The final stretch of the M1 upgrade between Bedfordshire, Milton Keynes and Northampton has started its phased opening.
A fourth lane will be available for the next four nights between junction 15 (Northampton) and 16 (Upper Heyford).
By 06:00 GMT on Friday the whole section would have the additional lane open, National Highways said.
A temporary 60mph (95km/h) speed limit will be in place while final technology tests are carried out.
Drivers will be able to use 46 new emergency stopping areas.