With the credit markets suffering as a result of the global financial crisis, the Metropolitan Transportation Authority has taken the unusual step of appealing directly to its riders for help in raising cash — but not, in this case, with a fare increase. The authority began running radio ads over the weekend, with former Mayor Edward I. Koch promoting the sale of tax-exempt bonds that it uses to finance its large-scale projects.

“These bonds help buy new equipment and maintain the network, which will improve reliability, safety and customer service,” Mr. Koch says in the ads.

The authority typically sells its bonds to large institutional investors; the appeal to the general public is unusual and intended to generate greater demand for its bonds at a time when the credit markets have been badly shaken. The ads tell interested buyers to contact their broker or financial adviser or to go to the authority’s Web site, www.mta.nyc.ny.us, where a prominent link will take them to the bond sale prospectus.