deaths per miles traveled. But what happens when the metrics are teased apart, and familiar data is charted in an unfamiliar way? Plotting the two most important variables against each other — miles traveled versus deaths per 100,000 population — yields a pattern that looks like a plateau followed by a steep drop. It evokes the theory of punctuated equilibrium, proposed by the paleontologists Stephen Jay Gould and Niles Eldredge, which suggests that instead of continuous gradual evolution, change occurs abruptly after periods of virtual standstill. “You see fatalities drop after a breakthrough in new technologies or behaviors, and then plateau until the next one,” said David L. Strickland, administrator of the National Highway Traffic Safety Administration. “It takes time for new safety technologies to work their way into the whole fleet of cars on the road.”

AMERICANS drive a staggering number of miles — close to three trillion every year, according to the government. (That is half a light-year, or 120 million trips around the world.) And although traffic accidents remain a major public safety problem, the biggest killer of people ages 5 to 34, vehicle travel is far safer than it was a few decades ago. Several factors appear to account for the sharp decline in fatalities. Technology (like anti-lock brakes and air bags) and road behavior (like wearing seat belts and driving sober) have both improved greatly since 1950. Americans almost always drive more each year than the previous one — at least until recently, when the recession curtailed road habits. And the auto fatality rate has been decreasing since the 1960s, when cars with massive engines carried their unbuckled passengers on primarily two-lane roads. The safety data is usually charted as

After the 1973 Arab oil embargo, President Richard M. Nixon sets a 55 m.p.h.

speed limit as national energy policy. A few years later, the Iranian revolution and the Iran-Iraq war curtail fuel supplies. People drive less (and more slowly); fatalities fall.

In 1984, New York becomes the first state to require drivers to wear

seat belts. Child car seats become the norm: by 1985, all states require them. Many states tighten laws against drunken driving, and by 1988 all states have set the minimum drinking age at 21.

Analysts note two important factors since 2004: safer vehicles and unemployment.

Economic downturns mean fewer people on the road, especially high-risk 16-to-24-year-old drivers. Also, most new vehicles have stability controls and better crash test ratings. Redesigned S.U.V.’s are less deadly in crashes.

By the 1990s, new technology like antilock brakes reduces

accidents, and air bags make them less deadly. But Americans also fall in love with high-riding S.U.V.'s, which tend to flip over and to endanger smaller vehicles.

In 1965, Ralph Nader publishes a best seller about

auto companies’ resisting safety features. The government creates the first agency devoted to highway safety. Auto fatalities hit a plateau.

American cars get bigger, faster and — with more V-8

engines in midsize cars — more deadly. The Interstate highway system grows, and speeds rise with it.

Seat Belts and Sobriety

Early estimates from 2012 show increases in driving and fatalities.

Air Bags and S.U.V.’s

Unstable Economy, Stable Vehicles

Energy Crises

Muscle Cars

“Unsafe at Any Speed”

1968

2011

2009

2008

2007

2004

2001

1997

1995

1992

1991

1990

1989

1988

1985

1984

1983

1982

1981

1980

1978

1977

1976

1974

1973

1972

1970

1969

1966

1963

1962

1961

1958

1956

1954

1953

1951

1950

Auto fatalities per 100,000 people

Vehicle miles driven per capita

Auto fatalities per 100,000 people

25

20

15

10

25

20

10,000

8,000

6,000