James Baker was pedaling to work along a slick, snow-covered road in Frederick County, Md., when a traffic light changed abruptly. He braked and skidded to the ground, unhurt but making a mess of his clothes before a long day of work and school.

He was on his bicycle that snowy morning last December, about an hour northwest of Washington, because the bus service in Frederick was so erratic. Routes were far apart and the buses often late, making a 30-minute bike ride, whatever the weather, a better option.

His commuting problems highlight a central theme for many low-income people trying to build a better life: A lack of reliable and efficient transportation is often a huge barrier.

In a large, continuing study of upward mobility based at Harvard, commuting time has emerged as the single strongest factor in the odds of escaping poverty. The longer an average commute in a given county, the worse the chances of low-income families there moving up the ladder.