Variable speed limits to smooth out traffic flows and open up the hard shoulder as an extra traffic lane. Completion is scheduled for spring 2014.
The managed motorway scheme - the first of its kind in the South West - on the busy Almondsbury Interchange will cover 3.4 miles of the M4 between junctions 19 and 20 and 3.1 miles of the M5 between Junction 15 and Junction 17.
Work on the M4 and M5 will include strengthening the hard shoulder, building emergency refuge areas, installing gantries and electronic signs, and installing sensors in the road to measure traffic flow. CCTV cameras will also be installed to monitor the motorway and emergency refuge areas.
It is one of 20 major road improvement projects due to start construction before March 2015, as part of the government’s £2.1bn road programme.
Roads minister Mike Penning said: "While this will be the southwest’s first managed motorway, experience on the M42 and M6 around Birmingham, shows that managed motorways deliver significant safety and journey time benefits.”
Preparatory work – including site clearance and the installation of CCTV cameras started in November 2011.
Total estimated project cost is £88.6m.
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