It's true that U.S. motor vehicle death rates have been trending downward for decades. Since the mid-1980s, the rate per registered vehicle has declined 43 percent. Traffic safety policies aimed at improving drivers and roadways have influenced this trend, but it's a mistake to attribute all of the death rate reductions to such policies. More sophisticated analyses are required to get a clearer idea of what's behind the reductions, and new Institute research helps to identify the reasons.
The researchers focused on two factors that have influenced the driver death rate per registered vehicle over 20 years (1985-2004). One is how vehicle use patterns change as vehicles age. The other is vehicle design changes — the introduction over time of different types of vehicles and more crashworthy ones to replace vehicles that weren't doing as good a job of protecting their occupants.
In the U.S. fleet these two factors can have countervailing influences. As vehicles age, their death rates go up. On the other hand, more crashworthy vehicles have been introduced, and their death rates are lower than in the older vehicles they replaced. Plus the types of vehicles in the fleet have shifted, and the shift from driving cars to SUVs can change the death rates. Separating these factors brings into sharper focus the effects of other influences on the U.S. motor vehicle death rate, including the effects of various traffic safety policies and programs aimed at improving drivers and roadways.
"While vehicle age effects have pushed the U.S. death rate upward, vehicle design improvements have tended to push the rate downward. The unknown is the effect of the other factors, particularly changes in traffic safety policies," explains Adrian Lund, Institute president and an author of the research report. "Once we adjusted for vehicle age and design, the effects of the other influencing factors became apparent."
The main finding is that from the mid-1980s to the mid-1990s traffic safety policies appeared to be having a positive effect, reducing death rates. But around 1994 this benefit ceased. Since then the death rate would have been on an upward trend if vehicle design improvements hadn't continued to push it downward.
Effects of vehicle age on driver death rates
The researchers computed death rates for vehicle models that didn't change in design over three model years — 1996-98 models during 1999, for example. This eliminated the effects of any design changes on the death rate because there were no such changes.
Computing the rates for several model year groups without design changes during individual calendar years, the researchers found that, on average, the death rate per registered vehicle increased 2 percent from the first to the second year a vehicle was driven, 5 percent from the second to the third year, and 3 percent from the third to fourth year. There was no change from the fourth to fifth year, a 1 percent increase to the sixth year, and a 3 percent increase to the seventh year of vehicle use.
Researchers don't know exactly why death rates go up as vehicles get older. It's probably not because of vehicle deterioration, at least during the early years of a vehicle's use. It probably has more to do with who drives older vehicles versus newer ones and how they drive them. When researchers adjusted for driver age and gender and for type of crash, the effects of vehicle age diminished or even disappeared.
Removing the design effects
The researchers separated out vehicle design effects on death rates by following the same vehicles over time. The rates still were affected by vehicle aging so, having already estimated the age effects, the researchers factored them out too. Then the data revealed that the downward trend in death rates would have ended in 1993. An upward trend would have begun if not for the vehicle design changes.
This highlights the importance of the design changes. They haven't just led to better crash test performances (see Status Report special issue: frontal crash test verifications, March 29, 2006). They've saved lives in real crashes faster than other effects could influence the death rate upward.
"The only problem is that people who aren't driving the newest vehicles aren't benefiting from the design changes," Lund says. "In fact, the risk for them is worsening."
Offsetting effects on death rates
Changes in traffic safety policies pushed death rates in both directions during the study period. For example, speed limits and travel speeds went up after the 55 mph limit was amended in 1987 and then up some more after the national limit was abolished in 1995 (see Status Report special issue: speeding, Nov. 22, 2003). This cost lives.
An offsetting trend has been increasing belt use. The biggest use rate increases occurred in the 1980s, when states began enacting safety belt laws. Buckling up leveled off in the 1990s, and for the past couple of years the rate has topped 80 percent (see "Washington state sets example for belt use," Jan. 11, 2003).
The death rate trend that would have been expected if vehicle designs hadn't changed goes down and then starts up, indicating that policies like belt laws might have been helping to lower death rates until the mid-1990s. Since then they haven't.
"We haven't seen the concentrated push in recent years for effective traffic safety policies that we saw in the 1980s," Lund points out. "Serious problems still are out there — faster travel speeds, for example — and we need to address them with the same resolve we applied to raising belt use and reducing alcohol-impaired driving in the 1980s and early 1990s. We also need to design roadways that are more forgiving of all the errors that motorists inevitably are going to make."
Influence of vehicle design improvements:
Driver deaths per million registered vehicles,
actual rates versus expected rates based on 1985 vehicle designs
Passenger vehicle designs have been continually improved. Without these improvements, the death rate would have stopped declining in 1994 and started going up. This reveals not only the importance of design changes but also the diminishing benefits of traffic safety policies in reducing deaths in recent years.