A pedestrian and two cyclists — one a 12-year-old boy — have been hit by a van just outside the West Beach caravan park entrance.

Police and emergency services rushed to the scene on Military Rd, near the mini golf course, just before 10.20am on Monday after reports of the collision.

media_camera Paramedics help the two cyclists who were run down by the out-of-control van on Military Rd, West Beach. Picture: AAP / Brenton Edwards

media_camera Emergency services at the accident scene on Military Rd, West Beach. Picture: AAP / Brenton Edwards

A white van travelling south down Military Rd struck the pedestrian before veering on to the other side of the road and hitting a father and son on bicycles.

The van then crashed into a Stobie pole.

The 44-year-old pedestrian from Mitchell Park has been taken to Royal Adelaide Hospital in a critical condition.

The father and son cyclists — 41 and 12, from West Beach — suffered non-life-threatening injuries.

Police are investigating if the driver of the van, a 55-year-old Fulham man, suffered a medical episode just before the crash.

Shocked witnesses told how the father cared for his son — who had tyre marks over his body — despite being in serious pain with an open leg fracture.

media_camera A van has hit a pedestrian and two cyclists on Military Road, West Beach. Picture: Rebecca Tagliaferro media_camera A van has hit a pedestrian and two cyclists on Military Road, West Beach. Picture: Rebecca Tagliaferro

Military Rd has now reopened between West Beach Rd and Africaine Rd to all traffic after being closed while Major Crash investigators examine the scene.

Rebecca Tagliaferro was driving north on Military Rd when she saw the white van driving “on the wrong side of the road” before it mounted the kerb and hit the father and his son on their bicycles.

media_camera Witness Rebecca Tagliaferro saw the crash and stopped to help the cyclists. Picture: AAP /Brenton Edwards

“The father was amazing ... he knew at the very last minute the van was coming towards them and he tried to pull his son (out of the way) but they just couldn’t get out in time,” she said.

“(The driver) just kept veering ... he took out one person then another two people on their bikes.”

Ms Tagliaferro stopped her car and rang triple-0 before helping the cyclists.

She said the father comforted his son despite suffering an open leg fracture.

“I stayed with the boy and held his hands,” she said. “He was only 12, I kept telling him how brave he was, he didn’t want his mother to see him like this.

“I’m in shock and I’m not even in the crash.”

Ms Tagliaferro said the boy also had tyre marks on his body and felt “a lot of pain in his legs”.

media_camera The father and son cyclists are taken to hospital by paramedics. Picture: AAP / Brenton Edwards

Costa Dinedios, 63, who was travelling with Ms Tagliaferro, said it was”traumatic” to see the cyclists hit.

“Suddenly there was this crash then they went down ... I just had a sick feeling,” he said.

“It looked like he had killed them.

“I went to look in the van and the driver was lying flat, he couldn’t get up.”