Karen Thorp, who lives off the Great West Road, was walking into Hounslow when she witnessed the landing of the air ambulance on the carriageway.
She described the work of emergency services, particularly the landing of the air ambulance in such a small space, as "amazing" in such sad circumstances.
Speaking to getwestlondon, Ms Thorp said: You couldn't see very much as police were keeping everybody away.
"You could see the ambulance crew were working on someone.
"Police were there cornering off everything and fire crews there to do anything they could.
"The air ambulance landing on the Great West Road was a skill as there's not a lot of area to land and it was amazing to watch.
“Sad the air ambulance was there but amazing how skillfully the pilots are, and they work in such difficult situations."
The tragic incident on Wednesday (September 13) afternoon saw one boy, believed to be around 11-years-old, suffering from a severe head injury flown to hospital.
A second pedestrian, a girl, was also treated for minor injuries and taken to hospital by road in an ambulance.
Investigation works continued to take place with the Great West Road only fully reopening at around 8.30pm.
Speaking about the traffic situation hours after the collision, Ms Thorp added: “Traffic is horrendous. The vehicle involved – a lorry – is still there.
“It's amazing in such horrible circumstances all these people pull together.
“It's a sad job and they don't get enough praise, especially police.
“They have all done an amazing job in a tough situation.”
Emergency services were called to reports of a road traffic collision on Great West Road at around 3.30pm.
The driver stopped at the scene.
No arrests have been made and police enquiries continue.
Follow our live blog for updates on the A4 crash.
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