As his vintage 1955 Ford station wagon rattles down California's busy Highway 8, Thomas Weller is on the lookout for a breakdown. But it's not his own junker, with 500,000 miles on it, that concerns Weller, 54. The self-employed auto mechanic cruises the freeways near his home outside San Diego in search of stranded motorists—so he can help them free of charge. "I didn't think there were angels who roamed the freeway," says Lin Manning, 47, a now-retired sheriff's sergeant whom Weller aided in 1982. "Maybe this is his mission in life."

Weller's unusual journey began 38 years ago, when he was 16 and his car slid off a highway near his hometown of Metamora, Ill., during a blizzard. Hours later he was discovered by a passing motorist, who towed his vehicle out of the snow. Declining to accept any money, the Good Samaritan told Weller that if he wanted to pay him back, he could do so by passing the favor along to someone else.

Since then Weller, who estimates he spends an average of $250 a month on fuel, has repaid that original favor some 3,000 times fa figure he calculates by the number of cards he has handed out; each reads in part, "I ask for no payment other than for you to pass on the favor"). Though the married father of two adult children has made a few dramatic rescues—like the family he hustled out of a stranded car minutes before another vehicle rear-ended it and both burst into flames—more typically he ends up changing flats, says his wife, Patti, 54. (Even in this cellular age, AAA doesn't always arrive that quickly.) "If I'm feeling down in any way or if I want a little excitement, I head out," says Weller. "Some people go hang gliding; I go play on the freeway."