Workers recently started to change speed limit signs on the first segment on the Fourth Avenue line in Brooklyn between 36th Street and 59th Street. Overall, officials plan to change the speed limits at 100 locations by the spring.

The team also found 267 faulty signals that were forcing train operators to pass at slower speeds. The equipment, known as grade time signals, was designed to halt trains that are moving too quickly. But officials kept adding more of them — eventually reaching 2,000, some of which were poorly maintained and misconfigured.

About 30 signals have been repaired in Brooklyn, from the DeKalb Avenue station to the 36th Street station, on the B, Q, D, N and R lines, and near the 9th Avenue station on the D line. Mr. Byford wants to eventually fix all of the faulty signals, though he cautioned that the work is complex and could take awhile.

“This is a great move and I think it’s one that a lot of people have been waiting on for quite a while,” said Benjamin Kabak, who writes the Second Ave. Sagas subway blog. “I think it can provide immediate dividends in terms of speeding up service.”

Mr. Byford, who started running the subway in January, is also pressing elected leaders to provide funding for his ambitious $40 billion proposal to modernize the subway. Installing modern signals is a key part of the plan. Last week, Mr. Byford announced the hiring of a signals expert named Pete Tomlin, who has worked on transit systems in Toronto and London, to oversee signal upgrades in New York.

Subway officials have blamed “overcrowding” and growing ridership as the main reason for delays. But Mr. Byford quickly disagreed and instead focused on finding the root causes for delays. Trains on New York’s subway system travel at about 17 miles per hour on average, the slowest of any heavy rail system in the United States, according to a 2010 analysis by a transportation planner named Matt Johnson. Trains on the Bay Area Rapid Transit in the San Francisco area, for instance, averaged 33 miles per hour, he found.

Mr. Byford is trying to correct problems that resulted from changes made after the 1995 crash. The top speed for trains on the subway is about 50 miles per hour, though most trains travel slower than that. When Mr. Byford rode trains with workers, they told him slow speeds were a major problem.