ARLINGTON, Va. — Seat/head restraint combinations in the Ford Freestar and its twin Mercury Monterey earn good overall ratings. Those in some Dodge Grand Caravan/Chrysler Town & Country models are rated acceptable, based on recent evaluations by the Insurance Institute for Highway Safety. However, the seat/head restraints in most current minivan models are marginal or poor, indicating they wouldn't provide adequate protection from whiplash injuries for many people in rear-end collisions.
The ratings are for seat/head restraint designs available in 14 current minivan models. Starting points for the ratings are measurements of head restraint geometry — the height of a restraint and its horizontal distance behind the back of the head of an average-size man. Seats with good or acceptable restraint geometry then are tested dynamically using a dummy that measures forces on the neck. This test simulates a collision in which a stationary vehicle is struck in the rear at 20 mph. Seats without good or acceptable geometry are rated poor overall because they cannot be positioned to protect many people.
Among the seat/head restraints that were tested dynamically, those in the Honda Odyssey are rated marginal overall. All seats in the Chevrolet Uplander (also sold as Buick Terazza, Pontiac Montana SV6, and Saturn Relay) and some in the Grand Caravan/Town & Country and Toyota Sienna are rated poor. These ratings are in addition to the good overall rating for the seats in the Freestar/Monterey and the acceptable rating for the seats in some Grand Caravan/Town & Country models. All of these seat/head restraint combinations earn overall ratings based on both geometry and dynamic test results.
Another minivan, the Kia Sedona, has been redesigned for the 2006 model year but isn't yet available. Results for the Sedona will be released early next year.
"Automakers are improving the geometry of their head restraints, compared with the last time we evaluated them," says Institute chief operating officer Adrian Lund. "Still, in this group of minivans the Fords are the only models with good dynamic performance for all of their seat designs. Many of the seat/head restraints we evaluated didn't even get to the testing stage because of marginal or poor geometry. These cannot begin to protect most people in rear-end crashes."
Some seats automatically earn poor ratings
The Institute doesn't test seats with head restraints that are rated marginal or poor for geometry because such seats cannot be positioned to protect many taller people. The seats that weren't tested in this group include all of those in the Chevrolet Astro, GMC Safari, Mazda MPV, and Nissan Quest plus some seats in the Grand Caravan and Toyota Sienna.
"It's disappointing that so many minivan seats are rated poor for rear impact protection," Lund says. "Drivers of minivans spend a lot of time on urban and suburban roads where rear-end collisions are common in stop-and-go traffic. Moms often are behind the wheel, and women are more vulnerable to whiplash injuries so they especially need good seats and head restraints."
Neck injuries are the most common kind reported in automobile crashes and are most likely to occur in rear impacts. Whiplash is the most serious injury reported in about 2 million insurance claims each year, which cost at least $8.5 billion. Such injuries aren't life-threatening, but they can be painful and debilitating.
Rear-end crashes are common events in urban and suburban traffic. For example, in one urban Virginia county 63 percent of daytime crashes on urban interstate highways in 2003 were rear impacts.
When a vehicle is struck in the rear and driven forward, the vehicle seats accelerate occupants' torsos forward. Unsupported, their heads will lag behind the forward movement of their torsos. This differential motion causes the neck to bend back and stretch. The higher the torso acceleration the more sudden the motion, the higher the forces on the neck, and the more likely a neck injury is to occur.
"The key to reducing neck injury risk is to keep the head and torso moving together," Lund explains. "To ensure this happens, a seat and head restraint have to work in concert to support the head, accelerating it with the torso as the vehicle is driven forward in a rear impact. This means the geometry of a head restraint has to be adequate, and so do the stiffness characteristics of the vehicle seat and head restraint."
A head restraint should extend at least as high as the top of the ears of the tallest expected occupant. A restraint also should be positioned close to the back of an occupant's head so it can contact the head and support it early in a rear-end crash.
"But good head restraint geometry by itself isn't sufficient," Lund says. "A seat has to be designed so it doesn't move backward and away from the head during a rear impact. A seat also needs to 'give' so an occupant will sink into it, moving the head closer to the restraint."
Sled test simulates rear-end collision
Overall seat/head restraint ratings are based on a two-step evaluation. In the first step restraint geometry is rated using measurements of height and distance from the back of the head of a test dummy that represents an average-size man. Seats with good or acceptable geometric ratings are subjected to a dynamic test conducted on a crash simulation sled that replicates the forces in a stationary vehicle that's rear-ended by another vehicle of the same weight going 20 mph.
A dummy specially designed to assess rear-end crash protection (BioRID) is used to show how a human would respond and measure the forces on the neck during simulated crashes. The sled is a movable platform that runs on fixed rails and can be programmed to re-create the accelerations that occur inside vehicles during real-world crashes.
"The sled test simulates the kind of crash that frequently occurs when one vehicle rear ends another in commuter traffic," Lund says. "People think of head restraints as head rests, but they're not. They're important safety devices. You're more likely to need the protection of a good head restraint in a collision than you are to need other safety devices because rear-end crashes are so common."
The Institute's dynamic ratings of good, acceptable, marginal, or poor are derived from two seat design parameters (peak acceleration of the dummy's torso and time from impact initiation to head restraint contact with the dummy's head) plus neck tension and shear forces recorded on BioRID during the test. The sooner a restraint contacts the dummy's head and the lower the acceleration of the torso and the forces on the dummy's neck, the better the dynamic rating. A seat/head restraint's dynamic rating is combined with its geometric rating to produce an overall rating.