Another stage of a £ 100 million congestion-busting programme on a Midland motorway is set to go live.

The M42 Active Traffic Management project on one of Europe's busiest stretches of motorway - between junction 3A with the M40 and junction 7 with the M6 - is to introduce mandatory, lane specific speed limits. It is due to go live on Monday.

The move, part of a package of measures to include opening the hard shoulder to traffic during congestion in 2007, is designed to manage traffic flows to prevent jams building up.

The speed limits will be controlled from the new traffic control centre in Quinton and displayed to drivers via the latest overhead gantry signs, spaced at intervals of just 500 metres along the 12 milelength of the pilot stretch.

The Highways Agency said the scheme represented onefifth the cost of widening the road, was quicker to introduce and should increase capacity on the stretch of motorway by up to 20 per cent.

Derek Turner, the HA's national traffic director, said: "This scheme is an important part of measures the HA is using to cut congestion in the West Midlands.

"This stage of the project introduces variable mandatory speed limits to the area for the first time and will help drivers by reducing stop-start traffic."

The system is already in operation on the M25 west of London and the HA said 60 per cent of motorists had noticed an improvement since its introduction.

It added that there was a six per cent decrease in ' stopstart' traffic as well as ten per cent fewer serious crashes, 30 per cent fewer minor accidents and up to eight per cent less emissions.

The switch-on of the mandatory speed limit system - which includes a wealth of early detection sensors and cameras to automatically slow the traffic down further back from accidents or queues - will happen overnight on Monday and be in full operation by the morning rush hour on Tuesday morning.

David Grant, who is managing the project for the HA, said it would be a "new driving experience" but added for the majority of the day it will not be used.

"However, when congestion builds up we will have a huge ability to be responsive to what's happening and communicate quickly with the driver," he added.

The ATM project covers one of the most heavily congested stretches of UK motorway. It takes 125,000 vehicles every day and serves increasingly busy infrastructure such as Birmingham International Airport and the National Exhibition Centre.

Advisory speed limits have been on the gantries since the summer and their becoming mandatory will be denoted by a red ring around the number.

A red cross will appear over the hard shoulder, meaning it is for emergency use only. In early 2007, that lane will be opened up at times of congestion.