"All-lane running" has been introduced between junction 10a for Wolverhampton and junction 13 for Stafford.
When trialled on the M25 near London, it "reduced congestion and led to more reliable journeys", said Highways England.
Lay-bys are replaced by emergency refuge areas - but some organisations say there are not enough of them.
The Central Motorway Police Group said the lack of lay-bys was "cause for concern" and there was "minimal scrutiny" of the scheme on the M6.
Edmund King, president of the AA said: "Laybys are every two and half kilometres - that is about one and a half miles.
"If you can't see a lay-by you have to stop in a live lane of traffic and that is so dangerous."
Matt Dallaway from the RAC said all-lane running as a concept was fine but he was concerned safety had not been tested.
MP's on the Commons Transport Committee have launched an inquiry.
Highways England said: "Our motorways are some of the safest in the world and the evidence shows collision and casualty rates on the M25 are down."
"As we gradually roll out these upgrades on other motorways we will continue to work closely with all the emergency services so we can ensure safety is maintained."
All-lane running is currently being installed on the M5 between junctions 4a for Bromsgrove and 6 Worcestershire North.
A scheme allowing motorists to drive on the hard shoulder of the M6 is "dangerous," motoring groups have said.