The agency believes safety and traffic flow will improve
The hard shoulders on the M4 and M5 around Bristol are to be opened up to cars during the rush hour - to ease congestion.
The Highways Agency will start work later this year on a 12-mile stretch running through the Almondsbury interchange.
The so-called managed motorways project affects junctions 19 to 20 of the M4 and junctions 15 to 17 on the M5.
By 2012 the hard shoulders will be opened as lanes at peak times.
Managed motorways use a range of techniques, including variable speed limits, to control traffic.
A pilot project on the M42 near Birmingham showed a 22% improvement in reliable journey times and a drop in accidents from an average of 5.1 a month to 1.8 a month.
Project manager Rob Cameron said: "This scheme will reduce congestion, make journey times more reliable and improve safety for road users.
"Our detailed surveys showed that hard shoulder strengthening work is not needed, which means road users won't be affected by additional roadworks during the busy spring and summer months."
The agency said that the use of the hard shoulder will also add more capacity at a lower cost than conventional road widening and with a lower environmental impact.
Emergency refuges for motorists who break down will be provided at regular intervals and the network will be closely monitored by staff at the agency's control centre.
Work will start in the autumn and the scheme is expected to be completed during 2012.