The one mile (1.6km) section of the A453, between Crusader roundabout and Farnborough Road, was opened in time for rush hour earlier.
After more than a decade of discussion, the £169m project to widen the whole road began in January 2013.
A further five-and-a-half miles (8.9km) from Mill Hill to junction 24 of the M1 is due to open around 24 June.
The road is to be called Remembrance Way in honour of the 453 British service personnel killed in Afghanistan.