Drivers face longer journeys after one of the two main link roads between Haywards Heath and Burgess Hill was closed for six weeks.
Vehicles now have to negotiate long diversions to avoid Rocky Lane after barriers went up on Monday.
The closure, which is causing congestion on other roads around the area, is to enable the continuing works on the fifth stage of the Haywards Heath Relief Road.
The latest construction phase now beginning is to realign Rocky Lane with a new roundabout. The work is expected to continue until February 24.
At a later date a new railway bridge will be completed to take the bigger relief road over the London to Brighton railway line.
When completed in its entirety it is estimated that the £15m relief road will divert eight million drivers a year away from Haywards Heath town centre.
At the same time, the final stage will allow people from the southern end of the huge Bolnore estate to access the town more easily.
Journeys between Haywards Heath and Burgess Hill will also improve for the occupants of the new estates now being built off Rocky Lane, as well as the recently completed Sandrocks estate and those at the former St Francis Hospital site.
Crest Nicholson has said it is committed to keeping local residents informed of the works in order to minimise disruption wherever possible. A newsletter is being distributed to 5,000 households in Haywards Heath.
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