Two motorways were shut on Saturday evening for several hours after human remains were found on the carriageway.
Several drivers called police from 6.40pm to report "something in the road" between junctions 20 and 21 of the M4 - between Almondsbury and Awkley.
The body is believed to be that of a man in his forties, whose next of kin have been told.
A police investigation to establish how the person came to be on the motorway continues.
The M4 was closed in both directions between junctions 20 and 22 due to the incident north of Bristol. The motorway has since been reopened.
The eastbound M4 was reopened around 1.30am on Sunday with the westbound closures removed about 3am.
In a statement, Avon and Somerset Police said: "Police are keen to hear from anyone who was travelling along that stretch of the M4 has any relevant information or dashcam footage."
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The M48 was also shut both ways between the M4 and junction 1 near the Severn Bridge.
National Highways later said the motorway was open again.
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