Two Los Angeles traffic engineers have pleaded guilty to charges that they hacked into the city's traffic control computer as part of a union dispute over wages.

Gabriel Murillo, 39, and Kartik Patel, 36, have both admitted that they broke into the Los Angeles Automated Traffic Surveillance Center, which controls traffic lights in a city with one of the highest rates of car ownership in the world.

The pair accessed the system illegally and shut down traffic signals at four critical points in the road network, causing crippling delays. It took four days to sort out the system and get it working normally.

The hack is thought to have been part of a pay-bargaining procedure between employers and the Engineers and Architects Association, which represents workers at the Automated Traffic Surveillance Center. But there is no suggestion that the union knew about or condoned the men's actions.

Defence attorney James Blatt, the lawyer for Murillo, told Los Angeles Times: "This was an emotional collective bargaining strike situation that should have been handled administratively.

"Mr Murillo and Mr Patel are outstanding citizens and have devoted a significant part of their professional lives to transportation safety in Los Angeles County."

The two men have been ordered to pay full restitution, and serve 120 days in jail or complete 240 hours of community work. Their home and work computers will also be monitored.

The case highlights the problems of employees gaining control of internal systems during disputes. In August the San Francisco state intranet was hijacked by a rogue systems administrator, which may ultimately cost a million dollars to repair.