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The return of the seat belt interlock: crazy rule or money saver?

NHTSA, the US National Highway Traffic Safety Administration, is looking at bringing back seat belt-ignition interlocks. These devices, which prevent cars from being driven unless each occupant is using a seat belt, are already allowed — but some automakers want to use them instead of complying with regulations, unique to North America, for protecting unbelted occupants.

BMW petitioned NHTSA last October to allow belt interlocks as a compliance option, citing the cost, weight, and fuel penalty required to pass the American beltless-occupant crash tests. NHTSA denied the petition, but may eventually let automakers do as BMW asked; NHTSA Administrator David Strickland said he would ask for research to look at opportunities for adjusting the regulations “if there is 100% certainty that everyone is wearing a belt.” Strickland said he sees BMW's point: “This could provide manufacturers design flexibility and options to not only improve the margin of safety in a crash, but could also relieve regulatory burdens and save significant costs.”

BMW's petition mentioned three types of ignition interlocks that might be used: one that prevents the car from being started unless all occupants buckle in, one that prevents the automatic transmission from being shifted out of Park, and one that would allow only very low speed driving (to go down a driveway or private road, or maneuver in a parking lot, for example) if the driver isn't using the seat belt.

The American requirements for unbelted occupant protection at issue were written into law in the 1970s and ’80s when most Americans didn't buckle in; but, in 2012, 86% of vehicle occupants in the US were belted in. That's an all-time high for the United States, but still below those of most of the developed world, including neighboring Canada, which sit in the mid to high 90s. The 14% of unbelted drivers and passengers tally up a whoppingly disproportionate 52% of those killed in car crashes in the US.

This is not the first time the idea has been considered. A seat belt/starter interlock was briefly required almost four decades ago. It was an idea put forth by then-Ford executive Lee Iacocca to stave off an airbag mandate; at that time almost zero Americans used belts, and airbags were incorrectly regarded as a replacement for seat belts rather than a supplemental restraint. So 1974-model cars came with an interlock: the belts in all occupied front seats had to be buckled or the engine couldn't be cranked.

The system was immediately and vehemently hated by the American motoring public. The mechanically-inclined quickly figured out which wires to cut and solder to defeat the system (Chrysler owners could simply unclip a connector, shown at right). Those without technical skill found that leaving the belts buckled and sitting on them did not work—there was a logic module that required the correct sequence (sit, buckle, crank). There was an underhood bypass button that could be pressed to give one "free" start in case of system fault. Taping the button down wouldn't work; it had to be pressed each time.

The components of the day were unreliable, especially given what automakers would pay for a system they did not want in the first place. Seat occupancy was detected with weight sensors; this made problems with bags of groceries, briefcases, packages, and pets. System faults, including failure of the bypass button, and nuisances were common.

Do you have a 1974 Chrysler, Dodge, Imperial, or Plymouth? If so, you can probably eliminate your seat-belt interlock by unclipping the wire underneath the driver’s seat.

Many Americans also hated seat belts, seeing them as an infringement on personal freedom, and even as unsafe (e.g., keeping people in a burning car). [Editor’s note: Some people had problems using the belts of 1974, whose cranky retractors required a very smooth touch and some degree of luck to prevent them from locking on every pull. These belts also sometimes tightened themselves in driving. Prior seat belts usually had no retractors, but had to be “manually sized” like airline seat belts, and when taken off, had to be stowed in clips above the door; the driver could not lean forward to release the emergency brake while wearing a seat belt.]

Now, we know from mountains of crash data and accident reconstructions that belt non-use affects everybody, not just the non-user; an unbelted driver is much more likely to be knocked unconscious or otherwise unable to control a car by a first impact, making further impacts (and death and injury of other people) much more likely. [Editor’s note: The costs to emergency rooms and health insurers from un-belted drivers can be quite high. People who would walk from even a fairly severe crash often end up in the critical ward for weeks if they were not belted.]

Shortly after a critical mass of congressmen bought new 1974 cars, the interlock requirement was repealed and a (rare) exemption was enacted to the prohibition on rendering inoperative a vehicle safety device or system.

Now we have much more accurate and precise ways of detecting seat occupancy; cameras able to monitor a driver for fatigue can certainly tell the difference between a driver and a dog or a stack of books. Today's automotive electronics are also much more dependable and flexible. Defeating a current-day interlock would be much harder than it was in 1974, but few would try; education and social and legal pressure have gradually brought most Americans to realize that seat belts save lives and prevent injuries and property damage.

It is difficult to think of a rational objection to the interlock as proposed by BMW. It helps to recall we have had interlocks preventing the car from cranking if it’s not in Park or Neutral, since even before the advent of US Federal Motor Vehicle Safety Standards in 1968. Few people object to these obvious crash-prevention devices in that context; with today's understanding and general acceptance of seat belts, a belt interlock ought to meet with good acceptance by those who were not too emotionally scarred from the 1974 systems.

The BMW point is an interesting one to ponder—not just in terms of crashworthiness requirements such as seat belts, air bags, knee bolsters and interior padding, but over a much broader scope. As driver assistance systems take over vehicle operation from the arbitrary, capricious, problem-prone “nut behind the wheel” (the driver), discussions of how much vehicle intervention is appropriate on behalf of the driver are discussions it's time to have.