Some lights around the Canford Bottom roundabout are being tested today as the £5.7million project nears its final stages.
Drivers should comply with any lights that are on around the roundabout near Wimborne, a Highways Agency spokesman said.
But the lights are just on for testing and it does not mean the whole junction is open, he added.
The revamped roundabout is due to be finished in time for the Olympics.
An opening date is expected shortly.