Anthony Downs, an economist whose wide-ranging studies of politics and urban affairs offered a rationale for why citizens would not vote in elections and explained why expanding the highway system would lead to more traffic jams, died Oct. 2 at a hospital in Bethesda, Md. He was 90.

The cause was sepsis, said a daughter, Katherine Downs.

Dr. Downs, the rare economist who was not an academic, spent most of his career working at two organizations: his father’s real estate research firm in Chicago and the Brookings Institution think tank in Washington.

Beginning in his 20s, he published numerous provocative works on modern society that helped shape how generations of scholars interpreted trends in political science, economics and urban studies. He made what is widely considered his most enduring contribution with “An Economic Theory of Democracy” (1957), a book in which he applied the principles of economics to the practice of voting.

Whether buying a car, purchasing household items or voting for a candidate, Dr. Downs wrote, most people make their choices largely on the basis of self-interest. As politicians compete for attention, the voters, in turn, base their decisions on what will give them the greatest benefit or utility, to borrow a term from economics. Paradoxically, Dr. Downs concluded, one of those choices is to not vote at all.

This idea, which became known as the theory of “rational ignorance,” suggests that many potential voters decide that the investment of time and mental energy to learn about political candidates and issues is not worth the effort — particularly in elections in which their votes are unlikely to affect the outcome.

In other words, if voting were subjected to a cost-benefit analysis, many people would decide that doing something else — working, playing, studying — is more important than casting a ballot.

“An Economic Theory of Democracy” became a key text in a field of economics called rational choice theory and has been translated into many languages. Other scholars built on Dr. Downs’s study to analyze voter participation and to predict what candidates could do to win more support.

“It was pathbreaking. It defined the field,” Tyler Cowen, an economics professor at George Mason University, said in an interview. “In political economy, which is the study of how our government works, he was one of the top four or five people of all time.”

Throughout his career, Dr. Downs defied simple political or ideological pigeonholing. Among his 24 books and 500 articles, he wrote about bureaucracy — not a universally bad thing, in his view — rent control (he opposed it), housing, suburban development and desegregation.

Some of his ideas appealed to conservative free-market economists and libertarians; others were more in step with liberals advocating greater governmental intervention in the economy; and some of his findings, as early as the 1960s, were a call of conscience to reform the country’s “fundamentally racist” social and economic structures.

Dr. Downs often did research as a consultant for real estate developers and governmental agencies, projecting growth patterns and the need for infrastructure and transportation. His studies of the country’s urban and racial inequities struck him at a deep personal level.

He served on a Senate commission and a White House task force analyzing inner-city life, then in 1967 and 1968 was on the staff of the National Advisory Commission on Civil Disorders, examining the causes of urban unrest in many cities. It was often called the Kerner Commission after its chairman, Illinois Gov. Otto Kerner Jr.

Dr. Downs wrote several chapters of the Kerner Commission’s final report, which became a bestseller after it was published in 1968. The report found that violence erupted as a result of poor housing, lack of jobs and educational opportunities, voter suppression, unfair banking and credit policies and discriminatory policing and judicial practices.

“Our nation is moving toward two societies, one black, one white — separate and unequal,” the report concluded.

“White society,” it went on, “is deeply implicated in the ghetto. White institutions created it, white institutions maintain it, and white society condones it.”

Dr. Downs expounded on this idea in a 1967 speech, which he called one of his proudest moments. Addressing the all-White Commercial Club of Chicago, he alluded to his work on federal panels investigating life in the Black inner city:

“I am privileged to have seen the truth concerning the relations between whites and blacks in our society more fully than most. And that truth is appalling. … I have been forced to conclude … that our entire social structure is fundamentally racist. That means that it is shaped by a pervasive anti-Negro prejudice that influences almost every one of us in this room far more than we care to admit.”

The next week, the club voted to admit Black members for the first time.

James Anthony Downs was born Nov. 21, 1930, in Evanston, Ill. His father was a consultant and entrepreneur who established the Real Estate Research Corp. His mother was a homemaker who translated books into Braille.

Dr. Downs graduated in 1953 from Carleton College in Northfield, Minn., then over the next three years received a master’s degree and a doctorate in economics, both from Stanford University. His dissertation was published as “An Economic Theory of Democracy.”

He began working at his father’s research company in 1959 and also taught at the University of Chicago from 1959 to 1962. He was at the Rand Corp. in California from 1963 to 1965 before working on the Washington-based commissions.

He moved from Chicago to Washington in 1977, when he joined Brookings. He retired at age 82. He was a resident of McLean, Va., where he was a member of St. Luke Catholic Church.

His wife of 42 years, the former Mary Katherine “Kay” Watson, died in 1998. Survivors include his wife of 22 years, the former Darian Dreyfuss Olsen of McLean; five children from his first marriage, Katherine Downs and Carol Downs, both of Roslindale, Mass., Christine Mann of Richmond, Calif., James Anthony Downs Jr. of Sherborn, Mass., and Paul Downs of Penn Valley, Pa.; three stepchildren, Howard Olsen of Burlingame, Calif., Spencer Olsen of Waxhaw, N.C., and Jennifer Yeamans of Livermore, Calif.; 13 grandchildren; and three great-grandchildren.

In later years, Dr. Downs became known for a subject he had been studying since the 1950s: automobile traffic. His 1992 book “Stuck in Traffic” — later expanded as “Still Stuck in Traffic” — was considered a landmark in applying economic principles to the daily commute of suburban motorists.

He devised a theory that building more and wider highways did not ease traffic congestion but, in fact, made it worse. The basic problem was that the number of cars was growing faster than the number of people.

“Since 1980,” Dr. Downs said on NPR’s “Talk of the Nation” in 2001, “every time the human population in the country went up by one person, the vehicle population of cars and trucks went up by 1.2 vehicles.”

As long as people wanted to live in single-family homes in the suburbs and drive to work, the traffic would only get worse. To Dr. Downs, traffic congestion wasn’t necessarily a problem but was the logical result of people’s decisions.

“It’s a means by which we achieve other objectives that are more important,” he told the Vancouver Sun in 2007, “such as having a wide range of choice of where to live and work.”

The only thing that would change drivers’ habits, Dr. Downs determined, would be to make them pay for the privilege. Thus, he advanced the idea of variable pricing on toll roads, raising the cost at peak times and lowering it during off hours. The practice, once considered outlandish, has been adopted in Northern Virginia and elsewhere throughout the country and the world.

“So get yourself an air-conditioned car with a CD player, a hands-free telephone, perhaps even a fax machine,” Dr. Downs wrote in 2001, “and commute with someone you really like. Learn to enjoy being stuck in traffic as another leisure activity, because congestion is here to stay.”