed this move with an unpopular 1973 mandate that all new cars without airbags or other passive restraints be equipped with an interlock that prevented a car from starting if front-seat occupants were unbelted.

Facing public backlash, Congress in 1974 banned NHTSA from requiring belt interlocks or even allowing automakers to use them to meet safety standards. Lawmakers restricted belt reminders, too.

A change under MAP-21, the 2012 highway reauthorization law, allows NHTSA to permit automakers to equip vehicles with belt interlocks as an alternative means to comply with certain federal safety standards (see "LATCH, belt reminders get lift; highway law supports teen driver laws, impaired driving research," Sept. 20, 2012). The technology prevents the use of a feature, such as the transmission, engine or entertainment system, if a belt isn't secured. MAP-21 also allows NHTSA to require belt reminders with auditory warnings that last longer than the prior eight-second limit.

Although NHTSA hasn't yet strengthened belt-reminder requirements or proposed a rule to permit belt interlocks, manufacturers are exploring the technologies.

General Motors is the first automaker since the law change to offer an interlock in the U.S., beginning with certain 2015 models sold only to fleets. The system isn't available on mainstream vehicles. GM's Seat Belt Assurance System prevents the driver from placing the vehicle in gear for 30 seconds after ignition or when placed in "park" unless the driver and, if present, right front passenger are belted.

Forgetfulness is a common reason why part-time belt users don't regularly use belts. Another reason cited is short trips.

Most new vehicles sold in the U.S. have enhanced reminders for the driver and front passenger that exceed federal requirements, and studies show that driver belt use is higher and fatality rates are lower in vehicles with enhanced belt reminders than in vehicles with reminders that meet minimum requirements (see "Safety belt reminder system in late-model Fords boosts buckle-up rate," Feb. 9, 2002, "Belt reminders in Hondas are persuading motorists to buckle up," June 13, 2006, and "Effective belt reminders don't need to be relentless," March 6, 2012). Studies of fleet drivers in Canada and the U.S. indicate that interlocks that prevent unbelted drivers from shifting into gear or driving faster than a set speed also increase belt use.

The goal of the latest IIHS belt study was to see if the GM gearshift interlock had a bigger effect on getting part-time belt users to buckle up than an enhanced belt reminder.

IIHS researchers recruited 32 part-time belt users in Maryland who had recently received a safety belt citation and reported not always using a belt. Every participant drove a Chevrolet Cruze with an enhanced reminder for one week. Half drove a different trim-level Cruze with the same enhanced reminder the following week, while the other half drove another Cruze with an interlock.

The enhanced reminder consisted of three 20-second cycles spaced one minute or more apart. Each cycle began with five auditory chimes played during a seven-second period and a red "telltale" belt icon in the instrument display that flashed for 20 seconds.

"The participants thought they were test-driving the Cruze to compare two trim lines of the car. They didn't know we were observing their belt use," says David Kidd, a senior research scientist with the Institute and the study's lead author.

The interlock increased the likelihood that a part-time belt user donned a belt at least once during travel — that is, from the time the car was placed into gear until it was last put in park — by 21 percent relative to the enhanced reminder. A second analysis examined if the interlock increased the amount of travel time the driver was belted compared with the enhanced-reminder group. By this measure, belt use in the reminder group decreased from 77 percent in week 1 to 69 percent in week 2. In the interlock group, belt use increased from 85 percent in the first week to 89 percent in the second week — a 16 percent rise in belt use compared to what would be expected based on drivers in the enhanced-reminder group.

"We were encouraged that the gearshift interlock was more effective at increasing belt use than the enhanced reminder. 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, waited for the system to deactivate or unbuckled during the trip at least once," Kidd says.

The researchers would have seen a bigger bump in belt use — 24 percent — if the drivers had been unable to circumvent the gearshift interlock.

If every vehicle in the United States had a gearshift interlock that increased front-seat belt use by 16 percent, at least 718 lives could be saved each year, the Institute estimates. An additional 358 lives could be saved if interlocks couldn't be bypassed by front-seat occupants determined to ride unbelted.

Kidd and his team also recruited 16 drivers who reported always using their belts to evaluate if they found the interlock acceptable. All of the full-time belt users experienced the interlock, mainly because they were in the habit of buckling up after shifting into gear.

"Part-time belt users are the population we want to reach with interlock technology," Kidd says. "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."

About 4 in 5 study participants agreed or strongly agreed that having a gearshift interlock was acceptable, and only 1 in 5 agreed or strongly agreed that they wouldn't enjoy driving their vehicle if it had a gearshift interlock. The responses were roughly the same among full- and part-time belt users.

Researchers also asked participants for their opinions of enhanced belt reminders in general and other types of belt interlocks. More than 80 percent of people surveyed favored enhanced belt reminders, but only 32 percent said they would support an ignition interlock. More than half of participants surveyed said they would support a gearshift interlock, entertainment system interlock or speed interlock limiting vehicles to 15 mph until the driver buckles up.

In an earlier IIHS survey, fewer than half of all full-time belt users said they would support using ignition interlocks to increase driver belt use, while 53 percent said they would support less-intrusive transmission or speed interlocks. However, few part-time belt users said they would support the technologies (see "Drivers back stronger belt reminders; European systems could be model," Jan. 24, 2013).

For drivers and their front-seat passengers, using a lap and shoulder belt reduces the risk of fatal injury in a crash by 45 percent in a car and 60 percent in a pickup truck, van or SUV.

Change in belt use with gear-shift interlocks
relative to enhanced reminders only

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