The £17.5m dual carriageway, to be called John Horrocks Way, will accept its first vehicles at 10.30am - but there will be no official ribbon cutting.
Lancashire County Council has decided against a civic ceremony because of strict “purdah” rules in the build up to the General Election in two weeks.
The low-key opening will initially bring an even more dramatic drop in traffic along Penwortham’s main road than the expected 70 per cent reduction predicted.
As vehicles start using the bypass, LCC will immediately close a small section of the A59 between the new road and Howick Cross Lane in both directions for up to three weeks to allow workmen to finish the junction for local traffic.
A spokesman said: “This can only be done once John Horrocks Way has opened.” Pedestrians and cyclists will be unaffected by the closure.
Changes at the other end of the town, at the bottom of Penwortham Hill, will also come into play.
The controversial stopping up of the slip road up to the flyover will coincide with the bypass opening. And a new layout for traffic going under the bridge will be opened at the same time.
The new 1.3km road is expected to take up to 22,000 vehicles a day out of the centre of Penwortham, reducing congestion and improving air quality.
A bypass was first suggested more than 30 years ago due to a traffic build-up along the busy A59 thorougfare in and out of Preston.
The project, which has taken almost two years to complete, will be followed by major works along the main road in Penwortham, with wider pavements and extra provision for cyclists.
The past two years have seen a boom in food and drink venues - and more are planned - in preparation for the drastic reduction in traffic.
The new road is named after a prominent member of the well-known Horrocks textile family. John Horrocks lived in Penwortham and was MP for Preston in the early 1800s. The idea to name the bypass after him came from schoolgirls Grace and Laura in a council competition.