“Whilst I can confirm a man has been arrested, I would ask that the wider public refrain from speculation; an arrest does not imply guilt and the man in custody is continuing to provide an account to us.”
The family of the dead man have been informed and are being supported by officers.
Police said they received a call at 06:50, shortly before the crash, reporting that a man who appeared vulnerable with grey hair and wearing dark clothing was walking alone on the A120 towards Colchester.
Officers said this description matched that of the man who died.
A white van was also found with significant fire damage in a layby on the Braintree-bound carriageway, close to Watery Lane, before Bradwell.
The force said it would like to hear from anyone driving on the A120 between Bradwell, Coggeshall and Colchester from 06:30 to 07:10 on Saturday who may have seen the man, or anyone who may have witnessed the van being set alight or someone accessing it.
According to the traffic-monitoring service Inrix, the road was reopened at about 17:25 on Saturday.