Part of the subpoenas also violated the Fourth Amendment right against unreasonable search and seizure, the judge ruled.
Christie cut ties with Kelly and Stepien in January after their names emerged in a series of e-mails about the allegedly politically motivated lane closures, which have presented the Republican governor with the biggest scandal of his tenure.
"Neither defendant has argued that the committee's investigation is anything but a legitimate, tailored inquiry into a matter of public interest," the judge wrote. "Yet the government's power to compel testimony has never been absolute."
Stepien's attorney hailed the opinion as "a complete vindication of Bill Stepien."
"In its zeal to achieve a blatantly political goal having nothing to do with Mr. Stepien, the committee disregarded the fundamental constitutional rights of this innocent man," Kevin Marino said in a statement.
Kelly's attorney, Michael Critchley, said: "For all the naysayers who criticized Ms. Kelly for asserting her constitutional rights, Judge Jacobson's decision provides a free tutorial on the protections the Fifth Amendment affords all citizens."
For its part, the committee will consult outside counsel to review its options, said its cochair, Assemblyman John S. Wisniewski (D., Middlesex).
To properly invoke the Fifth Amendment privilege, Jacobson said, Kelly and Stepien needed to show "substantial and real" danger of incrimination. The judge noted that the U.S. Attorney's Office is conducting a separate criminal investigation into the lane closures, and that federal law enforcement officials had attempted to interview both defendants.
"Under these circumstances, it is reasonable for Mr. Stepien and Ms. Kelly to fear that they currently face the hazard of prosecution in the concurrent federal investigation," Jacobson wrote.
Documents released by legislators in January show Kelly wrote an e-mail in August saying, "Time for some traffic problems in Fort Lee," the Bergen County borough near the bridge.
A law firm hired by Christie's office to investigate the lane closures released a report last month placing the blame on Kelly and David Wildstein, a former Christie appointee at the Port Authority of New York and New Jersey.
The report - criticized by Democrats and others as one-sided - said Christie and his current staff played no role in the traffic jams or any attempt to cover up what happened.
The judge said the subpoenas - which requested "all documents" regarding the lane closures, such as telephone and electronic records, dating back to Sept. 1, 2012 - were "overbroad." Document production therefore would have compelled testimonial evidence, violating the defendants' privilege against self-incrimination, Jacobson wrote.
Even amended requests for documents relating to 32 named individuals - including Wildstein - were too broad, she said.
Moreover, the panel failed to establish that it knew Kelly and Stepien possessed the documents it sought at the time it issued the subpoenas, Jacobson said.
"If the subpoenas had been limited to seeking only correspondence between the defendants and Mr. Wildstein, they may have been sufficient . . . because the committee already has possession of some such communications," Jacobson wrote.
The committee also argued that the Fifth Amendment privilege did not apply to records required by law to be kept by public employees. Jacobson disagreed, saying the law required only agencies or records offices to maintain documents, not individual employees.
In declining to find that Kelly and Stepien had failed without justification to comply with the subpoenas, Jacobson left the committee several ways to proceed.
Contrary to what the panel said during oral arguments in March, Jacobson found that it has the power to offer immunity in exchange for document production otherwise protected by the Fifth Amendment, in part because that is an important investigative tool.
Immunity provided by the Legislature could also protect Kelly and Stepien from criminal prosecution, and the legislative committee has said it does not want to interfere with the U.S. attorney's investigation. It was unclear Wednesday whether the panel would consider using that tool.
If Kelly and Stepien still refused to provide documents even with immunity, the panel could find them in contempt, the judge said.
Alternatively, the judge wrote, the panel "may wish to issue a new subpoena entirely, to address the constitutional concerns raised by the defendants and discussed at length in this opinion."
"There's more than one method to gather information in an investigation, and we will consider alternatives," Wisniewski said in a statement.
A Republican on the panel, Assemblywoman Amy Handlin, said the opinion sent "a clear message that the committee has overstepped its bounds."