But the number of cars on other parts of the motorway network has increased, according to new figures.
The statistics show a reduction of more than 20,000 vehicles a day on the M8 between the Kingston Bridge and the junction with the M73 in July, the first month the new motorway was open, fulfilling a key target of the project.
Up to around 60,000 vehicles used the new five-mile section connecting the existing M74 at Fullarton with the M8 in Glasgow, a figure expected to increase as traffic volumes go up in winter.
But the opening of the new road has led to higher traffic levels on approaching motorways west of the new road, with around 12,000 more vehicles per day on the M8 and 6500 more on the M77, compared with July 2010.
However, at the busiest section of the M8, between junctions 16 and 15 in Glasgow city centre, there were 26,000 fewer vehicles in July. Figures from earlier in the year, before the M74 extension was opened, suggest there would have been around 5700 fewer vehicles anyway, a slump thought to be caused by Britain's continuing economic downturn.
The figures, released under a Freedom of Information request, were seized upon by Transport Minister Keith Brown and Gordon Matheson, leader of Glasgow City Council, as proof the M74 project, which cost more than £650 million, had already delivered its core objectives.
Environmental groups who opposed the motorway were sceptical, pointing to the increased traffic volumes as evidence it was encouraging more people to drive.
Mr Brown said: "The new motorway was expected to reduce traffic on the M8 between Baillieston and Charing Cross by up to 20,000 vehicles per day. However, figures show the Completion is, in fact, exceeding expectations by achieving reductions of up to 26,000 vehicles per day."
Mr Matheson added: "Even at this early stage, this new road is meeting the targets that were set for it – and exceeding many."
Dan Barlow, head of policy for environmental charity WWF Scotland, said the M74 extension reflected part of an unsustainable transport system.
He said: "Evidence has long shown building new roads generates new traffic. It is therefore no surprise to see significant vehicle levels on the new M74 and increased traffic on the surrounding motorway network which far outweigh reductions seen on the M8."