Concrete barriers are to be installed on the M5 near Weston-super-Mare, replacing the worn out steel barriers.
A temporary 50mph (80km/h) speed limit will be in place while the work is carried out, but the limit will be suspended during the Easter bank holiday weekend.
A Highways Agency spokesman said the work, costing £3.8m, will begin on Friday and take three months.
A series of overnight slip road closures have also been scheduled.
"Concrete barriers reduce the risk of vehicles involved in accidents crossing from one side of the motorway to the other," said Dave Sledge, a Highways Agency manager.