A Berkeley transportation scholar offers an appealingly simple rule in Adam Nagourney’s dispatch on the sizeable subway and light-rail expansion in Los Angeles:

Robert B. Cervero, the director of the University of California Transportation Center in Berkeley, said that if the subway expansion cut commuting time as promised, it would indeed change ridership habits. Transit officials said the ride from Koreatown to Westwood by subway would take 24 minutes, compared with 50 minutes during the rush in a car or on a bus.

“The science of public transit is not too complicated,” Mr. Cervero said in an e-mail message. “It comes down to how time-competitive transit is with the private car. If it takes two to three times longer to get from Point A to Point B by transit, the vast majority of folks will drive. If it’s faster going by bus or train, then most will forsake their car and ride transit.”

Is it that simple? Social-science research proves over and over again that people are less rational with their money than we’d like to believe — which has forced the field of economics to reconsider once-cherished assumptions. It’s hard to believe people are any more rational with how they allocate their time.

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Don’t habit, social pressure, perceptions about what’s pleasant and safe all affect which mode of transport people choose?