This is the dramatic moment a road sign collapsed onto a runaway truck that smashed into a motorway post.

Dashcam footage shows the driverless Mitsubishi trailer truck rolling across a busy highway in Chonburi, Thailand, on September 3.

It wove between lanes before crashing into a road sign, causing it to collapse. The fallen sign triggered traffic congestion stretching more than one kilometre

Police arrived after receiving a report at 8:30 am local time. They coordinated with highway officials to clear the obstruction.

Driver Kajornsak Simasom, 34, said he left the truck to withdraw cash from an ATM.

He said: 'I parked the trailer truck on the left side of the road to withdraw money from the bank, but when I came back it had already started rolling away.

'I ran after it but couldn't catch up. It crashed into the signpost. I had pulled the handbrake and switched on the signal light, but I must not have fully engaged it. Luckily, it didn't crash into other cars or the damage could have been worse.'

Police said normal traffic flow has resumed on the motorway, adding legal action will be taken against Kajornsak.

Despite being one of the wealthiest countries in Asia, Thailand suffers from chronic underinvestment in its highways and education, contributing to its poor road safety levels. Only a handful of African countries are worse.

Thailand has one of the world's worst road safety records. Ministers have set the goal of reducing fatalities from 32.7 deaths per 100,000 people to 12 per 100,000 people by the year 2027.