Rush-hour driving has been reduced to a crawl on the road linking Glasgow to Stirling and the north east, since the £320million upgrading work began in January 2009.

Motorists have had to contend with diversions and closures leading to continuous tailbacks with a Glasgow to Stirling trip taking up to two hours. However, with the speed limit about to be set at 70mph on the Haggs to Auchenkilns stretch, the journey should take no more than 30 minutes.

Pete Rodger, of the Institute of Advanced Motorists, welcomed the move but added: "Motorists have been frustrated at being kept at a 40mph limit, especially at times when the traffic is light."

Government agency Transport Scotland said the scheme will cut journey times on the main route from the Highlands and the north-east to Glasgow and the south by 30% to 40% during peak periods.

The 40mph speed limit will remain in place on the southbound carriageway between Auchenkilns and junction four at Mollinsburn/Westfield and the upgrade is expected to be completed this autumn.