GUN BARREL CITY — For more than eight decades, Jim Howell was hardly one to cause a political ruckus. But this spring, he realized that a crude-oil superhighway ran through his backyard, just two feet below his patchy lawn and seven feet beyond a newly built porch.

“At first I felt guilty and stupid,” Mr. Howell said about discovering that the 20-inch-wide pipeline passed so close to his compact brick house.

Guilt became alarm as he read more about it and as people from ExxonMobil, its operator, showed up to add a row of yellow-and-black- striped warning markers. The pipeline, Pegasus, was the same one that ruptured about 280 miles northeast in March, spewing at least 210,000 gallons of heavy Canadian crude into neighborhood streets in Mayflower, Ark.