A 3-year-old boy suffered life-threatening injuries Thursday when a truck hit him in San Francisco's Outer Richmond District, authorities said.

A Dodge Ram 1500 was traveling north on 43rd Avenue just outside Golden Gate Park at 11:43 a.m. when the driver turned left on Fulton Street with a green light, said Officer Gordon Shyy.

The boy was riding a bicycle in a crosswalk on Fulton, traveling north across four lanes on a walk signal, when the driver ran him over and dragged him several feet, Shyy said.

Multiple family members were a short distance behind the boy when he was struck. He was transported to San Francisco General Hospital.

Once realizing he had struck the boy, the driver did a U-turn and returned to the scene. He is cooperating with the investigation, Shyy said.

Carlene Murray, 34, said she was playing just inside the park entrance with children from her nearby day care center when the crash happened.

She said the boy, who was not wearing a helmet, pedaled past on a red bike moments before he was struck.

"I called out to the mother and said, 'Your boy is riding toward the street,' " Murray said. "She acknowledged me, but didn't make any move to stop him."

Murray's assistant started running toward the intersection, but didn't make it in time. Murray said she turned around when she heard screams and saw the boy lying in the street.

She said she called police as her assistant began first aid.

"That intersection is notorious," Murray said. "I cross it at least a couple times a week with the kids and I never let them get more than a step in front of me.

"People just don't pay attention. The lights they have are OK, but they really need to put in those flashing yellow overhead lights to warn drivers about pedestrians."

Neighbors agreed with Murray, describing the stretch of Fulton Street as dangerous for both pedestrians and motorists.

Hannah Harrison, 23, who lives about a block from where the boy was hit, said her car was recently "totaled" by a drunken driver there.

"This street is bad," she said. "People fly down here at about 60 miles per hour."