Work starts on Tuesday and lasts until spring on the notorious Hatfield Peverel section of the trunk road dubbed the 'Worst Road in Britain' in surveys
The project will slot an extra lane between Hatfield Peverel and Witham South Junction taking some risk out of manoeuvres when joining and leaving the route.
The tarmac will also be resurfaced and gain extra street lighting and road signs.
Two lane operation in each direction will continue day times during the work which ends in March.
But there will be weekday closures between 8pm and 5am, 8pm to 8am on Saturday or Sunday mornings with a 50mph speed limit all the time.
The Government's highways agency, which is doing the work, also warns slip roads may be shut overnight with advanced warning, and clear diversion signs.