A cyclist has pleaded guilty to vehicular manslaughter after running down a 71-year-old pedestrian in what San Francisco's top prosecutor said appeared to be the first conviction of its kind in the US.

Under the unusual plea deal last week, Chris Bucchere, 37, would not serve any jail time but would be sentenced to three years probation and 1,000 hours of community service following the death of Sutchi Hui of San Bruno, district attorney George Gascon said.

"Our goal is to send a message to cyclists about safety," Gascon said. "Just because you are riding a bicycle doesn't mean all bets are off. All of the rules of the road that apply to everyone else apply to you too."

Bucchere, a software engineer from San Francisco had been riding recklessly and had run three red lights when he struck Hui as he and his wife crossed a street in the Castro district of San Francisco on 29 March 2012, prosecutors said.

Hui died four days later from his injuries. His wife was not hurt. Hui's family has filed a civil suit against Bucchere.

Bucchere's lawyer, Ted Cassman, did not return calls seeking comment.

Gascon said the victim's family did not want to see Bucchere imprisoned and prosecutors did not think a judge would sentence him to jail, so they offered probation and community service in the plea deal.

He added that they did not want to risk a possible not guilty verdict at a trial.

"We believe this is the best outcome for this type of case," he said.

Gascon said his office had carried out research and could not find any other cases in which a prosecutor had obtained a manslaughter conviction against a cyclist.

"To our knowledge, we believe it is the first in the nation," he said.

The incident drew widespread attention and criticism after Bucchere, while in hospital with his injuries, posted his thoughts online on the Google group Mission Cycling AM Riders.

Gascon said prosecutors argued during a preliminary hearing in March that the post did not show any remorse, as defence lawyers had argued.

"It gave us a window into his state of mind at the time," Gascon said.

Bucchere said in the posting that the traffic light was turning yellow as he approached the intersection.

"I was already way too committed to stop … I couldn't see a line through the crowd and I couldn't stop, so I laid it down and just plowed through the crowded crosswalk in the least-populated place I could find," the post said.

It later added: "I hope he ends up OK," in an apparent reference to Hui.

Bucchere is due to be sentenced on 16 August. A superior court judge could determine within six months if his conviction can be reduced to a misdemeanour.