The man was killed and a second man seriously injured when a lorry ploughed into a roadwork crew. The truck driver was also injured and was taken to hospital. He is regarded as a suspect, police said.
The A12 has been closed for several hours between Rotterdam and The Hague, leading to major jams.
Drivers were advised to detour via Utrecht – adding up to an hour to their journey – or stay home until the road is re-opened.
Jams also built up on the A27 and A15.