New York City subway riders are familiar with frequent service disruptions at night and on weekends.

But a report released Sunday by the state and city comptrollers says MTA needs to do a better job of keeping straphangers informed about changes that affect their rides. The report by New York State Comptroller Thomas DiNapoli and New York City Comptroller John Liu found that projects that disrupt subway lines often go over budget and that New York City Transit's efforts to publicize the service changes are inadequate.

It also found that at times service is disrupted when track work isn't happening. The report examined a random sample of 50 out of 3,332 service disruptions from Jan. 1, 2009 to Jan. 4, 2011 and found inefficiencies.

The auditors said, for example, that NYC Transit should recalculate how many buses it uses to fill in for out-of-commission subway lines. Subway diversion projects often go over budget, the report said. The auditors examined 12 contracts and found that four of them were over budget by a total of $26.6 million.

In its response to the audit, NYC Transit President Thomas Prendergast said that the agency's accounting system isn't set up for that type of budget monitoring, but it is working to ensure greater accountability and better budget estimates. The budget overruns mentioned in the audit may include costs not related to the diversions, he said.

The audit found that the track work often begins after the start of the diversion and ends early - with 10 to 27 percent of the diversion window lost. Prendergast said that the delays are caused in part by safety measures but added that the agency is working to improve efficiency. The report also found that NYC Transit does a good job of publicizing service disruptions on the website of its parent agency, the Metropolitan Transportation Authority, but should post more and better signs in stations and on trains.

Prendergast said that the agency has modified its signage since receiving the report. "We question whether Transit's budget is sufficient to alert millions of subway riders to diversions," the report said.